weed, common term for any wild plant, particularly an undesired plant, growing in cultivated ground, where it competes with crop plants for soil nutrients and water. In their natural habitat, wildflowers and herbs not only provide beauty but function in many useful ways, e.g., as a source of food for insects and animals and to enrich the earth, loosen hard-packed soils, and help prevent erosion. However, when they invade cultivated areas they often interfere with the desired crop by appropriating space, sunlight, moisture, and soil nutrients. Weeds may also harbor and spread insect and fungus pests. Dried weeds along roadsides are often the starting point for brush and forest fires. Their habits of growth and of propagation must be considered in attempting to eradicate them. Control methods include continual soil cultivation, blanketing the soil with some material (e.g., mulch) to thwart weed growth, and the use of various herbicides (see spraying). Plants which are cultivated in one region may become weeds when introduced in another, e.g., the oxeye daisy, imported to the United States from Europe; the Russian thistle, called tumbleweed in America; and burdock, which in Japan is grown as a vegetable. Crabgrass and ragweed are weeds well known to gardeners and to hay-fever sufferers.
See T. J. Muzik, Weed Biology and Control (1970); R. E. Wilkinson and H. E. Jaques, How to Know the Weeds (2d ed. 1973).