Select Page

Weed Flora


As horticulturists of any persuasion, we have a compelling environmental responsibility not to import, cultivate or promote known or potential weeds

Geoff Carr, 2002

Alien or naturalised plants have now become a significant proportion of the flora of all major countries, their effects being most pronounced on islands. This means that they have become a routine and costly part of plant management. They can be considered as one of the six major economic plant groups, the others being: medicinal plants and spices; agricultural cereals; horticultural crops; forestry plantation trees; ornamental plants.
Naturalised plants are part of the global redistribution and homogenisation of world vegetation that gathered momentum in the Age of Discovery when European countries were engaged in colonial expansion. Much of this was related to the the export of European agricultural practices to the temperate regions of Neo-European Africa, Australia, New Zealand, North America, South America. It was also associated with the redistribution of tropical crops, mainly in the eighteenth century and through the Caribbean and East Indies. Along with these crops came their associated weed flora as contaminants and, in an eighteenth century period of plant mania (botanophilia) centred mostly on England, ornamental garden plants became the subjects of plant exploration, a rapidly expanding commercial nursery industry, and an unprecedented period of plant introduction that would contribute to an expanding list of plants that were escaping from cultivation into the wild.

This article on weeds been written for horticulturists and gardeners. It gives an outline of the environmental, economic and social effects of weeds, especially those that have escaped from horticulture, and discusses the present and possible future ways of managing weed impacts.

World-wide, after land clearing and climate change invasive species present the greatest threat to biodiversity. Exotic invasions reduce ecosystem function and agricultural production as well as having negative impacts on human health and other social values.

In Australia, as land-clearing is progressively addressed, invasive species [37] have become the greatest threat to nature 39 and an enormous economic burden as weed control and lost production now costs Australia’s primary industries in excess of $4 billion p.a.[1]

Invasion ecology gathered momentum as a new science in the late 1950s. Globally, a major initiative was taken in 1997 through the establishment of The Global Invasive Species Program of the World Wildlife Fund which was supported by the United Nations Environmental Program.

In general, the problem of invasive species, both animals and plants, is getting worse. [10][11][104] The percentage of naturalised plants is increasing, both nationally and at a state level. As international trade, travel and tourism increase so does the artificial spread of species across the globe and to this scenario can now be added the uncertain impacts of climate change. Diseases such as AIDS and bird ‘flu’ illustrate clearly, in human terms, the potential dangers of modern transport systems. Stringent quarantine restrictions, increased public awareness, and additional government finance for invasive species management are still making little impact on this major threat to Australian and global ecosystems.

In Australia the weed problem is being addressed through The National Weeds Strategy of 1999. The Australian Weeds Committee ( acts as a coordinating body between government and other weed agencies while the Cooperative Research Centre for Australian Weed Management assist in coordinating national projects on particular aspects of weed research, awareness and management. State and Territory agencies also establish strategies and on-ground management, and in most States/Territories local government and other agencies also play an active role. In Victoria, for example, management of the risks posed by weeds is outlined in Victorian pest management: a framework for action: weed management strategy (2002) which was a follow-up to the Victorian Weed Strategy (1999).

Although the major programs listed above provide a framework for action, effective results can only be achieved with the cooperation of smaller groups: environmental agencies and organisations, land managers, Landcare, Friends groups as well as the nursery, landscaping and turf industries, botanic gardens, local government, community groups and individual gardeners. All have a role.

The vast majority of Australia’s environmental and economic weeds are plants that were deliberately introduced for ornamental horticulture, escaping from cultivation into the natural environment and agricultural land. It seems inevitable that many weeds of the future are already present in Australia, growing in our gardens.

Australia is a strongly urbanised nation with over 80% of the population living in urban seaboard cities. Being so distant from nature it is easy to forget our dependence on natural areas. Gardeners and horticulturists must play their part in protecting the wild environments that gave them their plants.


Environmental and agricultural weeds are now regarded as a primary threat to our natural and agricultural systems. What kinds of plants are causing these problems, where are they from and how did they get here? How do they invade natural areas and what can we expect in the future?

What is a weed?

For the purposes of this document a weed * is a plant growing where it is not wanted, generally with detrimental consequences. This is sometimes referred to as growing “out of place”. What is “out of place” is a subjective matter and will depend on the context – so we have environmental weeds, horticultural weeds, agricultural weeds, marine weeds 105 and so on. From an environmental perspective, agricultural crops could be viewed as weeds: from a farmer’s perspective, indigenous native plants growing in his crops are weeds. Whatever the perception, weeds usually require some form of action to reduce their impact.

The Australian National Weed Strategy of 1999 defines a weed as a plant that has, or has the potential to have, a detrimental effect on economic, social or conservation values.

Alien, naturalised and invasive plants

Plants that are growing in an area where they are not native are generally termed aliens, exotics or invasive alien species (IAS). Some of these will only survive for a short time because they cannot establish self-sustaining populations and only persist by new introductions: they are then known as casuals or casual aliens. Those aliens that have become established and are reproducing themselves without human assistance are said to be naturalised.

Every country will have some naturalised plants (see Figure 1) and their numbers are generally increasing. For example, In England there are 2,271 non-native species and hybrid plants in the wild, 73% of which have escaped from gardens 88. Australia now has a total of 3244 naturalised plants (Hosking, pers. comm, Nov., 2004), which is about 10% of the total flora (see Figs 1 & 2) and about 10 new plants become naturalised each year.[46] In New Zealand the proportion of naturalised plants is a staggering 43-50%.[7][76]

Although all naturalised plants can be regarded as weeds, the various species will pose different degrees of threat to agriculture and the environment depending largely, it seems, on where they are growing and their particular propensity to become weedy. For example, a study in 2003 concluded that of the 2660 species regarded as naturalised in Australia in that year, 798 (about30%) were considered a major problem and 1388 species (about 50%) a minor problem. [3]

Aggressively weedy plants that spread rapidly are said to be invasive. These plants are opportunistic and appear to thrive in the absence of their natural predators and diseases. This is dramatically illustrated by a number of Australian native plants that are relatively innocuous in Australia but are devastating environmental weeds in South Africa.

Where are they from?

In a general sense the weed flora of a particular region has arrived in that region as a result of commercial interactions and common cultural ties 5. To have survived it also probably came from a climatically similar area of the world. It is therefore no surprise to learn that the great majority of environmental weeds in Australia (over 98%) are from overseas, from the Americas (31%), Europe (27%), Africa (26%), Asia (10%) and 4% from elsewhere.[1] Australasia contributes about 2% of this number through the native species that have spread beyond their natural distribution, largely because of human activity.

Why were they introduced?

Some weeds have been introduced unknowingly with other goods – with livestock or as contaminants of grain and agricultural produce, in transported soil, ships ballast, on machinery, vehicles, shoes etc. However, by far the majority have been deliberately introduced for use in agriculture and horticulture, for fodder, erosion control, the aquarium industry, gardens etc.

As a result of Commonwealth policy supporting research into pastoral productivity, and over a period of more than 70 years in the 20th century, the Commonwealth Plant Introductions (CPI) scheme made 145,000 accessions consisting of more than 8200 species: among these were more than 2200 grasses and 2200 legumes representing about twice the indigenous flora in those families and about 22 and 18%, respectively of the total global flora in these groups 106.

In Australia at least 70% of the naturalised flora has been deliberately introduced (see Fig. 3) while in the USA this figure is as high as 83%.[34][46][74]

How do they escape?

Once introduced, there are many ways that plants can “escape” from cultivation. Their propagules may be spread on shoes, vehicles or machinery; populations can establish from dumped garden waste, from poorly managed or abandoned sites such as old cemeteries and homesteads, abandoned gardens etc. They may be distributed naturally as wind-blown seed, or by animals and birds with seed or fruits attached to their bodies, or eaten and distributed through their droppings.

The process of invasion

Plant invasions can be divided into four stages: arrival, establishment, local spread, and range expansion. The invasive success of a species can be positively correlated with propagule pressure, which is the frequency and abundance of propagules entering an area as part of the arrival phase. Land disturbance is thought to play an important role in the establishment phase of many species whether from fires, floods, logging or whatever. Once established the plant must spread locally, and its degree of success will depend on its dispersal ability and how effectively it can compete with the surrounding vegetation, this often being manifested as a high relative growth rate. Range expansion may follow as the plant spreads across the landscape, possibly into new habitats. This is more likely if a wide range of environmental conditions is tolerated, that is, if the plant has a broad ecological amplitude.

The future scenario

New introductions remain a major hazard: globally, 22,000 plant species are considered capable of becoming IAS in addition to the 4,000 species that already are 94 . Of the 24,249 introduced species that have not naturalised there are 4565 with records of naturalisation elsewhere in the world 99. Unchecked, alien species will continue to swamp native species, interfere with agricultural systems, hybridise with relatives from which they were formerly isolated, and have an increasing general influence on plant evolution.[111]

A list of plants introduced to Australia is now available from the Australian Weed CRC web site at It can also be searched on-line via the University of Queensland web site at>


Here are some of the different kinds of weeds: the categories are not mutually exclusive.

Aquatic weeds

Some of Australia’s worst weeds have taken over our waterways, blocking irrigation and drainage channels, hindering fishing and destroying wildfowl habitat e.g. Eichhornia crassipes, Water Hyacinth; Salvinia, Salvinia molesta, (probably introduced as an aquarium plant); Alternanthera philoxeroides, Alligator Weed. Most of Australia’s waterweeds were deliberately imported for use in garden ponds and aquaria.[18]

Agricultural (agrestal) weeds

These are weeds of agricultural land, impacting on pastures and generally reduce crop yield and quality. Well known examples are Echium plantagineum, Paterson’s Curse, and various brassicas. Because of their direct impact on our food supply these weeds have been given more attention than other kinds of weed. Agro-ecosystems are artificial, species-poor, environmentally homogeneous with predictable disturbance regimes while natural habitats are species-rich, environmentally heterogeneous and often with unpredictable regimes. Thus plants invading agro-ecosystems are mostly herbaceous species while those invading natural habitats have a broad range of life forms.

Urban (ruderal) weeds

Weeds mostly restricted to disturbed sites and waste places (e.g. Rumex spp., Docks; Taraxacum sp. agg., Dandelion; and many grasses). These weeds tend to grow in climatically comparable areas across the world as human commensals on rail lines, abandoned building sites sites etc., a kind of cosmopolitan urban flora: they can be unsightly and/or a potential fire hazard.

Garden weeds

These are unwanted garden plants that generally spoil the intended appearance of a garden and compete with the deliberately cultivated plants for water, nutrients, light and space. Examples include: Cerastium spp., Chickweed; Epilobium spp., Willow Herb; Galium aparine, Cleavers; and Oxalis purpurea, Oxalis.

Environmental weeds

Environmental weeds can be defined formally as alien plants that have invaded natural vegetation and are presumed to impact negatively on native species diversity and/or ecosystem function 3.


The global proliferation of environmental weeds (see Figure 4) has coincided with the huge explosion in human population that has taken place over the last 200 years and has lead to an increasing homogeneity of plant species in regions of similar climate. Not all naturalised plants are environmental weeds because some are restricted to farmland, roadsides etc. and do not invade native vegetation.

There does not appear to be any particular life-form predominant among these weeds; they may be trees, shrubs, climbers etc. A recent publication provides descriptions of over 400 species of the world’s major environmental weeds.[75]


It is estimated that Australia has 1388 environmental weeds 3 which is about 6% of the total flora (see Table 1). As part of the Australian National Weeds Program in 1998 two publications were produced that provided a comprehensive overview of the environmental weed problems in Australia at that time and these are available on the web 10, 18. About 60% of naturalised plants spread into native vegetation and about 10% are invasive: the number of naturalised weed species is increasing at the rate of about 10 species a year. Unless these weeds are managed, native plant communities will progressively disappear. Popular books are available describing and illustrating the environmental weeds of South-eastern Australia [13][21][22][96][97] and Australia’s wet/dry tropics 112, as well as a DVD produced in 2008 that covers the whole of Australia.[114]


The most recent published list of environmental weeds in Victoria appeared in 1992 and included 584 taxa (576 species) or 70% of the total of the State’s 825 naturalised plants. For each plant is given information on life form, geographic origin, means of introduction and dispersal, the vegetation formations invaded, and the potential risks5. A summary of the numbers of naturalised plants and environmental weeds World-wide and in Australia, New Zealand and Victoria is given in Table 1. In Victoria, the number of naturalised species is increasing at a rate of about 6-8 per year but know relatively little about them considering their economic and conservation significance.

Historically, environmental weeds have attracted less attention than agricultural weeds, the latter having an obvious and direct economic impact on humans through their interference with the growth of agricultural crops and grazing. Recognition of environmental weeds accelerated in the 1970s with the earliest publication of the expression “environmental weed” being in 1975 104.

Weedy Native Plants

Most of Australia’s weeds are from overseas (often referred to as “exotic”) but native plants can be weedy too when they are spread by humans beyond their natural range not only in Australia 56 but occasionally overseas.


Some of the worst environmental weeds in South Africa are from Australia (although probably introduced from English nurseries), notably: Acacia mearnsii, Black Wattle; A. cyclops, Round-seeded Acacia; A. longifolia, Sallow Wattle; A. melanoxylon, Blackwood; Hakea sericea, Silky Hakea and other hakeas (introduced for hedging or soil stabilization). All were introduced before 1865. The high amenity Pittosporum undulatum, Sweet Pittosporum, is invasive in several countries including S Africa, Azores, Hawaii, Jamaica and elsewhere63.

In the United States, which currently spends more than $A150 billion a year controlling infestations of non-native plants, the Australian Broad-leaf paperbark, Melaleuca quinquenervia, now infests more than 500,000 acres of the Florida Everglades and is banned from sale (see

In the UK over $A7.5 million p.a. is spent on controlling the Australian Swamp Stonecrop, C. helmsii, which has spread since 1970, choking ponds, threatening the survival of newts and frogs to become one of Britain’s worst weeds.

Australian eucalypts and acacias, promoted as solutions to problems such as erosion, dune stabilization and the like, are also reported as growing out of control in countries bordering the Mediterranean.

In focusing on plant imports it is also important to be good global citizens and avoid thet export of potential plant problems elsewhere.


In Australia native plants have become more prominent weeds over the last 40-50 years as revegetation schemes and native plantings become more popular56. In eastern Australia some of the worst weeds are fleshy-fruited bird-dispersed plants like Billardiera scandens, Common Apple Berry and, from Western Australia, Sollya heterophylla, Climbing Bluebell] 63. In Tasmania mainland wattles have been promoted by the nursery industry and some, like Acacia baileyana, Cootamundra Wattle, have not only become invasive but also shown the ability to hybridise with indigenous species 85.

A list of the top 10 invasive plants for sale in Western Australia contains 4 native plants. The national floral emblem Acacia pycnantha, Golden Wattle, is widely naturalised in WA and SA.

Planting natives has long been encouraged as an environmentally sound practice, but several ecologists have expressed the view that growing native plants is secondary to whether the plant is invasive or not 51, 64.

A few native species that are threatened in the wild have become weeds of roadsides and degraded land e.g. Lepidium hyssopifolium, Pepperweed.

There are several apparent patterns of native plant dispersal: from north to south, especially S Qld and N NSW to Victoria; from mainland to Tasmania; from temperate Western Australia to temperate eastern States especially Victoria; from mainland Australia to Norfolk and Lord Howe Islands 64.

Sleeper Weeds

Sometimes weeds lie dormant for a period and then suddenly proliferate when the right conditions arise. All these plants are known as sleeper weeds and it is very important that they are eradicated or managed before they multiply and spread. An example is Pinus contorta, Barge-pole Pine, planted in the 1940s and 1950s near Falls Creek, Victoria, in an attempt to ‘harvest’ rain and snow to assist with water supply for hydro-electricity generation. It is only since about 2000 that copious seedlings have been observed invading adjacent snow-gum woodlands in the Alpine National Park.

The delay in proliferation may be because they are not growing in an optimal environment; perhaps the climate is unsuitable, pollinators or vectors may not be present, or it may be a while before they can reproduce effectively. A lag phase of several decades or more can occur from the date of introduction and naturalisation to the time when growth becomes rapid and destructive. Trees are generally longer sleepers than shrubs which, in turn, lie dormant longer than herbs. It has been estimated that trees have, on average, a time lag of 170 years and shrubs 131 years 73.

Declared (noxious, proclaimed) weeds

Often referred to as “noxious” or “proclaimed noxious weeds”, these are plants that are banned under special weed legislation as plants that cause, or have the potential to cause, environmental or economic damage: they derive from the weed categories discussed above and are subject to government management. The book Noxious Weeds of Australia 15 gives a full account of declared weeds, their descriptions, history and control, although this is now a little outdated in terms of species covered.


In Victoria declared weeds have been proclaimed under the Catchment and Land Protection Act 1994. Under this Act there are no exemptions for plants growing in metropolitan areas as there was in the former Vermin and Noxious Weeds Act 1958. The most up-to-date listing of declared weeds in Victoria (proclaimed in the Victorian Government Gazettes of 18 December 1997 and 22 May 2003 under the Catchment and Land Protection Act) is available as a Landcare Note from the Department of
Sustainability and Environment (DSE) web site:

Victoria is divided into 10 catchment regions with Catchment Management Authorities that recommend to the Minister the plants that should be declared, and the categories under which they should be placed. Under the Act there are four categories of declared weed. There is variation across the State as to which of these categories different weeds may be declared according to the degree of threat posed:

1. State Prohibited Weeds
These weeds either already exist, posing a serious threat and therefore require   eradication, or they pose a potential serious threat should they invade. DSE is responsible for their control.
December 16, 2005 Festuca gautieri, commonly known as Bear-skin Fescue, was declared a state prohibited weed.
2. Regionally Prohibited Weeds
These are weeds that are generally of narrow distribution but with the potential to spread. There is the intention to control or eradicate these weeds. Land owners, DSE and municipal authorities are responsible for their control.
3. Regionally Controlled Weeds
These are usually widespread in a particular region and require constant control. Land owners are responsible for their control.
4. Restricted Weeds
This category includes plants that are a threat to primary production, Crown Land, the environment or community health and which have the potential to spread within the state or other parts of Australia if sold or traded in Victoria. This is a potential restriction on commercial sale of plants.

The following 12 species (all WONS) were Gazetted 27th October 2005, page 2389: Prickly Acacia, Pond Apple, Bridal Creeper, Cabomba, Rubber Vine, Hymenachne, Lantana, Mimosa, Chilean Needle-grass, Parkinsonia, Willows, Athel Pine or Tamarisk.

More information can be obtained from DSE and the Weeds Research Unit at Primary Industries Research Victoria (PIRVic) Frankston (formerly the Keith Turnbull Research Institute, Frankston). Two periodicals deal with invasive plants and animals, mostly from an economic perspective: the newsletter Under Control, Pest Plant and Animal Management News produced by the Victorian Department of Primary Industries, and the journal Plant Protection Quarterly.

Genetic Pollution

Though not strictly a weed issue, natural vegetation can be degraded by interbreeding with non-indigenous plant stock.

Pollen disperses more widely than seed and this has consequences for the genetic integrity of native populations when the pollen comes from introduced plants. Local populations have adapted to the local conditions and evolved resistance to disease etc. Just as global plant naturalisation tends towards species homogeneity, so breeding with plants from other populations leads to greater genetic uniformity and sometimes to extinction.

Genetic pollution (the mixing of foreign with native genes, known as introgression) generally occurs as hybridisation followed by repeated backcrossing to one or both of the parents. At the species level there may be hybridisation between local species and different species from outside the area. Hybrid vigour, the greater reproductive potential of the hybrids, may then result in a genetic ‘swamping’ of the original plants. Occasionally there is hybridisation between wild native plants and exotic (overseas) plants.

A particularly difficult problem to address is the mixing of genes below the species level by the interbreeding of plants from populations that would not normally meet. This can lead to the extinction of species as original populations are absorbed into the hybrid gene pool, and this is why it is important that revegetation is carried out using seed of known local provenance. To avoid genetic pollution it is important to collect seed close to the site where plants will be grown. For example, it is believed that some “indigenous” plantings of Lomandra longiflora in Victoria were grown from seed collected in New South Wales from genetic stock that is significantly different from many plants growing in Victoria. Once these populations interbreed to produce fertile offspring it becomes extremely difficult to trace the “natural” history of the species. In cases like this low risk overseas exotics may be “safer” than some “indigenous” stock. Because a plant is native to a particular area does not necessarily mean it is safe. With indiscriminate distribution of nursery stock plants can be widely distributed.

Garden plants and plants for agricultural use may be genetically ‘improved’ in many ways as, for example, in showing increased hardiness, drought tolerance or ornamental appeal. Unfortunately this may also improve their performance as weeds. It seems possible that such plants, if fertile, may breed with local and naturalised plants introducing the new characteristics. In Western Europe Populus nigra, the native Black Poplar, has a gene pool threatened by the widely cultivated hybrid poplars and P. nigra var. italica, Lombardy Poplar. In Australia plantation eucalypts may breed with the local native eucalypts.

An indirectly related problem is that of genetically engineered crops cross-pollinating with local crops or perhaps escaping and breeding with native plants. The escape of transgenes for herbicide, pest and disease resistance into surrounding weedy vegetation has been suggested as the source of herbicide-resistant “super weeds”.

Finally, there are a number of difficulties associated with the reintroduction of indigenous flora from seed Banks including:

• a narrowing of the genetic diversity available for reintroduction which reduces the robustness of the reintroduced population and may increase the negative effects of inbreeding after reintroduction
• a tendency to shift genetic diversity towards what is well adapted to cultivation
• a spread of genetic diversity (perhaps from mixing the gene pools of different wild populations) may result in individuals more poorly adapted to the particular reintroduction site.

Maximum genetic diversity is thus not always an appropriate goal; rather it should be maximum genetic diversity appropriate to a specific reintroduction site or sites. In other words the ex situ collection should be striving to maintain and/or assemble a genetically appropriate, reintroducable population that can function in a specific in situ environment.


Garden plants as environmental & agricultural weeds

Ornamental horticulture has been a major source of environmental and agricultural weeds. This article explores the introduction of garden plants to Australia and the way public and private gardens and the nursery industry have acted as a source for garden escapees. A few case histories of some of our worst weeds describe in detail how specific weeds have been introduced and spread into the wild.

Early introduction of ornamental and food plants to Australia

We are extremely lucky in Australia to have natural landscapes and areas of wilderness. In our admiration for the pastoral settings of countries like England, it is easy to forget that very little of their native vegetation remains – these are not natural landscapes but artificial man-made cultural landscapes. Australia, a continent with an extraordinary biodiversity and high proportion of endemism, has only been occupied by Europeans for a relatively short time but the impact on the environment has been vast.

The first known plant introduction

Aboriginals have occupied Australia for about 40,000– 50,000 years but there is no evidence that they introduced new plant species. The first confirmed naturalised alien plant was Tamarindus indicus, Tamarind, introduced by Macassans from the South Celebes when hunting Beche-de-mer (a sea cucumber in the genus Holothuria and from the southern Pacific and Indian oceans; used dried or smoked mainly as an ingredient in soup, especially in China and Indonesia) on the north coast in about 1700 47.

Colonial settlement

The history of plant introduction to Australia by the early settlers is well known: their desire for the familiar food and ornamental plants of their homelands; the interest in natural history and flourishing of acclimatisation societies, and the progressive increase in efficiency of plant distribution into and around the country.

In 1803, 15 years after the arrival of the First Fleet, Governor King listed 292 introduced plants of which about 9 were to become serious weeds 15. Between 1802 and 1804 Robert Brown, botanist with explorer Matthew Flinders, collected 29 species of introduced plants around Sydney. Most were naturalised and all were from Europe, probably arriving as contaminants in seed used for stock feed and crops. In 1850 concern about thistles was being expressed and, by 1869, there were 100 naturalised weeds ‘in the neighbourhood of Sydney’ 36 and 99 naturalised weeds reported in South Australia, including 14 garden plants. [35]

Gardens then were small and insignificant sanctuaries from the prodigious forces of nature, and settlers battled to keep unwanted plants creeping in from the surrounding wilderness. Only recently, it seems, have we become aware of how dramatically, profoundly and permanently this situation has reversed. Small patches of former wilderness are surrounded by large areas of encroaching cultivated land. It is nature, not us, that is in retreat as plants from our gardens threaten natural areas and the organisms they support.

It is easy to be critical of the early indiscriminate plant introductions, but in the context of the desperate lives of the early settlers it is hardly surprising that they were unaware or indifferent to the possible environmental consequences of their actions.

Botanical exploration

In Europe of the nineteenth century, with increasing affluence and leisure time, interest gradually turned more to ornamental garden garden plants. Intrepid colonial-style botanical and horticultural collectors gained public admiration for their exploits as they sought out beautiful, curious and new plants from dangerous and distant, often poorly explored lands – including Australia. These new plant introductions were treasured and served as trophies to impress friends and the general public. Plant collecting for economically important and ornamental plants has been a particular feature of western gardening since the earliest explorers.


We do not know precisely how many different plants have been introduced to Australia since European settlement, it is easier to estimate the number of exotics that are present here right now.

Numbers of plants introduced for ornamental horticulture

However, in 2004 it was estimated that there were about 2,7000 exotic species have been introduced to Australia of which about 25,360 species were for ornamental horticulture (see Figure 5) [68] indicating that gardening accounts for about 94% of the total number of plant introductions. As we have seen, of the total number of introduced exotics over 3,000 (about 10%) have naturalised. Contaminants are the most invasive group with 90% becoming naturalised. Garden plants are much less likely to naturalise than plants from other industry sectors but their vast numbers ensure that gardening has unwittingly played by far the most significant role in the importation and distribution of weedy plants. Further statistics emphasise this fact. For New Zealand, using nursery catalogues, seed catalogues, records from botanic gardens and many other sources it has been estimated that the number of alien plant introductions since settlement is 24,744 species in 3,863 genera. In the year 2002, of these species 1,769 had become naturalised.[77]

• 60-70% of the naturalised plants in Australia have escaped from gardens [11][46]
• In 1999 the Australian National Weeds Strategy Executive Committee announced a list of 20 weeds of national significance (WONS). These are considered the most damaging weeds in the country based on their invasiveness, potential for spread, and their socioeconomic and environmental impacts: fourteen of these plants (70%) are garden escapes
• About 40% of Australia’s current declared weeds are invasive garden plants [46]

Present-day figures indicate that, even with the wisdom of hindsight, we have a long way to go in increasing public awareness and managing weeds in horticulture.

• Between 1971 and 1995 about 200 of the 300 newly naturalised plants in Australia were introduced to the country as ornamentals [39]
• About 54% of the currently recognised 720 naturalised invasive garden plants were on sale in nurseries in 2002 [46]

As with other environmental issues it appears that attitudinal change is much more important in management than simple awareness. It seems extremely likely that many of the invasive garden plants of the future are currently in public and private gardens, and nurseries. The sheer number of alien species within the country is cause for concern.

Food Crops       221 85 38 3
Pasture            1086 348 32 13
Forestry 633 149 24 6
Gardening 25,360 1831 7 71
Accidental 207 186 90 7
TOTAL 27,009 2779 10 100

Numbers of plants available in nurseries

The Aussie Plant Finder (2004) by Margaret Hibbert [53] lists about 35,000 commercially available plants in Australia in a list sourced from about 400 nurseries Australia-wide. This number is much higher than the 27,000 total exotic plant species of Figure 5 because it is not restricted to species but includes exotic cultivars, native plants and native plant cultivars. The 400 nurseries represents only about 12% of the total number of Australian nurseries but targets those selling rare and unusual plants and so is probably our most comprehensive database of commercially available garden plants. This total number compares with over 73,000 plants available in the UK in 2004-5.[52] Certainly many of the taxa in the nursery industry would be cultivars and therefore of lesser environmental importance. The Greenlife Database held by the Royal Botanic Gardens, Melbourne with about 45,000 taxa records about 70% of these as cultivars. These figures suggest that the number of commercially available species (not kinds) is roughly the same as the number of Australian endemic species being approximately 13,000-16,000.

Numbers of plants grown in botanic gardens

The Census of Plants in Australian Botanic Gardens (a project of the Council of Heads of Australian Botanic Gardens CHABG) in 1992, listed all the vascular plant taxa, native and exotic, cultivated in the 6 major Australian botanic gardens. It included about 33,400 taxa (see CHABG web site).


Examples of well-intentioned but environmentally devastating plant introductions to Australia include: the purported spread of Blackberry seed, Rubus fruticosus sp. agg., in the wild by Ferdinand von Mueller; the escape, possibly from Darwin Botanic Gardens in the 1830s, of Mimosa pigra (Giant Sensitive Tree); and the escape from private gardens of Echium plantagineum (Paterson’s Curse, Salvation Jane). Unfortunately many of our most damaging environmental weeds are (or were) garden favorites: Asparagus asparagoides, Bridal Creeper; Chrysanthemum monilifera, Bitou Bush; Cryptostegia grandiflora, Rubber Vine; Lantana camara, Lantana; Opuntia spp., Prickly Pear; Salix spp., Willows also Hawthorn, Periwinkle, Erica, Watsonia, Gladiolus, English Broom, Gorse, St John’s Wort and many more.

Echium plantagineum – Paterson’s Curse (Salvation Jane)

First recorded in 1843 at the Camden garden of John Macarthur near Sydney Paterson’s Curse was presumably introduced as an ornamental. It soon appeared in nursery catalogues such as that of James Dickinson, Hobart, in 1845. By 1890 there were two major early infestations, one at Gladstone near Port Pirie in South Australia, the other at Cumberoona near Albury in New South Wales where it was assumed to have been introduced as a garden plant by the Paterson family – hence the name Paterson’s Curse. The name Salvation Jane is used in South Australia and refers to either its grazing value in drought years or the resemblance between the flowers and the bonnets worn by ladies in the Salvation Army. By 1900 the plant was recorded from many areas and generally widespread through South-eastern Australia. It is now found mainly on grazing land in southern Australia where it is regarded as important fodder for sheep. In Tasmania contaminated cereal grain appears to be the source of new infestations often appearing in the vicinity of poultry farms. (Historical information sourced from [15][38])

Paterson’s Curse has spread to most temperate regions of the world and is common in S Africa and S America, less common in N America and New Zealand; it has become an important weed only in Australia.

Lantana camara – Lantana

Lantana was introduced to Europe from C America in the 17th century as an ornamental and then distributed round the globe in conjunction with the spread of colonial empires. This plant is known as a classic example of long range dispersal of an ornamental in warm regions. In Australia, like Paterson’s Curse, it is recorded for the Camden garden of John Macarthur in 1843 and was soon provided in nurseries. By the late 1850s in Queensland it was recorded as forming impenetrable thickets along streams, on deserted farms and elsewhere until today when it occurs as a prominent weed between Cairns and Sydney, and also occurs on Lord Howe Island. It is now found in at least 47 countries and infests about 4 million ha in Australia. (Historical information sourced from [15])

Rubus fruticosus agg.– Blackberry

The first record of blackberry was in 1842 growing in an Adelaide garden although there is evidence of the plant in the 1830s. By the 1860s it was grown in botanic gardens and strongly promoted by Melbourne’s Government Botanist von Mueller – to control erosion, as a hedge, and as a source of food for travellers and explorers. By the 1880s it was widespread in Victoria and New South Wales with Mueller still recommending its naturalisation along rivulets.

The European Blackberry is now a weed of many temperate parts of the world and one of the most prolific weeds in Australia. There are also severe infestations in Chile, Hawaii and New Zealand (where it is probably the most troublesome weed). (Historical information sourced from [15])

Biological control has been attempted and in 1984 a rust fungus was illegally released, first in Victoria then in other states and further strains were explored in 1991. Some progress has been made but unsusceptible or less susceptible plants are arising: research continues.

Chromolaena odorata – Siam Weed

South American Chromolaena odorata, Siam weed, is rated as one of the world’s worst tropical plant pests occupying huge tracts of farmland and forest in Africa, Asia and the Indian subcontinent. It is poisonous to cattle, it can smother crops of sugarcane or banana plantations, and it can climb right into the rainforest canopy and swamp the native vegetation. It has been in Australia for about 10 years. Weed forms dense, impenetrable thickets several metres high which engulf native vegetation, especially along river banks and on forest fringes. When these dry off they add greatly to the bushfire hazard, posing a direct threat to the fire-sensitive rainforest. It grows faster than native grasses and vegetation and, like lantana, can completely replace the pasture grasses under eucalypts, leaving cattle and native wildlife with nothing to eat. It is not a serious weed in its native range and is considered to have been introduced to Calcutta in India as an ornamental in the 1840s, being transported through Asia during World War II. It spread to Africa in the 1940s and has caused havoc in East Timor after arriving there in the 1970s. It has been in Australia for about 10 years and is currently one of 28 alien plants on Australia’s Alert List for weeds that threaten biodiversity or cause environmental damage; the strategy is to eradicate Siam Weed wherever it is found.There are currently a limited number of populations in the early stages of their invasion having the potential to spread widely and inflict huge damage on native landscapes, fauna and flora. There are biological agents which might help to keep it in check, although they will not eliminate it. (Historical information from [15], current background from Weeds CRC press release).


Since the nursery industry is the source of plants for most gardeners, and is also responsible for large numbers of new plant introductions, it has been the subject of several recent studies.

