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

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.

The notion of "wanted" is of course entirely in the eye of the beholder. A weed in one situation might be a wildflower in another. Some people love dandelions for their yellow buttons, like gold coins on the ground. Children enjoy blowing the puffball seed heads that form on the dandelion; and adults might utilize the dandelion root as a herbal medicine. In fact you can even find dandelion greens for sale in certain restaurants or grocery stores in the United States. Yet the caretaker of a lawn will generally regard the dandelion as a troublesome weed.

Noxious weeds

The term Noxious weed is now used for especially difficult to manage weeds. These typically are invasive species that may be difficult to control, or may be health hazard to humans or stock animals or wildlife, or may be otherwise detrimental to an environment. Typically these plant species have been defined as noxious by legislation or government regulation. Typically, regulations make it an offence to cultivate, transplant, or disseminate the seeds of declared noxious weeds. In some cases it may be an offence even to permit them to grow (as on one's property) by inaction.

Although the Giant hogweed (Heraculeum mantegazzianum) is a noxious weed because it is difficult to manage in places where it has been introduced, it is also noxious because it exudes a watery sap that sensitizes the skin to ultraviolet radiation resulting in burns. Thus, it is both invasive and a potential health hazard.

Invasive species

(See main article at invasive species)

Many 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, eliminating more than 90% of the prickly pear infestation within 10 years. This case is frequently cited as an example of successful biological pest control.

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.

Weed control

(See main article at weed 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. However, to the extent that such chemicals leave a harmful residue in the soil, they can produce unanticipated adverse environmental effects, 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:

• 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
• Cow Vetch
• Hairy Vetch
• Milkweed
• Queen Annes Lace
• Wild Onion
• stickers
• Tumbleweed
• and many more

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