Any plant growing where it is not wanted. On land under cultivation, weeds compete with crops for water, light, and nutrients. On rangelands and in pastures, weeds are those plants that grazing animals dislike or that are poisonous. Many weeds are hosts of plant disease organisms or of insect pests. Some originally unwanted plants later were found to have virtues and came under cultivation, while some cultivated plants, when transplanted to new climates, escaped cultivation and became weeds in the new habitat.