WWF National list of naturalised invasive and potentially invasive garden plants

In 2004 the World Wildlife Fund (WWF) produced a list of 1036 naturalised invasive and potentially invasive garden plants 32. This list had started out originally as a list of 720 naturalised plants and several declared weeds produced for the Garden Plants Under The Spotlight (GPUTS) report[67] which, in turn, had been followed by a revised and updated list of 958 species. [48]

WWF Report – Jumping the Garden Fence: invasive garden plants in Australia and their environmental and agricultural impacts
In 2005 a report, Jumping the Garden Fence 46, was produced by the WWF and CSIRO extending earlier work and concentrating on the 720 naturalised and invasive garden plants (excluding the potentially invasive garden plants which would bring the total to 1036), and again pointing out the numbers of weedy plants that are on sale in nurseries including:

• 393 (54%) of the 720 naturalised invasive garden plants
• 72 (40%) of the 178 plants declared in one or more State or Territory
• 5 (25%) of the 20 WONS
• 62 (44%) of the 141 invasive garden plants that are agricultural or ruderal weeds
• 4 (14%) of the 28 species on CALEW
• 10 (20%) of he 49 weeds known to be impacting on rare or threatened Australian plants (ROTAP)
• 4 (8%) of the 48 weeds of greatest significance to the grazing industry, and 8 (33%) of the 24 emerging weeds that are a potential problem to the grazing industry

The 10 most serious invasive garden plants in Australia that are currently on sale was listed as:

Asparagus scandens, Asparagus Fern Cytisus spp., Broom
Pennisetum setaceum, Fountain Grass Gazania spp., Gazania
Gloriosa superba, Gloriosa Lily Bryophyllum daigremontianum x B. delagoense ‘Houghtonii’, Mother of Millions
Lonicera japonica, Japanese Honeysuckle Schinus molle, Pepper Tree
Vinca major, Periwinkle P Pittosporum undulatum, Sweet Pittosporum

In Victoria the 10 most invasive plants on sale were given as:

Eragrostis curvula, African Lovegrass Asparagus scandens, Asparagus Fern
Gazania spp., Gazania Equisetum spp., Horsetails
Nassella tenuissima, Mexican Feathergrass Oxalis spp., Oxalis
Schinus molle, Pepper Tree Vinca major, Periwinkle
Opuntia spp., Prickly Pear Erica lusitanica, Spanish Heath

In addition it was recommended that each State restrict the availability of the adjacent states’ declared weeds, even if not declared in the State in question.

Table 4
Numbers of plants in Australian botanic gardens, in the nursery industry, and on lists of national importance



TOTAL TAXA IN MAJOR URBAN BOTANIC GARDENS (INC. HYBRIDS AND CULTIVARS) 33,400, Naturalised 3244, Declared 429, Alert List 28, WONS 20, NAQs 41

c.35, 000 53
Garden thugs 958 48
Naturalised invasive and potentially invasive gardens plants 1036 32
Naturalised invasive garden plants 720 32

Some of the problems of managing weeds in the nursery industry have been attributed to poor labelling of plants. Invasive plants being may not be named or sold under poorly known names. Clear labelling with the botanical name would facilitate early detection of invasive plants in the trade.

WWF The Costs and Benefits of a Proposed Mandatory Invasive Species Labelling Scheme
This report (September 15, 2005) evaluates the arguments for and against a mandatory invasive species labelling scheme for ornamental plants and fish, and the issues and challenges that must be addressed for such a scheme to be fully effective.

WWF Report – Controlling the Sale of Invasive Garden Plants: Why voluntary measures alone fail
Also released on September 15, 2005 this report examines the reasons why voluntary measures alone are insufficient to control the sale of invasive garden plants. Although voluntary schemes favour good public relations and environmentally friendly branding they are hampered by: the disparate structure of the nursery industry which impedes the costly implementation and monitoring of schemes; the limited coverage of the national industry body (NGIA) which includes only about one third of businesses excluding many major retail outlets such as chain stores; an inherent resistance to regulation of stock. The report cites ineffectual voluntary schemes in New Zealand and Florida as predicted by a 2003 OECD study 84 and discusses how New Zealand has developed a statutory and cooperative policy, the National Pest Plant Accord, to identify and prohibit the sale of high-risk invasive garden plant species (see

The report suggests an approach combining a national legislative ban on high risk species, a consumer awareness campaign, and a national plant labelling scheme.

Weed proofing Australia

At the 15thAustralian Weed Conference in 2006 the WWF-Australia proposed a 10-point plan to control invasive garden plants. 95

1. Close the door to new weeds by a comprehensive permitted list/weed risk assessment system by 2006
2. Give garden industry and communities certainty about weed status of garden plants through a national list of invasive plant species
3. Better understand the extent and risk from continued trade in invasive garden plants
4. Increase knowledge of sterile garden plants and the dynamics of weed invasion
5. Build industry understanding of weed risks and costs and increase capacity for positive action
6. Mobilise the garden industry to respond to the invasive plant challenge
7. Provide protection for those industry leaders who comply with regulations by reducing transaction and compliance costs and establishing a streamlined national regulatory framework
8. Phase out the supply and trade of high risk invasive plants nationally
9. Increase product demand for low risk plants
10. Mobilise communities to search and destroy new infestations of escaped invasive garden plants


Until recent times gardeners in Australia and elsewhere have regarded the plant kingdom as an exciting and unrestricted palette of colours, textures and new species with which to decorate their gardens. But plant collecting no longer carries quite the same romantic appeal, and for sound environmental reasons. That desire for plants from wild, attractive, unexplored areas of the globe must now be viewed in the knowledge that these remote areas are rapidly diminishing. Plant collecting is nowadays most likely to be carried out for conservation reasons – to preserve the highly threatened world flora. With the benefit of hindsight it might be thought that we would now be much more cautious in distributing plants (or animals) around the world and Australia considering our unfortunate experiences with rabbits, Paterson’s Curse, blackberries, not to mention the equally unfortunate effects our flora has had in other parts of the world.

Present-day knowledge now firmly indicates that, for our own good, we need to source our garden plants with great care. How have we monitored the introduction of plants new to the country? For many years newly introduced plants were assumed “innocent until proved guilty”; that is, they were permitted unless the case against them was strong. The unfortunate history of weed invasions strongly suggests that this approach be reversed. Before plants are used for horticulture their clear benefits and lack of weed potential must be demonstrated; this is a situation where “guilty unless proved innocent” appears sound management.


Weeds impact on the environment, society, agriculture and the economy: they reduce farm and forest productivity, displace native species and contribute to land degradation thereby threatening the sustainability of natural ecosystems and primary production. For some images of weed impact see <>.

As we have seen, different weeds pose different threats and have varying impacts. Some reproduce but remain innocuous. Others spread aggressively to become invasive. Yet others (sleeper weeds) may remain quiescent for some time before suddenly increasing in numbers and becoming widespread, so there is also a variable period between introduction and naturalisation. We have also seen how, when impacts are sufficiently great, invasive plants are prohibited under legislation and are then referred to as declared (“noxious“ or “proclaimed”) weed and that, as a rough estimate, about 10% of naturalised plants become invasive39.

Environmental Impacts
Threat to sustainability
Humans have exacted a high price on the environment. In 1930 it is estimated that 10% of the planet’s primary productivity was directed to human needs, mostly food crops: by 2000 this had grown to 40%. In other words, towards half of the plant matter on the planet is now catering for human needs8. About 24% of the planet’s land surface is now devoted to agriculture 58. In Australia 60% of the land surface has been harnessed for agriculture (see Table 2) and approximately one third of the forests in Australia that existed prior to European settlement have been cleared for agriculture, forestry and mining 57. From an environmental perspective there is no difference between the loss of land to weeds or agricultural crops – the consequences are the same.

In Victoria over less than 200 years humans have been responsible for the introduction of over one quarter of the plants listed as growing “wild” in the state. Through the combined effects of agriculture, forestry and horticulture more than 65% of the area of Victoria carries wholly or predominantly exotic plant species9. Much of this land is dedicated to grazing and agricultural crops using species that require large quantities of water, fertilisers and other chemicals, all of which can detract from environmental values. All of these factors weaken the natural ecosystems that assist soil formation, nutrient cycling, air and water cleaning, cultural revitalization and many other services.

Against this background it can be understood that the impact of weeds is a major threat to environmental and agricultural sustainability. Globally the trend is towards a much diminished and homogenised flora as humans continue to disperse plants around the planet at an alarming rate. Farmers are now working hard to manage weeds with reduced herbicide use and ensure their farming practices are sustainable.

Habitat degradation
In a general sense, weeds interfere with fire regimes, food webs and nutrient cycling; they alter soil water levels and may contribute to soil erosion by eliminating surface vegetation. But more particularly the damage is done in many and complex ways. Weeds generally grow faster than native species and reproduce more effectively, competing successfully for light, nutrients, pollinators, water and space. Their dominance may be in part a result of the fact that they are away from their natural habitats and the pathogens and diseases that would normally restrict their growth. In extreme situations a monoculture eventuates as a particularly invasive species swamps everything around it, completely obliterating the native plants and the animals associated with them. When a single invader disrupts an entire ecosystem in this way it is referred to as a “transformer species” (for examples see Weeds of National Significance).

One example of current concern is the African grasses known as “Fireweeds”, including: Andropogon gayanus, Gamba Grass; Panicum purpurascens, Para Grass; Pennisetum polystachyon, Mission Grass; and Pennisetum ciliare, Buffel Grass ). These grasses build up huge fuel loads which on burning cause fires of such intensity and timing that native trees, shrubs and grasses cannot survive: they invade areas of high conservation value in the arid zone and monsoonal tropics of the Top End. As a result, vast areas of woodland in the north and centre of the continent are now at risk. Panicum maximum, Guinea grass was introduced as a pasture plant but in ungrazed areas it produces up to four times the fuel load of native grasses and destroys plants that normally thrive in the natural fires of lower intensity and frequency. There is a tendency for these grasses to do well in refugia – areas where a few species remain and are easily threatened. There is no effective control at present.

Ultimately severe weed invasion can lead to extinctions. Of the 88 known native plant extinctions, weed competition has been reported as the primary cause of 4. By far the largest number 77 were due to land fragmentation for grazing and agriculture49. In 2005 it is estimated that a further 57 species are threatened by weed invasions46. Native plants and animals directly threatened by weeds include the Mountain Pygmy Possum in NSW and Victoria; the Tussock Skink in Tasmania; Spiked Rice Flower and Hairy Quandong in NSW; the Spider Orchid in SA; the Richmond Birdwing Butterfly in Queensland and NSW; the Wing-fruited Lasiopetalum in WA; the Button Wrinklewort in ACT and NSW. In a study 108 of 945 threatened species, populations and ecological communities, it was concluded that invasive plants were a major factor affecting 419 of them, including 279 native plants and 62 native animals. The main threats came from 127 invasive plants, of which the worst were Lantana, Bitou Bush, blackberries, Kikuyu and Scotch Broom. Of the 127 invaders, 82 had gained their foothold in the Australian environment after escaping from parks, gardens and ornamental collections and now threaten at least 190 native species in NSW alone and maybe as many as 300. Fifty-six of these ‘killer weeds’ are still available for sale in garden centres across Australia, despite the fact they have previously been identified as highly invasive.

Genetic pollution
Pollen disperses more widely than seed and this has consequences for the genetic integrity of native populations when the pollen comes from introduced plants. Local populations have adapted to the local conditions and evolved resistance to disease etc. Just as global plant naturalisation tends towards a plant species homogeneity, so breeding with plants from other populations leads to greater genetic uniformity and sometimes to extinction.

Genetic pollution (the mixing of foreign with native genes, known as introgression) generally occurs as hybridisation followed by repeated backcrossing to one or both of the parents. At the species level there may be hybridisation between local species and different species from outside the area. Hybrid vigour, the greater reproductive potential of the hybrids, may then result in a genetic ‘swamping’ of the original plants. Occasionally there is hybridisation between wild native plants and exotic (overseas) plants.

A particularly difficult problem to address is the mixing of genes below the species level by the interbreeding of plants from populations that would not normally meet. This can lead to the extinction of species as original populations are absorbed into the hybrid gene pool, and this is why it is important that revegetation is carried out using seed of known local provenance.

Garden plants and plants for agricultural use may be genetically ‘improved’ in many ways as, for example, in showing increased hardiness and drought tolerance. Unfortunately this may also improve their performance as weeds. It seems possible that such plants, if fertile, may breed with local and naturalised plants introducing the new characteristics.

In Western Europe Populus nigra, the native Black Poplar, has a gene pool threatened by the widely cultivated hybrid poplars and P. nigra var. italica, Lombardy Poplar. In Australia plantation eucalypts may breed with the local native eucalypts 55.

Finally, in the new era of biotechnology, genetically engineered crops may cross-pollinate with local crops or, if capable of escaping and breeding with native plants, pose complex genetic problems. The escape of transgenes for herbicide, pest and disease resistance into surrounding weedy vegetation has been suggested as the source of herbicide-resistant “super weeds”.

There are a number of difficulties associated with the reintroduction of indigenous flora from seed banks including:

• a narrowing of the genetic diversity available for reintroduction which reduces the robustness of the reintroduced population and may increase the negative effects of inbreeding after reintroduction
• a tendency to shift genetic diversity towards what is well adapted to cultivation
• a spread of genetic diversity (perhaps from mixing the gene pools of different wild populations) may result in individuals more poorly adapted to the particular reintroduction site.

Maximum genetic diversity is thus not always an appropriate goal; rather it should be maximum genetic diversity appropriate to a specific reintroduction site or sites. In other words the ex situ collection should be striving to maintain and/or assemble a genetically appropriate, reintroducable population that can function in a specific in situ environment 69.

Social Impacts
The indigenous flora of a particular area is a fundamental part of its landscape and cultural heritage: it gives the region its special character and therefore instills in the people who live there a strong sense of place. Together, these factors help strengthen a feeling of community. Unique regional landscapes are also of great appeal to tourists. Naturalised plants can rapidly erode this uniqueness.

Weeds may block and pollute waterways, affecting the quality of drinking water and recreational values as well as increasing the cost of water management. In both urban and country areas they can be fire hazards, look unpleasant, and cause structural damage to buildings etc.

The Australian Society of Clinical Immunology and Allergy has indicated that the troublesome pollens triggering hay fever, allergies and asthma are often produced by Northern Hemisphere plants. Of the 25 major seasonal allergens in Australia around 20 of them are introduced species (Dr Tim O’Meara, Woolcock Research Institute at Sydney University). Olives are spreading in the wild and are also associated with allergies. Alien invasive plants that cause allergies in Australia through airborne substances include: Lolium rigidum, Annual Ryegrass; Paspalum spp., Paspalum; Holcus lanatus, Yorkshire Fog; Arctotheca calendula, Capeweed; Ambrosia spp., Ragweed; Plantago spp., Plantains; Parietaria judaica, Asthma weed; Ligustrum spp., Privet ; and the trees Betula spp., Birch and Olea europaea, Olive.

For humans or pets and livestock some species may cause chronic or acute poisoning, dermatitis or photosensitization. Toxic plants which pose a risk to children and pets include: Datura spp., Thornapple; and Ricinus communis, Castor Oil seeds; Solanum nigrum, Black-berry Nightshade, and Zantedeschia aethiopica, Arum Lily which, on Friday 1 September 2006, was subject to a Western Australia-wide ban imposed to prevent the further spread of the attractive but poisonous plant, which has invaded thousands of hectares of farmland, forests and wetlands in the south-west. Other weeds have spines or thorns that can cause injury.

Skin reactions may be produced on contact with Hypericum spp., St John’s Wort, and Ailanthus altissima, Tree of Heaven, causes dermatitis in susceptible people.

Economic Impacts
Although there are some beneficial impacts of weeds (e.g. providing habitat and pasture) these are small in relation to the negative impacts. Agricultural weeds compete with food crops and grasslands. They can: reduce the quality of meat, vegetables, hides and wool fibres; poison or injure stock; and harbour disease and vermin. Some cucurbits (melons) and Chondrilla juncea, Skeleton Weed, clog machinery and prevent efficient harvesting. They also interfere with fishing and waterway navigation; disrupt signals and railway lines; and impede hydroelecric and irrigation machinery. Weeds increase the risk of herbicide resistance in agriculture, and of fires in the bush.

Current estimates suggest that the cost to Australia’s primary industries in lost production and weed control now exceeds $4 billion p.a. 1, 71, 93 (see Table 2). This is a conservative estimate as it does not include the cost of losses of Ecosystem Services (see Environmenal Impacts) or the enormous contribution to weed control made by volunteers. Commonwealth, State and local government spend an additional M$116 on the monitoring, control, management and research on weeds. This total cost exceeds that of salinity management by 10-20 times and yet in 2004 invasive species commanded less than 10% of the resources devoted to salinity problems. These figures draw attention to the urgency of the problem and also underline the fact that prevention and early intervention are the most cost-effective way of dealing with weeds.

During the 2006-07 financial year, farmers spent $1,574 million controlling weeds, which is more than pests ($768 million) and land and soil problems ($649 million) combined.113

Costs of weed management are being incurred in many other countries. In the US the cost of dealing with invasive non-native plants has been estimated at US$35 billion 59.

Table 2 – Land use in Australia and cost of weed control 1,27, 71

Agriculture 60 3.93
National Parks and Nature Reserves 6 0.02
Indigenous land 14 0.08
Public land & private non-agric. land 20 0.003

How can weed management strategies be assesses in financial terms? A report on government expenditure on weed and pest animal management in Queensland cited the following benefit to cost ratios. 91 :
Prevention 31:0; Eradication 16:1; Containment 2:1

In 2006 a report on the economic impact of Australian weed biological control 93 examined the return on investment of the Australian weed control effort as a cost-benefit analysis (CBA). Results showed an overall benefit cost ratio of 23:1 – that for every $1 invested in the biocontrol effort a benefit of $23.10 is generated. This comprised 17.4 for agriculture, 3.8 for society and 1.9 for government. At the rate of current investment in biocontrol this equates to an annual net benefit of $95.3 million of which $71.8 million is expected to flow to the agricultural sector. The report recommended that, prior to control, there be careful recording of: the current distribution and density; historical and projected rate of spread; magnitude and extent of impact (on agriculture, the environment and society).


If we are to manage weeds effectively then we need a means of deciding when a particular species poses such a threat to the environment or agriculture that it either needs special management, or should not be grown at all.

The actual process of decision-making about a plant’s weed potential is known as the Weed Risk Assessment (WRA) or Weed Risk Assessment Procedure (WRAP). This usually refers to an elaborate formal procedure, as when plants being introduced to the country for the first time are rigorously assessed “at the border” by Biosecurity Australia. But it may be a more casual informal process that is carried out on plants already in the country, a post-border assessment, as when gardeners, based on the best current information they can obtain, decide not to grow particular plants.

As WRA is often a prohibitive, contentious and costly process, the assessment procedure needs to be as objective and transparent as possible because this will help gain its acceptance by a wide group of people – that is, the decision-making method must be evidence-based.

Each Australian State currently has a Weed Risk Assessment Officer and they lead the programs for their states as they decide on which plants pose the greatest risk.


How do we decide which species pose an unacceptable environmental or economic threat? Unacceptable to whom, when, where and why? And how, when we have decided which plants do pose an unacceptable risk, do we prioritise these plants for management? These questions fall within the broader general context of risk management 70.

How real is the risk?

Regardless of the discipline involved we can define risk as follows:


For many environmental scientists Environmental Risk Assessment (ERA) is routine work. The task is to define, as closely as possible, the boundaries of the above equation, ranking the consequences in some way.

Determining the relative weed threat posed by different plant species presents many problems. Here are some of the hurdles:

• It is a prohibitive and costly process and therefore generally unpopular to instigate
• Subjectivity of the decision-making
• Even with precise and reliable data, how do we decide what level of risk is unacceptable?
• Competing values e.g. botanical gardens must meet heritage, scientific and conservation values. In most situations there are also “management values” to take into account such as considerations of the particular environments or crops affected, ease of control, costs of eradication etc. How do we balance benefits against risks?
• How are borderline cases resolved?
• Which characters are to be chosen for the assessment and why; how are they to be weighted and why; and how do we know if the assessment is accurate and therefore reliable?
• How do we ensure that (scored) questions involved with assessment are answered the same way by different people?
• Difficulty gathering data
• Information about plants is often absent or difficult to interpret
• The weediness of a plant will depend on where it is growing – each locality is unique
• Probably as a risk assessment becomes more accurate and effective it also becomes more complex and impractical

In the face of these difficulties it is easy to do nothing – but inaction is a form of action. Ultimately land managers, scientists, horticulturists, quarantine inspectors and others must decide on how much risk is acceptable in their area of concern and then act accordingly. But how is this to be done?

Risk assessment is seen by some people as being based on value judgements and social context, and to others as objective measurement. Good risk analyses seem to include an amalgam of risk analysis methods and decision tools (and their constant review), flexible management, and stakeholder engagement: certainly they should be frank about what is unknown 80.

Measuring risk

It is tempting to think that plants are either weedy, or they are not, because that is the way we generally manage them. However, plants will display different degrees of weediness in different situations. So a WRA should indicate the potential degree of weediness. But then the degree of weediness will, in turn, depend on how amenable the actual growing site is to a plant’s weedy tendencies. If these two assumptions are accepted then we can make an important general observation about weeds:

To judge a plant as a weed has little meaning unless we also state the conditions and/or region where it may be weedy, and also provide some measure of the threat (risk) posed.

There needs to be a clear statement of the extent and kind of region/conditions to which the risk assessment applies (e.g. global, national, state, regional, local, wetlands, mallee, coastal dunes, Shire of Sherbrooke, temperate rainforest, Lofty Ranges). It also makes more biological sense if these areas are bioregions (e.g. ecosystems) rather than political regions (e.g. state boundaries).

Factors influencing weediness

What are the factors to be measured if we are to obtain an accurate prediction of weed potential for a particular plant?

Much research has been devoted to determining the biological and ecological characteristics that lead to environmental weediness, but no simple answers have emerged to the questions of which taxa will invade, how fast they will spread, what their impact will be, and how they are to be controlled. Part of the difficulty is being able to predict the effects of the receiving environment. Perhaps the most reliable single indicator of weed behaviour is a plant’s weed history 16, 6.

The book A Global Compendium of Weeds 6 states that “little has emerged that might help to identify species that have a high risk of becoming weeds, beyond the simple generalisation that those that have become weeds when introduced in one part of the world are likely to cause problems if introduced elsewhere”.

In spite of the above statement it is also sometimes pointed out that many of the invasive species in Australia could not have been predicted based on their histories e.g. Paterson’s Curse and also that within single genera the presence of one or more invasive species does not necessarily indicate that the genus as a whole poses a threat, although relatedness can sometimes be a useful indicator.

Disease is sometimes the major factor limiting invasive plant spread: if this threat is removed then plants bred for agricultural purposes, in particular, can spread very quickly although this will depend on the environmental conditions. It is now possible to identify environments where disease-resistant plants have a greater chance of invading local plant populations and to accurately predict where particular species are most likely to establish thus informing management strategies for the release of plants 109.

It is clear then that some plants possess intrinsic weedy attributes such as an enhanced reproductive capacity, dispersability etc. But these attributes cannot be isolated from their interaction with the characteristics of the receiving environment.

Predicting weediness also involves consideration of propagule pressure: the success of the initial invading population and the number of invasion attempts.

Inextricably combined with such assessments are management considerations such as the particular crops or ecosystems under threat, the cost and difficulty of control should the plant escape, priorities of resource allocation and so on. Because of the complexity of influencing factors, expert opinion rather than a written procedure has, in the past, played a large part in making decisions.

Weed scientists have established a number of useful generalisations about weed spread that have a strong bearing on weed risk assessment:

• The three most important factors in weed biology are: invasiveness, impacts and potential distribution
• Of these three categories it is impacts (e.g. dominance) that is the most difficult to predict
• The likelihood of a weed invasion depends strongly on the number of locations where it is growing and the range of environments within these locations: this is more important than the intrinsic invasiveness of the plant. Propagule pressure is also very important, being the frequency and abundance of propagules entering an area
• Land disturbance (whether from fires, floods, logging etc.) plays a very important part in the likelihood of establishment
• Once established the spread of a plant will depend on its dispersal ability and how effectively it can compete with the surrounding vegetation (this often being manifested as a high relative growth rate when in competition with other plants)
• Range expansion may follow as the plant spreads across the landscape, possibly into new habitats. This is more likely if the plant can cope with a wide range of environmental conditions, that is, if it has a wide ecological amplitude

Prevention is better than cure and weed scientists will no doubt try to improve their detection of ‘hot spots’ for invasion by certain species. Finally, it should be pointed out that although some measure of weed risk may be obtained by formal weed risk assessment it is much more difficult to be confident that a plant is safe (non-invasive).

Many screening techniques have been devised to assess a plant’s potential to become weedy (e.g. scoring systems, decision trees, process-based models). Currently-used models generally take the form of a table of scored (and generally weighted) weedy attributes, and decisions are then based on a plant’s total score 25. Scores for weediness are generally organised as accurately as possible into risk categories e.g. low, medium and high. The degree of detail entailed in these WRAs will depend on the requirements of the situation. Ideally the decision, the reason why it was reached, and any references used, should be recorded to assist assessments carried out elsewhere.

We can now consider the basic requirements of formal weed risk assessment.

Invasiveness, Impact, potential distribution

Weed risk is generally assessed by scoring plants for their weedy characteristics: the higher the score the greater the risk of the plant becoming weedy.

For weeds the general risk equation risk = consequences x likelihood may be broadly restated as:


As the figures obtained from this equation are only a general guide to weedy behaviour then it is more realistic and helpful from a management point of view if the plants are then placed into broad risk categories.

Assessments are more effective if carried out by several people including experts and stakeholders, as this encourages ownership and consistency of approach.

It is also important that the method used is repeatable and reliable. This means that any questions must be stated in a way that minimises individual interpretation so that different people will get the same result. Secondly, the scoring system needs “testing” for reliability to ensure that it is effective and realistic. As the results are subject to future review it is advisable to record the literature and any other sources of information that were used for the assessment.

A management strategy is then applied for each of the risk categories.


Biosecurity Australia

Stopping potential weeds at a country’s border is critically important and the first step in preventing invasion. However, the development of formal WRAPs used at country borders is, globally, still in its infancy with Australia, New Zealand and South Africa leading the way. In the UK in 2005 the Department of Environment, Food and Rural Affairs was funding research into a risk assessment methodology 72. Historically it was the tradition to permit all species unless they were known to be hazardous i.e. they were permitted unless on a “prohibited list”. The approach is now more circumspect with all species prohibited until it is demonstrated that they are harmless: they are then placed on a “permitted list”.

Australia’s border screening test is that used by the Plant Biosecurity section (formerly AQIS Policy) of Biosecurity Australia and Australian Quarantine and Inspection Service (AQIS) in the Federal Department of Agriculture, Fisheries and Forestry.

Live plants

Prospective live plant imports can be checked to see if they can be imported, and if so whether there are any special import requirements of any kind, using the Plant Biosecurity ICON database which is available on the web 44.

Although treatment is done prior to shipment the burden of risk and responsibility clearly lies with the importing country and newly arrived plants are inspected for pests and diseases, receive a mandatory treatment with methyl bromide, and at least 3 months in an official quarantine facility (depending on the type of plant). For potential disease-carriers such as vines the quarantine period is 2 years.


For seeds there is a separate process: the plant is first checked against a list of prohibited plants (Schedule 6) then a list of permitted plants (Schedule 5). In 2005 a WWF 41 report drew attention to the fact that Schedule 5 (the Permitted Seeds List) included 2916 genera encompassing thousands of plants with known weedy histories. A way around this was suggested in the report and in May 2005 an amendment to the Quarantine Proclamation Act was introduced to deal with part of this problem (see Resources for web address of Quarantine Amendment Proclamation 2005 (No. 2) registered: 30/05/2005). As a result of this Biosecurity Australia determined that 3335 of 4000 species with weed potential and claimed to be present or traded in the country were in fact not so.

If a species is included on Schedule 5 it means that the person importing seed of that species does not require a ‘permit to import’ from AQIS. The only exception to this is if the seed of a species has been genetically manipulated in any way and/or artificially selected for one or more of the following traits:

1 Tolerance of, or resistance to, herbicides
2 Enhanced tolerance of, or resistance to, environmental stress
3 Enhanced tolerance of, or resistance to, plant pathogens
4 Expression of toxic substances (including pesticides and poisons)
5 Enhanced growth characteristics (including growth rate, seasonality and fruiting or seeding density)

Biosecurity Australia is required to ensure that seed importers and researchers are not disadvantaged by regulation of seeds of species already present, or commonly traded, in Australia. International trade agreements also require Australia ensure that species already present in the country are not prohibited.

Border WRA

If the plant does not appear on any of the lists then it is passed through a formal WRA. This is a very sophisticated test developed in Australia and adopted by New Zealand and Hawaii with slight modifications. It is a scored, computer-based series of questions and has replaced a former “prohibited list”. It consists of 49 questions about weed history, biogeography and biology/ecology each answered yes/no/don’t know.45, 61 Plant Biosecurity assessments take an average of 2 days to complete per plant. A simplified form of this model is being tested at the Adelaide and Melbourne botanic gardens for their accesions. Nevertheless such models are extremely difficult to construct and inevitably contentious in many aspects. The present method, although very rigorous, leaves many plants “undecided” and therefore requiring further assessment.

AQIS produces information leaflets on actual and potential weed plants, the quarantine measures taken to exclude pests, and methods used for biological control and eradication of pest species.

The Quarantine Proclamation Act of 1998, amended in March 2005, includes a list of 60 plants that are quarantinable pests (mostly species but some are genera e.g. Aegilops spp.)


Once in the country the process of screening out potentially harmful plants must continue, as many invasive plants are already here.

In Victoria at DPI a review of current and potential noxious weeds is now in progress with a scored, evidence-based numerical ranking system consisting of 39-questions, 15 on invasiveness, 24 on degree of impact and including consideration of current and potential distribution. Also included in the review are plants on other important lists (WoNS, National Alert List and NGIV list) with others to be considered later. Plants that are ineradicable at a particular level are not named for that level.

Informal Post-Border WRA For Horticulturists

Of course it is not always possible or practical to fill out detailed scored assessments. Less stringent methods can be used to assess plants in horticulture.

WRA for professional horticulturists and botanic gardens
Horticulturists can be guided by weed lists compiled by organisations that have sufficient time to carry out thorough assessments. However, although a few well-established and authoritative lists can be used to regulate well-known and highly invasive weeds, at the local level information is often lacking.

However, there is still the need to be completely explicit in the methods used to nominate weeds for regulation. Organisations such as botanic gardens and local councils may be able to use a WRA method that is not too time-consuming.

Border protection by Biosecurity Australia surely requires the very best WRAs we can produce, no matter how complicated. However, there will always be pressure to reduce time and costs by keeping the process as brief and practical as possible. If horticulturists were to apply a similar but less sophisticated method then they would be looking for a WRA that, ideally, would be:

• Reliable (it must do the job it was intended to do)
• Relatively simple and easily understood
• Not time-consuming (i.e. does not need large amounts of data)
• Sufficiently objective as to be clearly defensible
• Treat all kinds of weeds equally (environmental weeds of equal importance to agricultural weeds)
• Allow ranking by some kind of index or score
• Frame questions such that different people would get the same result

In reality it must be expected that the accuracy of the assessment is probably proportional to the number and appropriateness of the questions and their scoring.

Home gardeners

The home gardener is unlikely to have the time or skills to apply a formal WRA like that discussed above. Nevertheless, s/he can be vigilant by being aware of those plant groups that might pose problems and knowing the characteristics indicating that a plant has weed potential.
Gardeners understand well the costly, labour-intensive and heart-breaking difficulties presented by weeds in the garden. Weeds in the bush confront us with similar but vastly greater and more far-reaching problems. Preventing the promotion, sale and cultivation of known weedy plants is surely a small price to pay for protecting our remaining native ecosystems and economy.

Probably the most helpful contribution that can be made by gardeners is to simply grow low- to no-risk plants.

Restricting the availability of actual and potential weeds is a controversial matter for many gardeners and nurseries because it entails withdrawing plants from the market for reasons that may not be immediately obvious, or that seem unduly draconian. Restricting the number plants available for gardening is also often perceived as interference with a basic freedom because we treat the garden as a place to do, more or less, as we please. Our short-term desire to grow whatever we like is often regarded as more important that the long-term consequences these actions may have for nature and our food supply. Although the sale of actual and potential weedy plants may be prevented these plants currently growing in gardens will continue to pose a threat.
It has been suggested that a priority alert list be developed to focus on early warning and surveillance programs that target high-risk invasive garden plants that the public can be encouraged to replace with low-risk species 100.

The prohibitive aspect of weed management in gardens can be softened by suggesting low-risk alternatives to invasive plants. At present about 7% of the flowering plants are known to have some weed potential: this means that the numbers of plants available for horticulture is still vast.

The following list is a very general guide to potential problem areas in horticulture. Specific weed problems, weed lists, and formal weed risk assessment are dealt with elsewhere. It is most important to realise that the following plants and plant groups are simply ones we need to keep an eye on. We need to be vigilant in the species we choose; this does not mean a total ban.

Styles and fashions

Some garden styles and fashions inadvertently promote potentially weedy plants 103. Grasses and grass gardens are a good example as the family Poaceae is a major source of many rapidly reproducing potentially weedy species. About one third of the grasses now growing in Australia are exotic species.

Attractive glossy magazines with quality photographs showing spectacular gardens in the UK can draw us inadvertently into hunting out plants that are not really appropriate for Australia.

Confronting severe droughts we promote xerophytes, plants that conserve water such as cacti and succulents. Unfortunately plants that are adapted to Australian hot and dry conditions will probably also be successful in the natural environment where conditions are also hot and dry, so we need to be very careful in the species we choose. Bamboos are among the most elegant of plants but many have invasive rhizomes e.g. Phyllostachys aurea. Gardeners need groundcovers that spread rapidly, especially in awkward spots like dry shade. But in this group are devastating weeds like the Periwinkle, Vinca major, and Houttuynia cordata and some mints that favour wet sites. As always it is a case of being vigilant and using common sense in plant selection.

Garden plants with weed potential

Weed management presents some difficult and paradoxical horticultural decisions and challenges as will be seen from some of the examples of weedy plants given below.

Plants that grow naturally in a similar climate
It is now well established that plants and animals may thrive away from their natural environments provided the climates and conditions are similar. In the new environment they are often free from natural diseases and predators. Some of the worst environmental weeds in South Africa are from Australia. In Australia we must be wary of plants from South Africa, California, the Canary Islands, the Mediterranean and, particularly in some areas notably hill districts, plants from temperate climates.

It makes horticultural sense to grow plants that do well in the local climate. It is therefore tempting to use “mediterranean” climate plants from South Africa, the Mediterranean, the Canary Islands and California in dry areas, and for temperate areas and hill country the more “temperate” plants from the UK, SW & Middle Europe as well as SE and NE USA. Unfortunately if the plants grow well in your local climate in the garden they may do equally well in the wild so it is quite likely that plants in your garden that are spreading prolifically by seedlings (or any other means) will do likewise in the nearby natural vegetation. Be aware of weeds that are specific to particular conditions such as creeks, coastal dunes, parks, farmland and bush and the ecosystems that are near the place of cultivation.

