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thumb Dandelions, shown here in proliferation, are commonly thought of as weeds.

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. Other weeds may also have medicinal properties; milk thistle is known for its ability to regenerate liver cells, and an extract from kudzu has been used to treat alcoholism.

1 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 HogweedHeracleum mantegazzianum Giant Hogweed Heracleum mantegazzianum is a member of the Parsley or Carrot family, ( Apiaceae) and is native of Central Asia. Many foreign plants were introduced to Britain in the 19th century, mainly for ornamental reasons. (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.

2 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 AustraliaAustralia is the sixth-largest country in the world (geographically), the only one to occupy an entire continent, and the largest in the region of Australasia. Australia includes the island of Tasmania, which is an Australian State. Its neighbouring count 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 controlBiological control of pests and diseases Overview A key belief of the organic gardener is that diversity furthers health. The more variety a landscape has, the more sustainable it is. The organic gardener therefore works to create a system where the insec.

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 ecosytemsIn ecology, an ecosystem is a community of organisms (plant, animal and other living organisms also referred as biocenose) together with their environment (or biotope), functioning as a unit. The term ecosystem first appeared in a 1935 publication by the, displacing species native to the target ecoregionEcoregions are defined by the World Wildlife Fund as "relatively large units of land or water containing a distinct assemblage of natural communities and species, with boundaries that approximate the original extent of natural communities prior to major l and causing harm.