Weeds are problem plants. They invade natural ecosystems, reduce agricultural production, are toxic to stock or can threaten other species (plants and animals).
Weeds are often introduced species, but they can also be native species growing outside their natural range.
Together with pest animals, weeds pose one of the most significant threats to biodiversity, after land clearing and habitat destruction.
Weeds compete with native plants for nutrients, sunlight and water. Some weeds can smother and kill native plants, prevent regeneration, or change the structure and function of an environment. This in turn can affect native animals that depend on native flora for food and shelter. Unlike native plants, introduced weeds have fewer natural predators or diseases in their new environment, giving them an advantage over native species.
Most of the state's environmental weeds (i.e. weeds that invade largely natural environments) were initially introduced as garden plants. In some cases, their seeds were dispersed across the landscape by birds, animals or the wind. In other cases they have escaped out of gardens or spread into bushland areas from dumped garden waste.