Weeds are estimated to cost our agricultural industry $4 billion annually and can also cause poisoning and other health issues for humans, pets and farm animals. The real cost of weeds to the environment is difficult to calculate, however it is expected that the cost would be similar to, if not greater than, that estimated for agricultural industries.

Weed problems are complex, and efforts to reduce their impacts must be coordinated across all sections of society. Land managers and land holders can spend considerable time and money each year in combating weed problems and protecting ecosystems and primary production on private and public land.

When managing weeds it is recommended to follow the invasive species adaptive management framework below:

What is a weed?

A weed is any plant that requires some form of action to reduce its effect on the economy, the environment, human health and amenity. Weeds are also known as invasive plants. Many plants introduced into Australia in the last 200 years are now weeds.

Weeds typically produce large numbers of seeds, assisting their spread. They are often excellent at surviving and reproducing in disturbed environments and are commonly the first species to colonise and dominate in these conditions.

A weed can be an exotic species or a native species that colonises and persists in an ecosystem in which it did not previously exist. Weeds can inhabit all environments; from our towns and cities through to our oceans, deserts and alpine areas.

Some weeds are of particular concern and, as a result, have been listed for priority management or in legislation.

Throughout Australia, weeds are spreading faster than they can be controlled and management of them is consuming an enormous amount of resources. Climate change poses an additional challenge to our ability to manage weeds.

A range of management frameworks has been developed to help coordinate the management of weeds at different levels of government throughout Australia.

Where do weeds come from?

Of the more than 28,000 plant species introduced into Australia from all over the world, approximately 2,700 have formed self-supporting populations in the natural environment.

Weeds are either native species that are colonising ecosystems in which they did not previously exist, or they are exotic plants introduced to Australia that have formed naturalised populations. Some introductions have been accidental, but most exotic plants have been introduced as garden plants, pasture grasses or other horticultural plants.

Of the ten new weeds recorded in Australia each year, two thirds are escaped garden plants. These weeds come from plant species that are already in Australia.

Sometimes plants that are now weeds have only become so years after they were first introduced. Sleeper weeds are plants from overseas that have established small populations but have not yet spread widely. These plants may be currently restricted in their range for a variety of reasons including the absence of suitable environmental conditions. If conditions change in the future these plants may spread and have the potential to cause extensive damage to Australia’s natural environment and agricultural systems.

Can native plants be considered weeds?

Not all of Australia’s weeds have come from other countries. Australian native plants can also become weeds when given the opportunity. This usually occurs when species move from within their natural habitat into new areas where they have a competitive advantage over indigenous plants, allowing them to establish naturalised populations. Native plants can also become weeds when characteristics within their natural habitat change, enabling them to better compete with other species and increase their population size and/or density.

All Australian states and territories have experienced native plant invasions. For example, in Victoria around 200 species have naturalised outside their native range. A common example of a native plant establishing itself outside its natural range is the Cootamundra Wattle Acacia baileyana.

Why are weeds a problem?

Weeds are among the most serious threats to Australia’s natural environment and primary production industries. They displace native species, contribute significantly to land degradation, and reduce farm and forest productivity.

Invasive species, including weeds, animal pests and diseases, represent the biggest threat to our biodiversity after habitat loss. Weed invasions change the natural diversity and balance of ecological communities. These changes threaten the survival of many plants and animals as the weeds compete with native plants for space, nutrients and sunlight.

Throughout Australia, weeds are spreading faster than they can be controlled. All Australian states and territories have experienced native plant invasions. Nationally the impact of invasive plants continues to increase with exotic species accounting for about 15% of all flora. This figure is increasing by about ten species per year.

Weeds typically produce large numbers of seeds, assisting their spread, and rapidly invade disturbed sites. Seeds spread into natural and disturbed environments, via wind, waterways, people, vehicles, machinery, birds and other animals.

Human activities and introduced animals, such as rabbits, cattle, horses, goats and pigs, can create good conditions for weed growth and contribute to weed spread. Weeds can contribute to soil disturbance, loss of native plant cover, and changed burning patterns. They also thrive where fertilizers and other wastes are washed into bushland, leaving extra nutrients in the soil.

Land management practices

Land management practices that clear native vegetation or degrade its condition favour weeds by:

  • providing opportunities for them to colonise new areas
  • reducing the ability of native vegetation to compete with and suppress invading species.

Land clearing, habitat fragmentation and over-grazing are examples of land management practices that adversely affect biodiversity and at the same time can benefit weeds. Prevention and early detection are important strategies to reduce weed spread.

Extreme events such as droughts, floods and cyclones

Extreme events including droughts, floods and cyclones can sometimes create ideal conditions for weeds to extend their range and invade new areas or out-compete native species in their existing range.

Dry soil conditions caused by drought prolong the longevity of weed seed banks; and importing fodder and grain into drought areas can bring new weed problems to the region. Drought can reduce the competitiveness of native vegetation, providing new opportunities for weed invasion.

Floods can spread weeds along watercourses into areas that were previously free of weeds. By washing away vegetation and exposing areas of disturbed soil, floods can also provide opportunities for new weed invasions by reducing competition from existing plants.

Cyclones can create new opportunities for weed invasion through associated flooding, soil movement and damage to native vegetation communities. During cleanup activities, it is important to limit further spread by minimising the movement of soil or plant material from one area to another.

The NSW Department of Primary Industries website provides detail on weed management following drought which you can view here – weeds and drought.

Fire

The interaction between fire management and weed management is a complex one. Some weeds are sensitive to fire, where fire can either be lethal or can suppress growth. Other weeds can actually benefit from fire where fire reduces competition and produces an environment in which the weed can spread rapidly. In these cases the new flush of weed growth following a fire can add to the available fuel load for future fires, potentially creating a cycle of high-fuel, intense burns followed by a period of immense weed growth.

Because of the wide range of interactions weeds and fire can have, the coordination of fire and weed management is complex. In particular, the use of fire as a weed control method needs careful research and planning.

Climate change

The characteristic of weeds to be able to respond rapidly to disturbances such as climate change, may give them a competitive advantage over less aggressive species. The impacts of climate change on single species and ecosystems are likely to be complex. Climate change, as well as the interactions between climate change and other processes (such as changes to land use and to fire regimes), may also turn some currently benign species (both native and non-native) into invasive species and may lead to sleeper weeds becoming more actively weedy.

Climate change is expected to increase the risk of invasion by weeds from neighbouring territories. Climate change may also favour weeds that have already established in Australia but are currently restricted in range, enabling them to increase their range. As climatic zones shift, weeds that are capable of rapid dispersal and establishment have the potential to invade new areas and increase their range.

Weeds that are well-suited to adapt to the impacts of climate change may not only fill gaps left by more vulnerable native plants, they may have an even greater effect by altering the composition of ecosystems and their integrity. In fact, climate change may favour certain native plants to such an extent that they then become weeds.

Increasing levels of carbon dioxide may also have an impact on plant growth rates, which may cause changes in weed spread.

Research into the potential impact of climate change on invasive species indicates that some weed species and pest animals may benefit from climate change. The Australian Institute of Alpine Studies found that climate change in the Australian Alps is expected to benefit some weed and pest animal species which may be currently excluded from higher altitudes due to the severe environmental conditions found at these altitudes.

Weed impacts

Weeds are one of the major threats to the natural environment. They are destroying native habitats, threatening native plants and animals and choking our natural systems including rivers and forests.

Directly or indirectly, all Australians are affected by weeds. Weeds reduce farm and forest productivity, invade crops, smother pastures and some can harm livestock. Land and water managers incur material and labour costs to control weeds – these costs are passed on to the Australian public through higher prices for produce.

The economic cost of weeds

Weeds reduce the quantity and quality of Australia’s agricultural, horticultural and forestry products, affecting both industry and consumers. It is estimated that weeds cost Australian farmers around $1.5 billion a year in weed control activities and a further $2.5 billion a year in lost agricultural production. The real cost of weeds to the environment is difficult to calculate, however it is expected that the cost would be similar to, if not greater than, that estimated for agricultural industries.

The most recent economic impacts of weeds was undertaken by the Centre for Invasive Species Solutions in 2018 and can be found here.

The impact of weeds on the environment

Weeds are one of the major threats to Australia’s natural environment. Major weed invasions change the natural diversity and balance of ecological communities. These changes threaten the survival of many plants and animals because the weeds compete with native plants for space, nutrients and sunlight.

Almost all of Australia’s native vegetation communities have been invaded, or are vulnerable to invasion by exotic species that could result in changes to the structure, species composition, fire frequency and abundance of native communities.

Nationally, invasive plants continue to invade the land with exotic species accounting for about 15% of flora. About one-quarter of them are either serious environmental weeds or have the potential to be serious weeds.

Weeds may out-compete native plants because:

  • they may not be affected by the pests or diseases that would normally control them in their natural habitats
  • the disturbed environment provides different conditions that better suit the invading weed.

As a result the weed may:

  • grow faster than native plants and successfully compete for available nutrients, water, space and sunlight
  • reduce natural diversity by smothering native plants or preventing them from growing back after clearing, fire or other disturbance
  • replace the native plants that animals use for shelter, food and nesting.

Weeds are often excellent at surviving and reproducing in disturbed environments and are often the first species to colonise and dominate in these conditions.

Weeds in the ocean spread over wide areas in a very short time. Introduced seaweeds came to Australia in the early 1980s in the ballast water of ships. They are now invading marine environments along the coast of south-eastern Australia.

The impact of weeds on agriculture

Weeds reduce farm and forest productivity, they invade crops, smother pastures and in some cases can harm livestock. They aggressively compete for water, nutrients and sunlight, resulting in reduced crop yield and poor crop quality. For example, prickle bushes such as Gorse, Blackberries, Prickly Acacia, Parkinsonia and Mesquite can invade vast areas of grazing land preventing productive use of that land.

Weeds contaminate produce, for example:

  • Burrs in wool contaminate fleeces
  • Grain milled with Saffron Thistle or Amsinckia results in discoloured flour
  • Animals that eat specific weeds, such as wild garlic, produce tainted milk and meat
  • Spines on fruit of Caltrop and Spiny Emex can damage the feet of stock animals
  • Paterson’s Curse irritates the udders of dairy cows and can kill horses
  • Hemlock can be lethal to both stock and people.

Weeds can also affect the operation of farm machinery.

Farmers spend a large amount of time and money managing weeds. Despite control efforts, a recent Australian Bureau of Statistics survey of issues facing the agriculture industry found that weeds were the most commonly reported natural resource management issue affecting landowners. Weed-related issues affected 73% of Australian agricultural establishments during 2004-05. This compares to 46% of farmers reporting soil and land issues and 38% reporting water issues.

The impact of weeds on human health

Weeds can also cause human health problems. Many common weeds such as Parthenium Weed, Ragweed, Rye Grass and Privet cause asthma and other respiratory problems, especially in children. Some weeds can also cause skin irritation and some are poisonous.

Some water weeds such as Water Hyacinth (Eichhornia crassipes) and Cabomba (Cabomba caroliniana) can affect the quality of our drinking water if infestations are not managed within water supply dams.

There are no common characteristics of a poison or harmful weed that would help distinguish them. But as a general rule, plants with a bitter taste, unusual smell, milky sap or red berries may be poisonous with some plants having poisonous roots and bulbs.