What is a weed?
To a gardener, a farmer or a botanist, a weed may mean different things. Broadly, a weed is a plant growing outside its natural environment and having some sort of adverse impact. The majority of weeds are from overseas but 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 in moving to a new environment.

Types of weeds
DECLARED NOXIOUS WEEDS have been proclaimed under the Noxious Weeds Act 1993, generally because they are serious economic pests, toxic to stock or a danger to human health. They are usually weeds of agriculture. There is a legal requirement on landowners to control these weeds. The species listed as noxious vary between different parts of NSW. A list of the species which are noxious in your area can be obtained from the website www.agric.nsw.gov.au/noxweed/

There are several categories of noxious weeds:

• W1 weeds must be continuously suppressed and destroyed, and Council must be notified within three days of detection of the plant on a property (e.g. many water weeds),
• W2 weeds must be continuously suppressed and destroyed,
• W3 weeds are widespread species whose total destruction is impractical. Landholders are required to prevent their spread, and reduce numbers to the satisfaction of Council,
• W4 weeds vary in the action required. Most commonly they must not be propagated or sold, but there is no legal requirement to remove existing plants. Check with your local Council or NSW Agriculture for the specific requirement relating to these weed species.
  

ENVIRONMENTAL WEEDS are plants which invade native vegetation, and which may replace local native plants causing loss of habitat for native animals. Some environmental weeds have been declared noxious (such as bitou bush), but most have not and there is no legal requirement to control them. 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, particularly to the farmers and local authorities who bear that cost. However, the impact of weeds on natural vegetation has only recently been recognised. Weed invasion is one of the greatest threats to some types of native vegetation, particularly when that vegetation is close to towns and farming land.

Weeds can come to dominate the vegetation in these areas, preventing regeneration of native plants, 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. They 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. They 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?
Two things are needed for weeds to invade vegetation: 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 are:

• 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.

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

• dumped in garden refuse,
• 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.