Weed is the generic word for a plant growing in a spot where it is not wanted. The most prominent use of the word is in connection with farming, where weeds may damage crops when growing in fields and poison domesticated animals when growing on pasture land. Many weeds are short-lived annual plants, that normally take advantage of temporarily bare soil to produce another generation of seeds before the soil is covered over again by slower growth; with the advent of agriculture, with extensive areas of ploughed soil exposed every year, the opportunities for such plants have been greatly expanded.
Other plants have become weeds by being transferred by human action to locations where they have no natural grazing predators; the classic case is the prickly pear (Opuntia stricta), which overran vast areas of Australia until a moth, Cactoblastis cactorum was introduced. This is frequently quoted as the classic example of successful biological pest control, eliminating >90% of the prickly pear infestation within 10 years.
In cases like the prickly pear in Australia, the weeds are termed invasive exotics (or exotic invasives). This term is applied when a plant is an introduced foreign plant that then invades and disturbs natural ecosytems, displacing species native to the target ecoregion.
In order to reduce weed growth many weed control strategies have been developed. The most basic is ploughing, which cuts the roots of annual weeds. In modern times, chemical weed killers have caused environmental damage, and efforts are being made to reduce the use of such substances (see for example genetic engineering, organic gardening).
Plants that are often considered weeds include: