Weeds impact on primary industry by increasing the cost of horticultural production, reducing productivity of pastoral land, impeding stock mustering and choking and polluting water supplies. Weeds are spread by climatic conditions such as high winds and flooding, by domestic, feral and native animals, via contaminated stock, grain, machinery, and vehicles, and directly by people on their clothing and shoes. Many plants that are now recognised as weeds were either accidentally or intentionally introduced. Accidental introductions include weeds that may have been brought in with grain or transported on machinery. Intentional introductions include plants that were brought in for their attractive flowers or perceived economic value.
Some weeds are poisonous, eg.
Some introduced species, such as mission grass, produce a large flammable biomass that cures later in the season than native grasses. Thus, fire regimes can be altered to more intense late dry season fires. Some weeds already cover large areas but have not yet reached their potential ecological limits of distribution. It is difficult to estimate the cost of the environmental and social impacts of weeds. The lack of human and financial resources for weed control means that weed species can potentially become uncontrolled on a large scale.