Plants able to survive extreme conditions such as drought
Cacti and succulents, grey foliage plants, some culinary herbs, grasses

Plants in groups with a known weed history

Iridaceae, Iris family
ManySouth African species with known weed potential
Poaceae, Grass family – many are seedy annuals (about a third of Australia’s grasses areintroduced)
Asteraceae, Daisy family – many are seedy annuals (constitute about 40% of all agricultural weeds)
Bulbs – collectors may acquire these, often in ignorance, by ordering through the internet from the USA and Britain. They escape detection by customs. These are often bulbs from southern Africa, which readily adapt to the Australian climate, soils, and frequent bushfires. Fire-adapted bulbs are often the first to recover and take over after fire. This is most noticeable in WA which has infestations of freesias, lachenalias, and watsonias. In pasture many bulbs are toxic to stock.

Other families with weedy members include: Papaveraceae, Poppy Family; Brassicaceae, Brassica Family; Amaranthaceae, Amaranth Family (see below**).

Plants that produce prolific numbers of propagules

These plants are even more dangerous when their propagules are easily transported.

Seeds – The following botanical families contain high weed representation Asteraceae, Daisy family; Amaranthaceae, Amaranth family; Asparagaceae, Asparagus family; Brassicaceae, Cabbage family; Chenopodiaceae, Chenopod family; Ericaceae, Erica family; Fabaceae, Pea family; Pinaceae, pine family; Poaceae, Grass family; Papaveraceae, Poppy family; Polygonaceae, Rhubarb family
Stem fragmentsTradescantia, Clematis vitalba, Delairea odorata, Hedera helix, Ipomoea, Vinca
Leaf propagules Bryophyllum, Crassula, Opuntia
TubersHelianthus tuberosus (Jerusalem Artichoke), Anredera cordifolia
Corms and cormletsCyperus rotundus (Nutgrass), Watsonia, Chasmanthe, Crocosmia, Sparaxis,Gladiolus, Ixia, Romulea, Tritonia
BulbsAllium, Oxalis
SuckersAilanthus, Populus, Ulmus, Robinia pseudoacacia
Plants with fruits spread by birds and animals
Pittosporum, Cotoneaster, Crataegus, Prunus, Pyracantha, Lantana, Arbutus unedo, Olea europaea, Schinus areira, Chrysanthemoides monilifera, Coprosma, Ilex, Leycesteria formosa, Ligustrum, Hedera helix, Lonicera, Sollya heterophylla

Less obvious characteristics include the ability of a plant to rapidly colonise and dominate disturbed or new sites and the relationship between the plants and its pollinators, seed and fruit vectors.

Plants grown from seed or bulbs

Monitoring of the contents of seed packets is currently inadequate. All mail entering the country is scanned by AQIS but small consignments of seed may be undetected and the online sale of weeds both internationally and nationally requires more stringent controls.[86] Wildflower seed mixes, especially those from overseas, may contain plants not listed on the packet. Some packets do not list their contents or may provide inaccurate lists. Care is needed in growing any plants from seed as contaminants may have crept in. Being aware of these potential problems can assist weed monitoring.

Mail order bulb catalogues which include well known weeds have resulted in complaints to the Department of Agriculture in WA. Chincherinchee, Ornithogalum thyrsoides, is spreading between Kojonup and Albany and can poison livestock. Community groups around Perth and in the south-west have been working to eradicate infestations of several common garden bulbs that have escaped into bushland including: Baboon-flowers, Babiana spp.; Black Flag, Ferraria crispa; freesias; Sparaxis spp.; watsonias; and Soldiers, Lachenalia spp.

Information about controlling bulbs and other weeds in bush in WA is available from The Wildflower Society (Tel. 06-9383 7979) also [89][90]



There is now a global recognition of the environmental, social and economic threats posed by invasive species and this has lead to a range of conventions, treaties and strategies at all political levels, from international to local.


Article 8 of the 1993 Convention on Biological Diversity (CBD) calls on national governments “to prevent the introduction of” and “control or eradicate those alien species which threaten ecosystems, habitats or species”. Before the CBD there was the 1952 International Plant Protection Convention which was ratified by 111 governments and directed at preventing the introduction and spread of pest plants. Ratification of the CBD entails obligations for the signatory governments of the countries involved.

It was only as late as the mid 1990s that international awareness targeted invasive alien species (IAS) as one of the most significant threats to biodiversity worldwide.

The Global Invasive Species Program

The Global Invasive Species Program (GISP) was established in 1997 to address the global threat and support the implementation of Article 8 of the CBD. In 2001 GISP produced the Global Strategy on Invasive Alien Species to increase awareness and provide advice on policy formulation together with the more practically orientated Invasive Alien Species: a Toolkit of Best Prevention and Management Practices.


The National Weed Strategy

The National Weeds Strategy: a Strategic Approach to Weed Problems of National Significance (NWS) was launched in June 1997 (with a second edition in 1999) 29, by three Ministerial Councils: Agriculture and Resource Management Council of Australia and New Zealand; Australian and New Zealand Environment and Conservation Council; and Forestry Ministers Council. This report takes a strategic approach to weed management problems of national significance, addressing environmental and agricultural weeds equally. The NWS describes the nature of the problem, discusses why existing weed management measures are not adequate, lists the roles and responsibilities of government, community, landowners and land users. It lists three goals with underpinning objectives and strategies, which are to be addressed by the Executive Committee and government at all levels in tackling this form of land degradation. The NWS is available on the web at

Australian Weeds Committee

This body is charged by the National Weed Strategy to reduce the detrimental impact of weeds on the economic, environmental, social and cultural sustainability of Australia’s productive capacity and natural ecosystems by providing an inter-Governmental mechanism for identification and resolution of weed issues at a National level for Australia. The object is to ensure an integrated approach to all aspects of weed management

Cooperative Research Centre for Australian Weed Management

Cooperative Research Centres were established in 1990 and funded by the Commonwealth Government to coordinate academics and the public and private sectors towards common goals. The CRC for Australian Weed Management (Weeds CRC) provides specific information on weedy species and their management 28, and hosts a mailing list ‘Enviroweeds’; its focus is on research.

The National Weed Detection Network

The Weed CRC and National Heritage Trust have funded a National Weed Detection Network (NWDN). The project started in July 2004 and will last for four years as a community-based weed detection network. The aim is to detect new weed incursions at a stage when eradication or containment is possible, thus minimising both control costs and impacts. The project is being piloted in Queensland by the Weed CRC in collaboration with the Queensland Herbarium. Volunteers, called “weed spotters” employ fortuitous surveillance (spotting weeds while engaging in other activities). Specimens are identified by botanists, fully documented and recoded with herbarium specimens and the Herbarium notifies government of any new naturalisations, new occurrences of declared weeds and any new and emerging weeds (see


To ensure protection against the damage done by weeds one obvious approach is legislation, but this is difficult to coordinate and enforce at whatever political level it is applied. The World Wildlife Fund has produced a series of reports examining the effectiveness of National and State legislation in dealing with weeds, and in particular those emanating from horticulture.

WWF Invasive Plants of National Importance and their Legal Status by State and Territory

This 2004 report12 examined the State and Territory legal controls on nationally important weeds – those on the following three lists: the National Environmental Weed Alert List, Weeds Recommended For National Eradication, the Northern Australia Quarantine Strategy target plant list, and Weeds of National Significance. The report concluded that State and Territory legal controls on invasive plants are extremely variable. At a national level, preventative legal measures on Invasive Plants of National Importance are poor. Only 2 States, Queensland and South Australia, prohibit the sale of all 20 weeds of national significance. For naturalised non-native plants recommended for national eradication or containment there is a strong State/Territory bias toward agricultural over environmental weeds. Combined State and Territory government legislation forms an inadequate statutory framework and far stronger national legal controls are needed under the Environment Protection and Biodiversity Conservation Act 1999 to prevent the sale and importation of invasive plants of national importance. The reasons why, in 2005, State and Territory weed legislation in Australia is generally ineffective in both stopping the legal importation of new invasive plant species and blocking the major pathway for the intentional spread of those already in their jurisdictions is further discussed in 58 which is available on the web. It then proposes a way forward to overcome these weaknesses.

WWF Garden plants that are invasive plants of national importance: an overview of their legal status, commercial availability and risk status

This report 33 is an overview of garden plants that are invasive plants of national importance (IPNI, those listed as WONS, NAQs and CALEW – see Glossary), assessing their legal status, commercial availability and risk status. The report points to irregularities (“loopholes”) in legislation that have allowed IPNI at both Commonwealth and State/Territory levels to be commercially traded through the gardening industry and therefore to potentially become widely distributed. It concluded that inadequacies were often the result of poor communication between government departments. For example, plants may be prohibited as imports and appear on one or more of these lists and yet be commercially available and promoted in the media (e.g. Rhodomyrtus tomentosa). Horsetails, Equisetum spp., are declared weeds in 6 States (this prohibits their sale and requires full destruction of plants when found), and also appear on the CALEW but can currently be legally imported. Asparagus asparagoides, Bridal Creeper, appears on the WONS list of 20 most important National weeds. In 2001 all States and Territories agreed to prohibit the sale of WONS but Bridal Creeper is only a declared weed in 5 States and Territories excluding some areas of NSW where the plant is commercially available.

Key findings of the second 33 report included:

• 9 (32%) of the 28 plants on CALEW can be legally imported
• 16 (57%) of the 28 plants on CALEW are naturalised garden plants. Half of these can be currently imported and 6 are available for sale
• 6 (21%) of the 28 plants on CALEW are available for sale in Australia

In Victoria, in 2002, at least 43% of its environmental weeds were available in the nursery trade34.


Victorian Pest Management

The overarching Victorian government policy document on weeds is Victorian Pest Management- a Framework for Action (2002) 31 (VPMF) and its offshoot the Weed Management Strategy (2002). These documents concentrate on weeds declared under the Catchment and Land Protection Act 1994 and translate into regional Weed Action Plans that are still being developed.

Weed policy direction in Victoria is established by the Department of Sustainability and Environment (DSE) and implemented by the Department of Primary Industries (DPI).

There is a statutory requirement to manage environmental weeds in national parks under the Flora and Fauna Guarantee Act 1988 which refers to the management of ‘threatening processes’.

Advice on the management of weeds (herbicides, integrated weed management, hand weeding, biocontrol) is available from DPI through the Customer Service Centre on phone 136 186 or through Landcare Notes available at For a general overview of control methods for environmental weeds see 13, for biological control see 14.

In 2006 $9 million was allocated over a period of 3 years to a Tackling Weeds of Private Land
(TWoPL) initiative complementing other State Government pest management programs and fostering partnerships with key stakeholders: the garden industry; local government; linear reserve managers (esp. railways and roads) catchment management authorities; fodder industry.

Weed Alert Rapid Response Plan

Weed Alert Rapid Response Plan (WARRP) is a program established by DPI to identify potential, new and emerging weeds in Victoria and to set in train a process for their eradication and communication about them. In Victoria when a new incursion has been identified information about the new weed is provided to the Weed Alert Network of Weed Spotters 101.


This is a program to reduce the transport of weeds by contractors, service providers and government agencies. It uses vehicle log books to asist purchasers and contractors to assess the weed status of an area where produce is being obtained or where works are to be done. The program includes an education package (see


Whatever means of weed risk assessment is adopted, from a management perspective the outcome is the same – a list of plants that must be dealt with in some way. What kinds of lists are available and how reliable are they?

Some lists are statutory; they are legally binding. Others have governmental or scientific authority; but these lists are relatively few. How are we to assess and respond to the many others?

Some plants are considered so invasive that they pose a serious threat almost anywhere they might be grown; they have a broad ecological amplitude. Others may be devastating weeds but only under a narrow range of conditions.

In the desire to establish firm guidelines for weed management it is tempting to compile “definitive” lists for management action. Lists like this should be tempered with the knowledge that plants behave differently in different areas: the degree of environmental and/or agricultural risk posed by a given plant will vary from place to place. These points have already been mentioned:

• Lists are directed at different levels of the ecological hierarchy – global, national, state, regional, or local – we need to know the geographic/bioregional scope of the list
• We need to have some estimate of the degree of risk posed by plants
• The degree of risk posed by any plant will depend on where it is growing


Weedy where?

The list of Australia’s 20 worst weeds, the WONS, appear appropriate for total exclusion from cultivation in this country. And yet it might be pointed out that Australia’s worst weed, Parkinsonia aculeata, Parkinsonia, from tropical America, is not naturalised in Victoria and, being a tropical plant, would pose little or no threat if grown there. In such a case it may be also be pointed out that this is such a devastating weed where it does grow, that cultivation anywhere might only increase the probability of it being transported accidentally or deliberately to new sites. This situation becomes even more important when we are considering growing potentially devastating weeds that are not yet established in our region. A current illustration of this exists where plants that are declared weeds in one state are readily available in nurseries just across their borders 58. Clearly for legislation to be effective a nationally coordinated approach to the problem is called for and at least adjacent states could be cooperating with one-another and coordinating their efforts.

But what if the tropical plant in question were not a devastating weed but only a minor one, and we wanted to grow it in southern Australia? There is no simple answer here. Ultimately decisions must be based on estimated risk.


High risk plants

It would be a great advantage if the lists: global, national, state, regional, local represented a nested hierarchy of plants posing high (unacceptable) risk, that the plants in a list at any level of the hierarchy also include all those in the levels above. So, weed management would be assisted if a list of global weeds were compiled of those plants considered to pose such a serious threat (no matter where they are grown) that they should be banned or seriously discouraged. Such a list would be based on the best WRA achievable. A national list would then be compiled in the same way and would, of course, include all the plants on the global list. This process could then proceed down the hierarchy. It should be noted that some plants on the global list may not be in the country, but that only emphasises their high potential threat.

Further lists could be assembled to indicate plants with moderate risk, and so on. Of course these lists would be less authoritative and allow


The Global Invasive Species Database
The Global Invasive Species Database (GISD) aims to improve awareness of all invasive alien species just plants) and to encourage their effective management. It is managed by the Invasive Species Specialist Group (ISSG) of the Species Survival Commission of the IUCN-World Conservation Union. The GISD was developed as part of GISP. The focus is on species that threaten native biodiversity and its scope is all taxonomic groups from micro-organisms to animals and plants in all ecosystems. Species information is either supplied by or reviewed by expert contributors from around the world. It is being constantly updated.

The world’s worst agricultural weeds
A 1977 publication 65 lists the 18 most serious agricultural weeds world-wide, in approximate order of impact, as:

Cyperus rotundus, Nutgrass (world’s worst weed) Sorghum halepense, Johnson Grass
Cynodon dactylon, Couch Imperata cylindrica, Blady Grass
Echinochoa crusgalli, Barnyard Grass Chenopodium album, Fat Hen
Echinochloa colonum, Awnless Barnyard Grass Digitaria sanguinalis, Summer Grass
Eleusine indica, Goose-grass Convolvulus arvensis, Field Bindweed

The above “top ten” is distinguished by being clearly the most widespread with the greatest impact on the largest number of crops. The remaining eight are:

Portulaca oleracea, Purslane Amaranthus hybridus, Pigweed
Eichhornia crassipes, Water Hyacinth Cyperus esculentus, Nut Sedge
Avena fatua, False Oats Paspalum conjugatum, Ti Gras, Sour Grass
Amaranthus spinosus, Spiny Amaranth Rottboellia exaltata, Corn Grass, Itch Grass

The Global Compendium of Weeds

The Gobal Compendium of Weeds is a compilation of plants known to have naturalised somewhere in the world. This project is an extension of the Exotic Species Database for Australia compiled by Rod Randall 99, supported by the CRC for Australian Weed Management and nearing completion in 2006. From the Exotic Species Database database which has been compiled from nursery stock lists, a botanic gardens plant inventory, books, pamphlets etc. it is estimated that 27,338 species of exotic plants have been introduced to Australia, numerically about 40% more than those species naturally present. It is also estimated that a total of about 36,414 species are found in cultivation, to some extent, somewhere in the country of which 25,714 are exotics and 10,700 natives. Of the 672 natives that have naturalised outside their native range 591 (88%) are also cultivated. Of the 24,249 introduced species that have not naturalised there are 4565 with records of naturalisation elsewhere in the world. The final form of the database is not decided but it is expected that one version will be a PDF document and another a delimited file that can be imported into any database. Both will be available from the CRC for Australian Weed Management.

A recent publication provides descriptions of over 400 species of the world’s major environmental weeds 75


Invasive Plants of National Importance
Invasive Plants of National Importance (IPNI) have have been defined as those invasive plants that the Australian Government should have an interest, in cooperation with the States and Territories, to ensure a robust nationally coordinated and effective prevention, eradication and control response 33.

The only national list of invasive species accepted collectively by the Australian, State and Territory governments is the 20 species listed on the WONS (see below). In the absence of a more comprehensive list the WWF has compiled a set of lists that are either official government lists, or lists recommended by scientists. These lists were divided into three categories:

Quarantine Lists

Species that are a high invasion risk, some present already.

1. Northern Australia Quarantine Strategy Weed Target List

Northern Australia Quarantine Strategy Weed Target List (NAQS) is a list of 41 species that are regarded as serious threats to Australia’s productivity, export markets and the environment (available on the site

Alert Lists

Naturalised invasive species currently within a restricted range and whose eradication is feasible and cost-effective.

1. Commonwealth Alert List of Environmental Weeds

In 2000, the Department of the Environment and Heritage worked with experts to identify plant species that are in the early stages of establishment and have the potential to become a significant threat to Australian biodiversity if not managed. Those species that were identified have been placed on the National Environmental Alert List. The Commonwealth Alert List of Environmental Weeds (CALEW)42 list is made up of 28 non-native weeds that have established naturalised populations in the wild.

