Weed, a plant that grows where it is not wanted. In general, plants are called weeds when they deprive more desirable plants of food, water, sunlight, and space. Some weeds are injurious to humans and domestic animals. Among these are poison ivy, giant ragweed, and locoweed. Some weeds impede the flow of water through irrigation ditches; some cause damage to streets and sidewalks.

Some plants generally regarded as weeds are actually useful or beneficial. Many provide food or shelter for wild animals. Some, including purslane, dandelions, and pokeweed, are edible. Jimsonweed, wormseed, boneset, and some other weeds are medicinal plants.

Weeds are vigorous plants and are particularly intrusive and difficult to destroy. A number have persistent underground rootstocks, corms, bulbs, or tubers. Others spread by horizontal underground stems called rhizomes, which send up new shoots each spring. Weed seeds are spread in various ways. Some are blown by the wind; others become attached to clothing or an animal's fur and are carried from one place to another. Seeds of many types of weeds in North America were brought there accidentally from Europe or Asiafor example, in batches of crop seeds contaminated with weed seeds.

The cockleburThe cocklebur is a weed with spiny, prickly burs.
Weed Control

Methods of controlling weeds include cultivating the soil, rotating crops, and planting crops in close rows so that weeds are shaded and die. Some farmers burn sections of their fields to destroy weeds. In vegetable gardens, mulches of straw, sawdust, or paper can be used to prevent weed growth. Such insects as the Klamath weed beetle and such fungi as the rust fungus are sometimes used to destroy weeds.

Chemicals used to kill weeds are called herbicides. Most are highly specific, killing only certain kinds of plants while not harming other kinds. For example, some herbicides kill only broad-leafed weeds while others kill only grassy weeds. The most widely used agricultural herbicides are atrazine for corn weeds, pendimethalin for corn and soybean weeds, trifluralin for soybean and cotton weeds, fluometuron for cotton weeds, and 2,4-D (2,4-dichlorophenoxyacetic acid) for wheat weeds. Herbicides kill weeds by suppressing respiration, destroying chlorophyll, inhibiting protein synthesis, interfering with cell division, destroying the root system, or causing dehydration.

Although all herbicides should be used with caution, most are low in toxicity and decompose in the soil after one growing season. Some, however, contain the toxin dioxin, which is potentially harmful to humans and other animals. Their use was banned in the United States in 1979. These herbicides include silvex and 2,4,5-T (2,4,5-trichlorophenoxyacetic acid). Other herbicides, such as alachlor (which controls corn, soybean, and peanut weeds), are suspected cancer-causing agents, and their use in the United States has been restricted by the Environmental Protection Agency.