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.