Scientific Name Common Name (Where Known) Extent In Australia Potential Distribution
Acacia catechu var. sundra Cutch tree NT QLD, WA
Acacia karoo Karroo thorn QLD, NSW, SA, WA
Asystasia gangetica ssp. micrantha NSW QLD, NT, WA
Barleria prionitis Barleria QLD, NT WA
Bassia scoparia (weedy form) Kochia Tas, WA NSW, Vic, SA
Calluna vulgaris Scotch heather Tas NSW, Vic
Chromolaena odorata (weedy form)
Siam weed QLD NT, WA
Cynoglossum creticum NSW Vic, Tas, SA, WA
Cyperus teneristolon NSW QLD, Vic, SA, WA
Cytisus multiflora White spanish broom Vic NSW, Tas, WA
Dittrichia viscosa WA NSW, Vic, Tas, SA
Equisetum spp. Horsetail NSW, Tas, Vic
Gymnocoronis spilanthoides Senegal tea plant QLD, NSW
Hieracium aurantiacum Orange hawkweed, orange paintbrush
Tas, Vic NSW, SA
Koelreuteria elegans Chinese rain tree, flame gold,golden rain tree
Lachenalia reflexa Lachenalia WA NSW, Vic, Tas, SA
Lagarosiphon major Lagarosiphon Tas, NSW Vic, SA, WA
Nassella charruana Vic NSW, SA, WA
Nassella hyalina NSW, Vic
Pelargonium alchemilloides WA NSW, Vic, Tas, SA
Pereskia aculeata Leaf cactus QLD, NSW WA
Piptochaetium montevidense Vic NSW, SA, WA
Praxelis clematidea QLD NT, WA
Retama raetam White weeping broom SA, WA NSW, Vic, Tas
Senecio glastifolius Holly leaf senecio, water dissel
NSW, WA,Vic, Tas, SA
Thunbergia laurifolia Laurel clock vine QLD NT, WA
Tipuana tipu QLD NT, WA
Trianoptiles solitaria Vic NSW, Tas, SA, WA

Also available at

2. Candidate eradication and containment list impacting natural ecosystems
This 3 is a list of 34 naturalised species compiled by scientists to include species that pose a direct threat to natural ecosystems because of their potential impact on native species: it has no official status.
3. Candidate eradication and containment list impacting agricultural ecosystems

This3 is a list of 27 naturalised species compiled by scientists to include species that pose a potential threat to agricultural ecosystems should they ever spread further: it has no official status.

4. Bureau of Rural Sciences eradication candidate list of sleeper agricultural weeds

A list 66, compiled by scientists, of 17 species for cost-effective eradication before they become major agricultural weeds: it has no official status.

Control Lists

Invasive species that are naturalised and widespread, pose a major threat to the environment or agriculture and whose containment or control will protect values of national environmental significance.

Weeds of National Significance

To implement the NWS it was decided to rank weed problems of national significance. The National Weeds Strategy Executive Committee was established in 1997. The Committee concluded that the greatest impact from weed problems within Australia was related to the effect and spread of specific individual species. On this basis, the assessment procedure developed would focus on determining Weeds of National Significance (WONS) 17, 43 and prioritise weeds over a range of land uses at the national level in an attempt to draw together meaningful indicators for future weed decision-making as well as a method for prioritising weeds at the State, regional and local levels17. The selection of twenty WONS is a test case for improved coordination among stakeholders responsible for weed management within Australia. The list below is ranked in order of importance. Figures in brackets are the percentage of land surface occupied by each weed, the total Australian land area occupied being 70.9% 17.

1 Parkinsonia aculeata Parkinsonia (12.4) 11 Cabomba caroliniana Cabomba (0.5)
2 Prosopis spp. Mesquite (5..3) 12 Nassella neesiana Chilean Needle Grass (0.2)
3 Rubus fruticosus spp. agg. Blackberry (9.0) 13 Tamarix aphylla Athel Pine (1.0)
4 Lantana camara Lantana (5.1)
14 willows except weeping willows, pussy willow and sterile pussy willow Salix spp. except S. babylonica, S. X calodendron and S. X reichardtii (0.8)
5 Cryptostegia grandiflora Rubber Vine (7.7) 15 Nassella trichotoma Serrated Tussock (2.2)
6 Chrysanthemoides monilifera Boneseed (3.0)
7 Acacia nilotica ssp. indica Prickly Acacia (2.3) 16 Parthenium hysterophorus Parthenium Weed (5.6)
8 Hymenachne amplexicaulis Hymenachne (1.0) 17 Annona glabra Pond Apple (0.4)
9 Salvinia molesta Salvinia (5.0) 18 Ulex europaeus Gorse (3.0)
10 Mimosa pigra Mimosa (1.0) 19 Asparagus asparagoides Bridal Creeper (5.0)
20 Alternanthera philoxeroides Alligator Weed (0.4)

The list of WONS is available at
A document addressing management strategies for WONS is available at:

Table 7
Land areas occupied by transformer weeds in Australia 40 cited in 39

Rubus fruticosus spp. agg., Blackberry 8 M
Acacia nilotica subsp. indica, Prickly Acacia 6.6 M
Lantana camara, Lantana 4 M
Cryptostegia grandiflora, Rubber Vine 700 000 (over 20% Qld)
Mimosa pigra, Mimosa, Sensitive Tree 80 000

Each State has lists of declared weeds. In Victoria further restricted lists may be produced (see Declared Weeds, Part 1).

Regional and local – Bushland Friendly Gardens
The Weeds CRC has a web page on Bushland-friendly Gardens that synthesises published weed information for 22 “clickable” climatic regions of Australia that are recognised by the horticultural industry ( Master lists of weeds for each region were compiled by synthesising existing lists and brochures of problem plants in that region using the assumption that the more lists on which a plant appears, the more it is likely to impact. Each region generates two lists of commonly available garden plants that should be avoided in the selected region. Where a plant species appeared on three or more regional lists it was placed into the ‘serious environmental weeds’ list – these being plants to be removed as soon as possible. If a plant appeared twice, it was placed on the ‘environmental weeds to avoid’ list – recommended not to be planted, or at least managed with great care. The Weed CRC believes that the final weed lists for each region are a fair representation of the potential and actual impacts for each species. The site has a bibliography of regional lists and also indicates literature that suggests alternative species to plant.

Tackling invasive plants, as with many pervasive ecological problems, requires an integrated and coordinated approach at many levels; from global to local, industrial to scientific.

Management in the field
Like the weeds in our gardens, environmental and agricultural weeds require management. The broad plan of action is prevention, containment, control and eradication. This means early detection and rapid response, ideally by preventing their occurrence but, if they have established, controlling them using environmentally safe methods – chemical, mechanical, biological etc.

Effective weed risk management (WRM) in the field is hampered by the multiplicity of land-holders, both public and private, together with the many additional organisations that have indirect influence over this land. Clearly a coordinated effort is needed. In Victoria for example organisations whose activities can influence the weed flora include: the nursery industry, Parks Victoria; Victorian Catchment Management Council and the Catchment Management Authorities; Victorian Farmers Federation; private landholders; VicRail and so on. One current difficulty is the integration of weed mapping into a common system acceptable to the various stakeholders. Procedures and methods used to reduce the impacts of weeds include:

For potential new weeds not yet in the country:

• preventing the entry of weed species into Australia (border protection – Biosecurity Australia)

For new arrivals, minimising their spread by :

• early detection and eradication of those both in the wild, and in cultivation (agriculture and the horticulture industry)

For established weeds:

? using existing control measures such as herbicides (world-wide herbicide weed resistance in crops began 1970-1980), cultivation, and biological control

These key procedures are supported in the following ways:

• legislation to control movement of weeds; and
• research including development of new detection and eradication methods
• campaigns to educate the public and industry
• national coordination

Clearly prevention is better than the expensive control and eradication programs that are necessary once weeds become established. Historically government has been reactive, responding to weed problems as they have arisen. It is now urgent that we become proactive, ensuring that potential weeds are refused entry into Australia and that the demand, supply and control of those already present is closely monitored.

As with many other ecological problems the management of invasive plants must be tackled at all levels of governance, from global to local and organisational. Individuals must also accept that they have a role, no matter how small.

Establishing a global and national framework of intention gives guidance and authority to the groups who deal directly with the weeds.

Weed distribution

Where weeds are well established through most of their potential range resources are best directed to eradication of early invaders rather than attempting eradication or banning their availability. Figure 6 illustrates the typical pattern of invasive weed spread, indicating the cost-benefit of tackling eradication or control early in the invasion process.

Figure 6 – Pattern of weed spread

Prioritising for management
In assessing the allocation of resources to weed management it is clearly important to assess costs against potential benefits. A plant that has spread throughout its potential range is a lesser priority than a potentially invasive plant that is newly arrived. Weed risk assessment follows the three key parameters by considering: invasiveness (how fast-spreading is it?), potential distribution (is it localised at present but could spread over a wide area?), impacts (what is its potential for adversely impacting of social, environmental or agricultural values?).

Contraceptive spray
The Weed CRC together with CSIRO scientists and Prof Ed Newbigin of Melbourne University Botany Department in August 2005 announced a research program aimed at producing a non-toxic spray that mimics chemicals produced by the plant itself to prevent self-fertilization. This is generally a protein that allows the plant to detect and ignore its own pollen. It therefore acts like a chemical contraceptive. Work would begin on the agricultural weed Raphanus raphanistrum, Wild Radish, and results were expected within three years 81.

Nursery Industry
In 2000 a draft national strategy on invasive garden plants called Garden Plants Under the Spotlight20 (GPUTS) was produced by the Weeds CRC with State Government agencies and the Nursery Industry Association of Australia NIAA (now Nursery and Garden Industry Australia, NGIA). It made a number of recommendations for raising industry and community awareness about invasive garden plants and strengthening legislation. Weed work continues at NGIA with: the development of an Invasive Plants policy Position including a commitment to developing industry-based communications and awareness programs; appointment of an Environmental Policy Manager; accreditation programs, Technical Nursery Papers 19,20,110 and newsletters, articles etc.; ESM programs; “Grow Me Instead!” pamphlets; cooperation with other organisations.

Garden Thugs
From this strategy, a list of 52 ‘Garden Thugs’ was assembled with the intention that they be removed from trade around Australia voluntarily 19. The strategy has not been finalised.


Ailanthus altissima Tree of Heaven Allium triquetrum Angled Onion
Alternanthera philoxeroides Alligator Weed Anredera cordifolia Madeira Vine
Arundo donax Giant Reed Asparagus asparagoides Bridal Creeper
Asparagus scandens Asparagus Fern Cabomba caroliniana Cabomba
Chrysanthemoides monilifera Boneseed Cortaderia spp. Pampas grass
Cotoneaster spp. Cotoneaster Crocosmia x crocosmiiflora Montbretia
Cryptostegia grandiflora Rubber Vine Cytisus scoparius English Broom
Delairea odorata Cape Ivy Dipogon lignosus Dolichus Pea
Eichhornia crassipes Water Hyacinth Elodea canadensis Canadian Pondweed
Equisetum spp. Common Horsetail Erica lusitanica Spanish Heath
Genista monspessulana Cape Broom Hedera helix Ivy, English Ivy
Hydrocotyle ranunculoides Water Pennywort Hypericum perforatum & vars St John’s Wort
Ipomoea indica Purple or Blue Morning Glory Lagarosiphon major Oxygen Weed
Lantana camara Lantana Leycesteria formosa Himalayan Honeysuckle
Mimosa pigra Mimosa Myriophyllum aquaticum Parrot’s Feather
Opuntia spp. (not Opuntia ficus-indica, Cylindropuntia spp., Tephrocactus spp.) Prickly Pear
Parkinsonia aculeata Parkinsonia Pinus radiata Monterey Pine
Pittosporum undulatum Sweet Pittosporum Polygala virgata Purple Broom
Prosopis spp. Mesquites Rosa rubiginosa Sweet Briar, Eglantine
Salix cinerea Grey Sallow Salix fragilis var. fragilis Crack Willow
Salix nigra Black Willow Salix rubens Willow
Salix hybrids Willow Salix x sepulcralis Willow
Salvinia molesta Salvinia Senna alata Candle Bush
Sparaxis bulbifera Harlequin-flower Tamarix ramosissima Saltcedar
Tamarix aphylla Tamarisk Tradescantia albiflora Wandering Jew
Ulex europaeus Furze, Gorse Watsonia bulbifera Bulbil Watsonia
Ziziphus mauritiana Chinee Apple

This list was no doubt given impetus by a list of 958 “garden thugs” eventually published by Randall in 2001 48.

In Victoria DPI is currently working with the Nursery and Garden Industry of Victoria (NGIV) to produce a list of invasive garden plants that may be restricted and removed from sale (Daniel Joubert, DPI Frankston pers. comm.). A ‘Voluntary List’ of about 58 plants has been agreed between the NGIV and the State Government as a recommendation for members not to grow or sell (see below).

Botanical Name Common Name
Acacia nilotica * Prickly Acacia
Ambrosia spp. All Ragweeds
Annona glabra.* Pond Apple
Anredera cordifolia Madeira Vine
Asparagus asparagoides * Bridal Creeper
Bassia scoparia , B. sieversiana , Kochia alata, K. scoparia var.
culta, K. scoparia var. pubsecens , K. scoparia var. subvillosa
Moq., K. scoparia var. trichophila (Stapf), K. sieversiana , K.
trichophila Stapf. Kochia
Cabomba spp. (all) * Cabomba
Calystegia silvatica Greater Bindweed
Carthamus glaucus Glaucous Start Thistle
Cenchrus incertus Spiny Bluegrass
Centaurea maculosa Spotted Napweed
Chromolaena odorata Siam Weed
Cryptostegia grandiflora* Rubber Vine
Disa bracteata African Weed-orchid
Gymnocoronis spilanthoides Senegal Tea
Hedera helix English Ivy
Hymenache amplexicaulis * Hymenache
Hypericum calycinum Grow under permit Large Flowered St John’s Wort
Hypericum canariense Canary Island St John’s Wort
Hypericum humifusum Grow under permit Trailing St John’s Wort
Lantana camara* Lantana
Miconia spp. * Miconia
Mimosa pigra * Giant Sensitive Plant
Nassella spp.* Needlegrass
Onopordum spp. Onopordum Thistles
Onopordum tauricum Taurian Thistle
Opuntia aurantiaca Tiger Pear
Parkinsonia aculeata* Parkinsonia
Rubus alceifolius Giant Bramble
Rubus argutus Florida Blackberry
Rubus rugosus Keriberry
Sagittaria graminea Sagittaria
Sagittaria montevidensis Arrowhead
Sagittaria platyphylla Delta Arrowhead
Sagittaria pygmaea Dwarf Arrowhead
Salix aegyptiaca * Asian sallow
Salix alba * White willow
Salix cinerea * Common Sallow
Salix exigua * Sandbar Willow
Salix fragilis. * Crack Willow
Salix glaucophylloides * Dune Willow
Salix humboltiana * Pencil Willow
Salix spp. (all except S. babylonica L., S. calodendron [= S. x
calodendron = S. caprea x S. purpurea x S. viminalis, S. x reichardtii
[= S. caprea x S. cinerea, S. alba var caerulea ) * Willow, sallow, osier
Salix matsudana * Tortured Willow
Salix nigra * Black Willow
Salix purpurea and cultivars * Purple Osier
Salix seringeana * Seringe Willow
Salix viminalis * Basket Willow
Salix x chrysocoma * Golden Weeping Willow
Salix x dasyclados.* Willow
Salix x pendulina. * Wisconsin Weeping Willow
Salix x rubens * White Crack Willow
Salix x sepulcralis. * Weeping Willow
Salvinia. spp. (all) * Salvinia
Scolymus maculatus Spotted Thistle
Tamarix aphylla * Athel Tree

* Plant is Weed of National Significance (WONS)

A number of additional Nursery Papers have been produced covering such topics as advising the public about weeds, providing alternatives to garden escapes, and preventing the introduction of new weeds: these can be accessed from the NGIA web site. A Fact Sheet for the horticultural media is available from

Sustainable Gardening Australia

One response by the nursery industry has been the formation, in 2004 of Sustainable Gardening Australia (SGA). SGA is a not-for-profit association totally committed to achieving real, continually improving and easily understood environmental solutions for gardeners. Nurseries and Garden Centres on the SGA program have voluntarily removed from sale 10 of the worst weed invaders of their area and are also encouraged to tag other potentially invasive plants with “SGA Weed Warning” labels. Included in the work of SGA are a Green Gardeners course with Holmesglen TAFE and the landscape industry (inc. certification for designers and landscapers); an online course in Sustainable Gardening.

Bushland Friendly Nursery Scheme

On the Northern NSW coast a Bushland Friendly Nursery (BFN) Scheme has been introduced and there is the possibility of the scheme a national program. The schemes establish weed lists for the local area, specifying plants that should not be sold, propagated or knowingly distributed. Unfortunately such a system based on goodwill can be easily abused by introduction of plants from elsewhere.

Botanic Gardens

For many years Botanic Gardens were part of an international network exchanging seed lists (Index Semina) – this being the main means of plant acquisition, especially the rare and unusual species. Seed exchange is now restricted. Firstly, there is the legally binding Convention on International Trade in Endangered Species of Wild Flora and Fauna (CITES). Secondly, under Article 8 of the 1993 Convention on Biological Diversity (CBD) national governments are called on “to prevent the introduction of” and “control or eradicate those alien species which threaten ecosystems, habitats or species”. The Global Invasive Species Program (GISP), was established in 1997 to address the global threat and support the implementation of Article 8 of the CBD. As a result of these international initiatives there is now a common agreement among many botanic gardens to carefully monitor the acquisition and use of genetic resources, one aspect of which is obtaining consent from the country and/or organisation of origin to ensure potential benefit sharing (including non-monetary benefits), this also being a requirement of the CBA.

The major Australian botanic gardens have not produced Index Semina for many years (the last seed list produced by the Royal Botanic Gardens Melbourne was in 1994) and seed acquisition from overseas is dealt with by special request and supervised with caution.

Historically there is no doubt that botanic gardens were a point of entry for plants new to the continent. The escape of Blackberry, Rubus fruticosus, and Sensitive Tree, Mimosa pudica, have been attributed to the Melbourne and Darwin botanic gardens respectively. Some of the newly acquired plants would have passed to public parks and gardens, the nursery industry and from there to home gardens.

Internationally botanic gardens are still in the early stages of developing WRA models. Chicago Botanic Garden has recently developed an invasive screening protocol using an integrative approach that combined regional invasive species lists, commercial availability, a WRA and on-site evaluation 60.

The Australian Botanic Gardens Weed Network
In October 2004 the Council of Heads of Australian Botanic Gardens (CHABG) endorsed a proposal for the collaboration of Australia’s Botanic Gardens in developing weed policies, management and weed risk assessment. A network of about 70 public gardens, zoos and arboreta has been established to achieve these objectives 102.

Plants posing high risk are removed altogether. Direct weed management strategies include: pruning, using barriers to roots, removing seed heads; not selling the plant or allowing it to be moved out of the gardens; restricting access to plants through the use of barriers and enclosures; use of signs to indicate weed problems and enforcement procedures e.g. signs at gates indicating that removing plants or parts thereof is a criminal offence.

Local Government

The Municipal Association of Victoria encourages local councils to include invasive species management as part of their biodiversity conservation policies (see Victorian Local Government Weed Management Report: Programs, Resources and Management Approaches, 2003. also 98.

Community groups

Voluntary community groups managing small areas of public land are working hard to monitor and manage the weeds on their sites. There is an excellent weed management manual prepared by the Weed CRC as a training aid for private landholders, conservation groups, catchment management groups, local, state and territory governments and industry available at

Ideas for preventing weeds in your area are given in the Suggestions for Home Gardeners. The “Bradley method” of weeding by starting in the least infested areas and working towards the greatest infestation has been an inspiration to many community groups when tackling what seems an insuperable task 87.

Another innovation has been the Plant Swap Meet, where pest plants are handed in by residents in exchange for native plants. Swap Meets have also been used successfully as part of a “Connect with Nature” campaign in New Zealand.

Weedbusters and Bushcare

Weedbusters is an awareness program that works with the community to achieve sustainable land and water management, primarily through increased public involvement in weed management, education and awareness projects. Weedbusters started in 1994 in Queensland, spreading to become national in 1997 with encouragement and support from the Australian governement, all State and Territory Governments and the Cooperative Research Centre (CRC) for Australian Weed Management. Weedbusters is now a year-long campaign, culminating in Weedbuster Week – a week of highlights and celebrations. The Weedbusters program has spread to New Zealand and South Africa, and other countries have expressed interest in running their own Weedbuster campaign. (see


Landcare groups are working on solutions to the degradation caused by loss of native vegetation. Groups are working in rural, urban and bushland areas to assess weed problems as part of an integrated approach to sustainable land use. “Landcare groups look at the causes of weed infestation and then tackle the source of the problem as well as the weeds themselves. They remove weeds from farming areas, bushland and coastlines. Volunteers use techniques such as burning, chemical control and hand removal. It may involve changing the way that land is managed – perhaps by reducing the access of stock, people and vehicles.

Weed Spotters

Weed Spotters is a group of environmentally-concerned people who are contacted and asked to be on the lookout for potential, new and emerging weeds. Weed Spotters not only look out for weeds in the “field” but also in nurseries and garden centres, the media, email discussion groups, the internet, books etc. In Victoria in November 2004 there were over 600 Weed Spotters registered. Prospective new members in Victoria can contact Kate Blood ( or write to her at DPI, PO Box 7, Beaufort, Vic 3373.

In Northern Australia two AQIS officers Barbara Waterhouse and Andrew Mitchell patrol for new plant invaders. Their beat covers an area half the size of Europe, extending across the country from Broome to Cairns and well into Papua New Guinea and Asia. To help them in their colossal task is a network of “weed spotters”, dedicated landholders and government agencies that collect unfamiliar plants they find. Working with other scientists in the CRC for Australian Weed Management and Herbaria, Waterhouse and Mitchell have shown time and again that early detection of a new invasion is the way to go – controlling, maybe even eradicating, new landscape destroyers before they can get a foothold (extract from a Weed CRC news release on the web).

The Environmental Weeds Action Network (EWAN)
This group is a community initiative to tackle the problem of environmental weeds in bushland and waterways in Western Australia. It brings together community members in both urban and rural areas, bush regenerators, local government, weed scientists and ecologists to save indigenous flora from the threat of weeds. EWAN was formed at a public meeting in February 1996 by members of community groups concerned about lack of communication between community groups and government agencies.


Weed Warriors
DPI Victoria now has a schools education program called Weed Warriors that involves children and parents in raising biological control agents in the classroom that can then be released in local weed infestations. The students learn about local weed problems and help to reduce local infestations.

Royal Botanic Gardens Melbourne

The Royal Botanic Gardens Policy for Conservation of Plant Biodiversity 30 is an organisational declaration of support for the principles of plant conservation and the preservation of biodiversity. The Royal Botanic Gardens encourages environmentally responsible horticulture and does not wish to contribute in any way to weed invasions of ecosystems.


Until recently Botanic Gardens were part of an international network exchanging seed lists – this being the main means of plant acquisition, especially of rare and unusual species. However, seed exchange is now restricted. Firstly, there is the legally binding Convention on International Trade in Endangered Species of Wild Flora and Fauna (CITES). Secondly, under the CBD there is a common agreement among many botanic gardens concerning the acquisition and use of genetic resources, including consent from the country and/or organisation of origin and potential benefit sharing (including non-monetary).

The Weed Strategic Plan

As an organisation accessing and displaying a wide range of plants, many of these being rare or unusual (it manages over 10, 000 different kinds) there is always the possibility for acquiring or displaying plants that have the potential to become environmental, agricultural, or other weeds, or to exhibit other undesirable traits.

The RBGM Weed Strategic Plan 2003-2008 (WSP) outlines ways in which the RBG can address weed problems through environmentally sound horticultural practises and, by setting an environmental “best practise” example, encourage environmentally responsible attitudes in other organisations and the general community by:

• producing lists of environmentally safe species and cultivars
• educate and inform similar organisations, the public and nursery industry about invasive garden plants that become weeds
• assemble information on and lists of weeds, especially in the local context
• monitor all existing stock as well as stock entering and leaving the RBG
• assess the threat that popular or common horticultural plants may present to related native plants in the wild (conspecifics and congeners) including the possibility of hybridization threatening wild plant populations and their genetic integrity
• ensure that plants suggested as environmentally safe “alternatives” have been given full screening

The Weed Strategic Plan can be accessed at


One interesting initiative in the United States is for organisations to become signatories to a Voluntary Code of Conduct (VCC). The organisations that are signatories to the VCC include government, botanic gardens, home gardeners, landscapers and the nursery industry

In the United Kingdom a Horticultural Code of Practice called Helping to Prevent The Spread of Invasive Non-Native Species was produced by the Department of Environment, Food and Rural Affairs in March 2005 72 (available at It provides advice and guidance on the safe use, control, and disposal of invasive non-native plants for everyone engaged in horticulture and related activities that involve the use of plants.

The following suggestions are adapted for Australia from the American and English Voluntary Codes of Conduct.

Suggestions for the home gardener
As already discussed the environmentally-aware home gardener and nurseryman would only grow and supply plants with low environmental or agricultural risk, being especially cautious about plants in the groups already discussed.

• Try and be aware of lists of plants that have an invasive record and to ask for non-invasive species when buying plants
• Try to develop garden designs that use non-invasive species.
• Do not swap or give away plants you know to be invasive
• Help alert other members of your local community, societies, garden clubs, family and friends about the risks posed by weeds and encourage speakers to keep groups up-to-date
• Be aware of inadvertent promotion of invasive plants through the media and assist the media to avoid future such promotions.
• Avoid transporting known invasive plants.
• Dispose of plants known to have any weedy tendencies in a responsible way.
• Seek information on the most environmentally-friendly and effective way of controlling weeds.
• Seek information about invasive plants in your area: this may be through the local council, botanical gardens, conservationists and government agencies.
• Encourage the nursery industry to label plants clearly with botanical names somewhere on the label with an indication of possible dangers to the wider environment.
• Beware of releasing aquarium weeds into waterways or drains.
• If you send seeds or cuttings abroad (whether as gifts, for science, for landscaping or for trade) check to see if the plant has the potential to become invasive.
• Avoid dumping garden waste in the bush or over the fence.
• Be aware of potential seed spread when transporting plants by covering trailers etc.
• Join volunteer community weed groups to help eliminate weeds in your area.
• Be extremely cautious in bringing in seed from overseas and be aware that this must be cleared by Biosecurity Australia.
• Participate in early warning of weeds by observing the plants that occur in your local area and be aware of the agency to be contacted with the information.
• Assist garden clubs and societies to develop weed policies
• Replace invasive plants in your garden with safe alternatives
• Seek additional information from local, State and Federal government authorities – also see

The national Weed Alert Hotline phone number 1800 084 881

Voluntary Codes of Conduct for Government
• Require risk assessment for government-led or financed plant introductions to ensure that no new harmful plant species are introduced, intentionally or unintentionally.
• Do not distribute existing holdings of invasive plant species to areas where they can potentially do harm; eliminate these holdings or maintain new or existing holdings using appropriate safeguards.
• Coordinate and facilitate collaboration in databases, early warning systems, monitoring, and other means of preventing invasive plant species problems.
• Lead and fund (subject to budgetary considerations) the development of environmentally sound methods to control harmful invasive plant species, seek control of such species on public lands and promote their control on adjacent private lands.
• Develop and promote the use of non-invasive plant species within all government units and to the public.
• Facilitate, lead, coordinate and evaluate public outreach and education on harmful invasive plant species.
• Encourage that employees and management participate in ongoing training programs on invasive plant species.
• Foster international cooperation to minimize the risk of the import and export of potentially invasive plant species.
• Develop partnerships and incentive programs to lessen the impact of invasive plant species and provide non-invasive restoration materials.
• Provide a forum for regular evaluation of the effectiveness of these voluntary codes of conduct towards preventing the invasive plant species problem.
• Enforce invasive plant species legislation at all levels.

Voluntary Codes of Conduct for Nursery Professionals

• Ensure that invasive potential is assessed prior to introducing and marketing plant species new to North America. Invasive potential should be assessed by the introducer or qualified experts using emerging risk assessment methods that consider plant characteristics and prior observations or experience with the plant elsewhere in the world. Additional insights may be gained through extensive monitoring on the nursery site prior to further distribution.
• Work with regional experts and stakeholders to determine which species in your region are either currently invasive or will become invasive. Identify plants that could be suitable alternatives in your region.
• Develop and promote alternative plant material through plant selection and breeding.
• Where agreement has been reached among nursery associations, government, academia and ecology and conservation organizations, phase-out existing stocks of those specific invasive species in regions where they are considered to be a threat.
• Follow all laws on importation and quarantine of plant materials across political boundaries.
• Encourage customers to use, and garden writers to promote, non-invasive plants.

Voluntary Codes of Conduct for Landscape Architects/h6>
• Seek out education and information on invasive species issues:
• Work with local plant ecologists, horticulturists, nurseries, botanic gardens, conservation organizations and others to determine what species in your region either are currently highly invasive or show aggressive potential. Investigate species under consideration that may present a threat.
• Increase interaction with other professionals and non-professionals to identify alternative plant material and other solutions to problems caused by harmful invasive plants.
• Take advantage of continuing education opportunities to learn more about invasive species issue
• Identify and specify non-invasive species that are aesthetically and horticulturally suitable alternatives to invasive species in your region.
• Eliminate specification of species that are invasive in your region.
• Be aware of potential environmental impacts beyond the designed and managed area of the landscape plan (e.g. plants may spread to adjacent natural area or cropland).
• Encourage nurseries and other suppliers to provide landscape contractors and the public with non-invasive plants.
• Collaborate with other local experts and agencies in the development and revision of local landscape ordinances. Promote inclusion of invasive species issues in these ordinances.
Code of Conduct for Botanic Gardens and Arboreta
• Conduct an institution-wide review examining all departments and activities that provide opportunities to stem the proliferation of invasive species and inform visitors. For example, review or write a collections policy that addresses this issue; examine such activities as seed sales, plant sales, book store offerings, wreath-making workshops, etc.
• Avoid introducing invasive plants by establishing an invasive plant assessment procedure. Predictive risk assessments are desirable, and should also include responsible monitoring on the garden site or through partnerships with other institutions. Institutions should be aware of both direct and indirect effects of plant introduction, such as biological interference in gene flow, disruption of pollinator relationships, etc.
• Consider removing invasive species from plant collections. If a decision is made to retain an invasive plant, ensure its control and provide strong interpretation to the public explaining the risk and its function in the garden.
• Seek to control harmful invasive species in natural areas managed by the garden and assist others in controlling them on their property, when possible.
• Promote non-invasive alternative plants or, when possible, help develop non-invasive alternatives through plant selection or breeding.
• If your institution participates in seed or plant distribution, including through Index Seminum, do not distribute known invasive plants except for bona-fide research purposes, and consider the consequences of distribution outside your biogeographic region.

Consider a statement of caution attached to species that appear to be potentially invasive but have not been fully evaluated.
• Increase public awareness about invasive plants. Inform why they are a problem, including the origin, mechanisms of harm, and need for prevention and control. Work with the local nursery and seed industries to assist the public in environmentally safe gardening and sales. Horticulture education programs, such as those at universities, should also be included in education and outreach efforts. Encourage the public to evaluate what they do in their own practices and gardens.
• Participate in developing, implementing, or supporting national, regional, or local early warning systems for immediate reporting and control. Participate also in the creation of regional lists of concern.
• Botanical gardens should try to become informed about invasiveness of their species in other biogeographic regions, and this information should be compiled and shared in a manner accessible to all.
• Become partners with other organizations in the management of harmful invasive species.
• Follow all laws on importation, exportation, quarantine, and distribution of plant materials across political boundaries, including foreign countries. Be sensitive to conventions and treaties that deal with this issue, and encourage affiliated organizations (plant societies, garden clubs, etc.) to do the same.


The management of garden plants that have the potential to become agricultural and environmental weeds is particularly controversial and poses many difficult questions for gardeners.


Here are some FAQs concerning weeds, each with a brief discussion. Please note that there are no “right” answers to these questions.

Disputes about weeds (often concerning whether a plant is to be prohibited or not) seem to boil down to what constitutes an acceptable level of risk which, in turn, depends on how accurate our weed risk assessment is and how much we value the natural environment. It is also often a matter of reconciling environmental values with competing values: socio-cultural, scientific, heritage, economic, health, amenity, conservation, management etc. It does appear that the more objective the decision-making process and more complete the information used to mount a case, the greater likelihood there is of conflict resolution.

The questions posed here were initially compiled with botanic gardens in mind but the issues raised have general application.

1. Surely the only way for a botanic garden to be completely environmentally responsible would be to convert the site into an indigenous flora reserve and to not grow any exotic plants at all?
In principle this may be true. However, most of the plants in botanic gardens pose little, if any, environmental threat. Nevertheless, there is no doubt that the botanic gardens should minimise its environmental impacts and often substantial areas are devoted to the conservation of indigenous vegetation. Also Botanic gardens have an educative and scientific role and through the display of plant diversity, including exotic plants, promote a concern for plant life, biodiversity, and conservation, and a scientific approach to conservation issues.

2. Surely any plant can be given weed status?
Since all plants (other than sterile hybrids and ones that are maintained in cultivation by human propagation) reproduce effectively under appropriate conditions (otherwise they would not exist) it may be argued that they all have the potential to find areas where they will flourish. In practise many garden plants, especially hybrids and cultivars, reproduce very poorly and pose no environmental threat. A WWF/CSIRO report Jumping the garden fence46 states “the majority of plants available for sale by nurseries are not likely to become invasive in Australian ecosystems”.

3. Should botanic gardens be exempt from the conventions and regulations usually applied to weed management?
Growing plants that have a well-known weedy history sends an environmentally irresponsible message to the public.
Botanic gardens are scientific institutions that display plant diversity, often within a cultural landscape of local to national significance. Scientists and the general public are curious about plants that are poisonous or display other undesirable attributes, including those that are weeds. These plants may be needed for scientific research and therefore may be allowed “scientific, heritage, conservation or educational exemption” where the risk is not high. In cases like this the weed potential can be acknowledged through appropriate labelling and public education. Botanic gardens are also relatively intensively managed and so plants may be monitored more closely than in many other gardens. Nevertheless, “scientific exemption” cannot override environmental responsibilities and so there should be precautionary environmental management of the plants (removing seeding flower heads, growing in an enclosure to exclude vectors or root travel etc.). Such plants are best considered on a case-by-case basis.

