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Biosecurity matter (weeds)

What is a weed?

To a gardener, a farmer or a botanist, a weed may mean different things. Broadly, a weed is a plant that is growing outside its natural environment and has some sort of adverse impact.

The majority of weeds are from overseas, however, some native Australian plants can also become weeds within Australia. Many Australian plants are also serious weeds in other countries where they have been imported as garden plants, or for economic uses such as the wattles planted in South Africa for tanbark.

Whatever their origin, they spread 'like weeds' because they arrive in a growth environment which is favourable, often because they have left their natural pests and diseases behind them moving to a new environment.

Types of weeds

Declared weeds

Declared weeds have been proclaimed under the Biosecurity Act 2015 and Council's weed control programs, generally because they are serious economic pests in agricultural settings, present a high threat to biodiversity, are toxic to stock or a danger to human health. Biosecurity is everyone's responsibility and there is a legal requirement to control and/or to prevent the spread of these weeds.

The species listed as biosecurity matter can vary between different parts of New South Wales.

Environmental weeds

Environmental weeds are plants that invade native vegetation and may replace local native plants, causing loss of habitat for native animals.

Where environmental weeds are presenting a risk to assets, a risk assessment may be carried out and the infestation treated as biosecurity matter with an attached control requirement.

Many plant species are both agricultural and environmental weeds, depending on where they are growing.

Why do weeds matter?

The huge financial cost of weeds and weed control to agriculture is well known, especially to the farmers and local authorities who bear that cost.

Weed invasion is also one of the greatest threats to some types of native vegetation, particularly when that vegetation is close to towns and farming land.

Weeds can dominate the vegetation in these areas, preventing native plants from regenerating, and even killing them in some cases.

Weeds may reduce the habitat available for native animals, reduce biodiversity and alter the visual character of the landscape. Weeds can increase the fuel load, making areas more fire-prone, or they can make areas impossible to burn so that species which are dependent on the occasional fire can no longer survive there. Weeds can even change the soil by secreting chemicals from their leaves or roots so that other plants cannot grow in it.

Why do weeds invade?

To invade vegetation, weeds need a source of propagules (seed, bulbs, pieces of stem or root) and suitable growing conditions (light levels, soil moisture, nutrients).

Disturbance is not essential for invasion, but it increases the likelihood of it by creating bare ground, changing soil conditions and stimulating seed germination.

Possible sources of disturbance include:

  • flooding, which can remove vegetation and deposit sediment
  • earth moving or cultivation
  • removal of the existing vegetation cover by herbicides or fire
  • trampling by livestock
  • nutrient enrichment with fertilisers or manure
  • enhanced soil moisture due to runoff from nearby paved surfaces or compacted soil
  • pulling up or digging out other weeds
  • spraying with knockdown herbicides that bare the ground.

Seed and other propagules can be moved around in a number of ways. They can be:

  • dumped in garden refuse
  • distributed through physical and online markets
  • blown on the wind
  • spread by birds or other animals, if packaged in edible fruits
  • carried on animals and waterbirds, attached to fur, feathers or in mud on their feet
  • carried in water
  • carried in the gut of livestock which have fed on weedy pasture or hay
  • spread on cars, mowers, agricultural or earth-moving machinery
  • imported in soil, mulching materials, potted plants, hay and other stock feeds
  • carried on clothing or in mud on boots.

We can help you

If you need more information about biosecurity matter (weeds), please contact Council's Invasive Species Supervisor, Paul Martin, on: