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This article is about unwanted plants. Other meanings: the town of Weed, California; Weed music distribution service and Marijuana.

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.

Noxious weeds

A Noxious weed is a plant that is growing in such a manner that it is causing problems by interfering with other desirable plants or contaminating the harvest of useful plants. Noxious weeds are especially problematic weeds because they spread rapidly, are hard to kill, are toxic to cattle and/or humans, or cause other serious problems. Generally (but not always) noxious weeds are invasive species. In the US and Canada, government agencies, usually within the federal, state or provincial departments of agriculture maintain lists of noxious weeds, and the term has legal significance. It is usually unlawful to transport these weeds or their seeds.

The teasel, for example, was a plant widely grown because it was used to manufacture woolens. It is no longer needed for this purpose but grows everywhere ("Growing everywhere" does not make it a noxious weed). Thus it has become a noxious weed.

Invasive species

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 species (or exotic invasives). This term is applied when a plant is an introduced species that invades and disturbs natural ecosytems, displacing species native to the target ecoregion and causing harm.

Control

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 known as herbicides have been widely used; they have caused some environmental damage, and efforts are being made to reduce the use of such substances (see for example genetic engineering, organic gardening).

Weeds that aren't weeds

The notion of 'wanted' is often in the eye of the beholder. Some people love dandelions for their yellow buttons, like gold coins on the ground. Some people love to blow the puffball seed heads that form on the dandelion. Dandelion root is used in herbal medicine. In fact you can even find dandelion greens for sale in certain restaurants or grocery stores in the United States. The dandelion is a prime example of how the notion of 'weed' means different things to different people.

Plants that are often considered weeds include:

• Dandelion
• Giant salvinia
• Goutweed
• Japanese knotweed
• Kudzu
• Water hyacinth
• Johnson Grass
• Dock
• Plantain
• Purple dead nettle
• Henbit
• Shepherd's purse
• Cleavers
• Thistle
• Lambsquarters
• Violets
• Quackgrass
• Crabgrass
• Sunflower
• Hairy vetch
• Milkweed
• Queen Annes Lace
• Wild Onion
• and many more

See also: weedy species, weed control, herbicide, pesticide