4. Plants with berries are known to be quickly spread by birds and animals – should they be banned from the nursery industry and private gardens? Botanic gardens?
Plants that produce fleshy fruits are without doubt a high-risk category. Certainly they should be discouraged and/or carefully monitored. Whether botanic gardens should grow them is a question of whether there is a case for “scientific exemption” and whether they can be managed to prevent spread (see 3).

5. I know that Agapanthus is an environmental weed but since everyone grows it and I can buy it in almost any nursery there is no point in removing it from my garden?
A question like this is similar to saying “everyone else steals so why shouldn’t I?”
It is clearly true that one extra person growing Agapanthus is hardly likely to increase the environmental threat posed by this plant. However, if the case against these plants is sound then there is little option but to be environmentally responsible, at least dead-heading plants that may pose a threat (although spread appears to be mostly vegetative through the dumping of garden waste etc. 107). Situations like this create the opportunity to provide information to people who are sympathetic to good environmental intentions but who are unaware of weed histories or methods of management.

6. Do rare and unusual plants pose any environmental threat?
There is no reason in principle why rare plants should pose more environmental danger than any other plants. Collectors and botanic gardens enjoy the challenge of growing novelties and displaying plants that are rare in the wild or rarely cultivated. In botanic gardens especially such plants contribute to the diversity and variety of the collections. They can also be the subjects of scientific research or evaluation for their cultivation potential. There is, however, the problem that plants rare in the wild or in cultivation are difficult to assess for their weed potential because they have not been “trialled” through years of cultivation. An absence of records or performance in a few gardens is not evidence that the plant is innocuous. It is environmentally irresponsible to grow and share plants new to cultivation without a very careful weed assessment. Such plants must be passed through the Standard WRAP and their weed potential assessed according to their known biology.
The serious environmental weeds Coprosma repens, Mirror bush¸and Pinus radiata, Monterey Pine are uncommon to rare in their countries of origin (New Zealand and U.S.A. respectively), so rarity in country of origin does not necessarily imply that there is no weed potential.

7. Is it safe to grow plants with known weedy history if they are out of their natural climatic range?
Just because a plant is extremely weedy in Queesland (or wherever) surely that does not mean that it is necessarily weedy in my region?
It is often pointed out that plants that have weed potential in particular climates may be cultivated outside their natural climatic range without displaying any tendency to naturalise. For example, Lantana camara is generally cultivated safely in Victoria but is a widespread weed in the warmer climates of New South Wales and Queensland.
These cases are extremely hard to assess. A good weed risk assessment, in developing an evaluation of the weed risk involved, will take account of both the local conditions and those conditions under which the plant thrives. Transport by humans and pilfering from public gardens are all part of this risk equation (see 14).
For botanic gardens there is the question as to whether such plants should be on public display. Firstly, although very unlikely, it is possible for propagation material to be taken from plants to climates where they may proliferate (see 14); secondly, displaying such plants does not set a good example to the viewing public; thirdly, the likelihood of continuing substantial warming of Victoria’s climate means that plants currently weedy in warmer zones are likely to become progressively more weedy in southern Australia. It seems that each case must be considered individually and an attempt made to assess the seriousness of the risk posed.

8. Is it safe to grow plants with a known history of weediness provided they are away from natural areas?
It is sometimes suggested that the main danger to natural areas occurs when exotic plants are grown in areas abutting natural vegetation. The former Noxious Weeds Act in Victoria allowed noxious weeds to be grown in the Melbourne Metropolitan area – presumably at least partly because they were considered to pose little threat when not directly adjacent to native bushland.
Unfortunately, although it may be true that the threat posed is small, there is still the potential for plants to be transported (shoes, vehicles etc.), accidentally or deliberately, to areas where they will thrive. It is extremely difficult to decide how real such a threat is. Even in metropolitan Melbourne and other rural centres, there are lots of small bushland reserves and ribbons of linear reserves. These remnant areas are threatened by invasive garden plants carried by wind, water, birds and other vectors including dumped garden waste. Gardens anywhere are surrounded by remnant natural areas and agricultural areas which are all vulnerable to invasive garden plants.

9. If I were to remove all the potential weeds from my home garden it would mean removing about half the plants – are you seriously suggesting that is what I do?
Half the plants seems a high estimate. Nevertheless, if we are genuine and resolute about protecting the natural environment then this might result in a significant change to current horticultural practices, not only those relating to invasive plants. Develop a replacement program over a couple of years. Perhaps replace one or two every six months removing the most invasive and/or damaging first. See it as an opportunity to reinvigorate the garden.

10. The same species can reproduce more effectively in some conditions than others – how does this affect its weed risk assessment?
This question is rather similar to 7. Some plants appear to produce more seed as their growing conditions more closely approximate the conditions of natural populations – what might be called climatic fecundity. Ceratonia siliqua, Carob, produces few seed pods in Melbourne but in gardens closer to Mildura the seed pods become an ornamental feature, the same is true of Jacaranda mimosifolia. At other times it appears that certain variants within a species are more fertile than others and for less obvious reasons. Agapanthus seedlings are rarely sighted in the Royal Botanic Gardens Melbourne and yet there is little doubt that large populations have resulted from seed in naturalised populations on the Bellarine and Mornington Peninsulas. Is it possible that a wide variation in fecundity is part of the biology of certain plants?

11. If a plant has high heritage value but is an environmental or agricultural weed, should we preserve it? How do we resolve competing environmental, heritage and commercial values?
There are occasions when there is a conflict between environmental/economic and heritage values. An old Peppercorn tree, Schinus molle, might have great historical value but also be the source of potentially environmentally damaging seed. Should it be removed? Some important agricultural, horticultural and forestry plants are also weeds (e.g. Phalaris aquatica, a pasture grass; Pinus radiata, a timber tree; Olea europea, commercial olive). Here are competing commercial and conservation interests.

In cases like this the more objective information that is accumulated the greater likelihood of conflict resolution. Weed Risk Assessment gives an indication of degree of environmental risk (as likelihood of spread, and consequences should spread occur) and therefore suggests the level of precaution needed to minimise impact. In South Australia local government planners are recommended to reject planning approval for those olive orchards posing very high environmental risk, lower risk orchards are expected to put in place appropriate environmental precautions to avoid feral plants (Virtue).
Another example: in times of drought the turf industry is likely to suffer severely if it is tackled on water consumption (cool-season grasses) and also weediness (warm season grasses). What can be done?
In unfortunate cases like this every opportunity could be taken to alleviate the problem. Trying to use non-weedy grasses, reducing areas of turf if that is possible, trialling and introducing sterile cultivars and so on.
Many herbs and crop plants are weedy; how are we to deal with them? Both the carrot (Daucus carota) and wheat (Triticum aestivum) are quite widely naturalised in Australia. Here, although there are certainly enviromental threats, human interest would seem to demand some license – to ban the growth of wheat or carrots would be seen as particularly draconian if not “ridiculous”. It is not so clear with, say, Globe Artechoke (Cynara scolymus) and Olives (Olea europaea) because their need in horticulture seems less and their environmental threat possibly greater than that of wheat and carrots. Nevertheless, removing them from gardens would be seen as quite harsh. Perhaps in an environmentally responsible institution like a botanic garden they could be grown with signs indicating their environmental threat. There are more difficult cases like mints (Mentha), Black Mustard (Brassica nigra), the chamomiles, onions, Nasturtium, Salad Burnet and many more, all of which are quite widely naturalised in Australia. Decisions here are likely to be a compromise between the agricultural/environmental risk posed by the plant on the one hand with its food/cultural/social value on the other, taking into account such factors as how necessary the plant is in the gardens, how invasive it might be, and how great the impact if it escaped.

12. We like to cultivate exotic plants that grow well in the local area, but if they grow well then surely they also have an increased weed potential. Does that mean we should always select plants that don’t grow well?
Many garden plants do not pose a threat. Plants grown should be selected carefully and monitored for their weed potential. If they start rapidly reproducing in the garden, then chances are they may escape and invade elsewhere. If a plant shows these tendencies, it might be time to consider replacing it. Also see 2.

13. What can I do in my garden to help? Who can I talk to for more information?
In Victoria you could check that the plants do not occur on the state noxious weed list or the nursery industry “garden thugs” list, or any lists compiled by your local council. You can speak to your local council, DPI or check the web sites listed in this document. To get a feel for whether particular plants might be weedy you can check the on-line Global Weed Compendium. In the home garden, replace invasive garden plants with safer alternatives, dispose of garden waste responsibly (don’t dump it in bushland or over the back fence), use your circles of influence to tell others about the issue and what they can do to help. If you see invasive garden plants in local bushland and on roadsides etc., tell the responsible authority and ask for something to be done about it.

14. Any accessible plant in a public park or garden may be stolen. Propagation material can also be taken away to places where the plant could become a weed.
This must be taken into account as part of the weed risk assessment which, in turn,will include consideration of the potential impacts should the plant escape to become weedy.

15. Humans are part of nature and therefore the spread of alien plants across the globe as a result of human activity is part of a natural process. The concern with weeds is a form of xenophobia and an irrational and emotive attempt to keep the world in a pristine condition.
This booklet has outlined the economic, environmental and social reasons why invasive plants are detrimental to humans and other organisms. Tackling these issues is sound management for the future.
Biological invasions (like extinctions) are natural events but it is the rate and numbers, as well as the large distances and agency, that separates human-driven invasions from colonizations by other agencies.



Blood, K., Taylor, U., Nugent, T. & Timmins, S. (1998). Weed Navigator. Weeds CRC, Adelaide.
Blood, K. (2003). Weed identification, recognition of key environmental weeds, and where to find information resources. In Proceedings, First Biennial Conference, Developments in Weed Management, 20-21 August 2003, held at All Seasons International Hotel, Bendigo. (Weed Society of Victoria Inc.: Frankston, Victoria) pp. 16.


• Under Control , Pest Plant and Animal Management News. Victorian Department of Primary Industry.
• Plant Protection Quarterly. R.G. & F.J. Richardson, Meredith, Vic.
• Weedscene. Newsletter of the Weed Society of Victoria Inc.
• Weedwatch. Newsletter of the Cooperative Research Centre for Australian Weed Management. Available on-line at


Centre for Biological Information Technology (2004). Declared plants of Australia: an identification and information system.
An interactive identification tool for declared plants of Australia using Lucid software. $80.
see also


Web site of Biosecurity Australia plant section. For policies, reviews, import restrictions etc.

Site of the Council of Heads of Australian Botanic Gardens. A combined inventory of plants in the 6 major Australian
botanic gardens, as at 1992, can be accessed here.

A Horticultural Code of Practice called “Helping to prevent the spread of invasive non-native species” published by the
UK Department for Environment, Food and Rural Affairs in March 2005.
Excellent American site with invasive plant Codes of Conduct for government, botanic gardens, home gardeners, landscapers and the nursery industry


New South Wales
North America
Western Australia

Australian Federal Department of Environment and Heritage. An important account of weeds in Australia. includes: description, control methods including biological control; National Weeds Strategy; weeds of national significance; weeds on the national environmental alert list; Australian government funding; Natural Heritage Trust; links; publications.

Presents literature on weed identification, weed management and control, and environmental and agricultural weeds.
A weed management manual prepared as a training aid for private landholders, conservation groups, catchment management groups, local, state and territory governments and industry. Mostly environmental weeds. Many publications are available from the Australian Weed Management CRC site
An excellent Victorian site with links to major legislation, discussion of individual weeds, important literature etc.
conferences and workshops focusing on alien plant invasions which have been held worldwide since 1992. Topics of discussion have included: the nature of invasions, the impact of alien plants on the biodiversity of native communities, available control and management options, and specific case studies of ecological traits


Federal Noxious Weed Disseminules of the U.S. Julia Scher
Released in 2005 an identification key to the 96 taxa on the USA federal noxious weed list, and 10 taxa regulated under the Federal Seed Act. Good pics, facts sheets.

International Phytosanitary Portal – the official web site for the International Plant Protection Convention
Discussion paper re quarantine policy as it relates to the WTO environment

Australian Quarantine and Inspection Service (Biosecurity). ICON is AQIS’s import conditions database. It contains the Australian import conditions for more than 18,000 foreign plant, animal, mineral and human products and incorporates prohibited weeds.
Commonwealth alert list of 28 environmental weeds
Provides a list of the currently declared noxious weeds in Victoria as in Landcare Notes (1998). Declared Noxious Weeds. KTRI ISSN 1329-833X
The Victorian DPI site describing and listing weeds in Victoria.
Provides an on-line database of all noxious weeds in Australia. These can be listed on a state
A searchable database of the noxious weed lists for all U.S. states and six southern provinces of Canada.
The United States Natural Resources Conservation Service. Includes: Federal Noxious Weed List ; State Noxious Weed Reports; State and Federal Composite List of All U.S. Noxious Weeds; Invasive Plants of the U.S. ; Introduced Plants of the U.S.
Australia’s Virtual Herbarium for records of plants in Australian Herbaria and their natural distributions.
The IUCN-ISSG’s Global Invasive Species Database
The INVADERS Database is a comprehensive database of exotic plant names and weed distribution records for five states in the northwestern United States.
American listing of international databases of weedy plants
First edition of publication below produced in 2007. Hussey, B.M.J., Keighery, G.J., Dodd, J., Lloyd, S.G. and Cousens, R.D. (2007). Western Weeds, A guide to the weeds of Western Australia. 2nd Edition. The Weeds Society of WA Inc.

The Australian Network for Plant Conservation
A new site emanating from the Eden project in the UK with information on plant conservation in horticulture.
The New Zealand Plant Conservation Network. Includes weed list and pictures of most of the New Zealand flora.

PLANT QUARANTINE$file/Quarantine1998WD02.pdf
This is a 157 page pdf file of the Quarantine Proclamation under the 1908 Quarantine Act and amended 11 Mar 2005.
Page 86 has a list of plants that are quarantinable pests. Schedule 5 is a list of permitted seeds.

This is the site of the Golobal Invasive Species Programme including a downloadable summary document outlining a global perspective of invasive alien species.
Weeds Australia and National Weeds Strategy. A site managed by the National Weeds Strategy Executive to promote access to key weed policies, regulations, current issues, national initiatives, research, extension, training and personnel.
Portal to the Australian Quarantine and Inspection Service. The site explains the functions and operations of this government service.
Australian Weed Management CRC
Council of Australian Weed Societies. Links, conferences and publications.
Weeds of National Significance.
This is the web site for local government and includes their latest 2003 report on weed management. Search on “weeds”.
National Weeds Strategy Executive Committee site with useful Guidelines for Developing Weed Strategies
A report examining the State and Territory legal controls on nationally important weeds, namely the Alert List of Environmental Weeds, weeds recommended or national eradication, the Northern Australia Quarantine Strategy target plant list, and Weeds of National Significance.
The American invasive species program
Schedule 5, Permitted Seeds as at 25-5-2005
New Zealand’s National Pest Plant Accord. Procedure for monitoring plants available in commerce in New Zealand.

Weeds on the Australian National Environmental Alert list.
Possibly the most comprehensive and erudite account of environmental weeds in Australia.
The INVADERS Database is a comprehensive database of exotic plant names and weed distribution records for five states in the northwestern United States.
Naturalised plants in New Zealand.
This website provides information available in the book A Global Compendium of Weeds by Rod Randall. It lists about 20,000 taxa of plants giving information about the “weedy” characteristics of each based on information from nearly 300 references.
National Plant Germplasm System, maintained by the U.S. Dept. of Ag., good for taxonomic information including economic importance, distributional range, references and other databases with information.
Missouri Botanical Garden’s VAST (VAScular Tropicos) nomenclatural database good for publication references and location records. The location data can be used as input to CLIMATE for determining the potential for a species to occur in Australia.
U.S Department of Agriculture Plants Database. A list of vascular plants, mosses, lichens, etc. of the U.S. and its territories. It has useful links to other sources of information on individual species.
Flora of Australia Online. The number of families available online is currently limited.

Australia-wide weed discussion group.
People concerned that Australia is not responding adequately to invasive species formed the Invasive Species Council to lobby against invasive species of all kinds. The ISC became incorporated in July 2002, and held its first public event on 26 August 2002 in Melbourne. It has an informative newsletter, the Feral Herald.

Mountain Invasion Research Network.
Weed Society of Victoria. Other weed societies include The Weed Society of New South Wales (Inc), The Weed Society of Queensland (Inc), The Weed Management Society of South Australia, The Weed Society of Western Australia (Inc) and the Tasmanian Weed Society (Inc). The umbrella organisation is the Council of Australian Weed Societies (CAWSS).
Global Invasive Species Information Network
The Center for Invasive Plant Management
Environmental Weeds Action Group. Community based network in WA


National Herbarium of Victoria
Plant identification as part of the Weed Alert Rapid Response project.

Australian Research Centre for Urban Ecology (ARCUE)
A division of RBG ARCUE researches all aspects of urban ecology.

Department of Sustainability and Environment (DSE)
Responsible for the management of crown land that is not national park.

Department of Primary Industry (DPI)
DPI conducts weed management in Victoria. [

Municipal Association of Victoria (MAV)
This is the umbrella organisation for local government.

Print Friendly, PDF & Email