- This article is about unwanted plants. For other meanings see Weed (disambiguation)
Weed is the generic word for a plant growing in a spot where it is not wanted. Weeds become of economic significance 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 expanded greatly.
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; herbalists know milk thistle for its ability to regenerate liver cells, and an extract from kudzu has been used to treat alcoholism.
The term Noxious weed now applies to especially difficult to manage weeds. These typically are invasive species that may be difficult to control, or may be a health hazard to humans or to stock animals or to wildlife, or may be otherwise detrimental to an environment. Typically, legislation or government regulation defines certain plant species as noxious. 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.
- 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.
Auckland, New Zealand is often described as the weediest city in the world.
- 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).
More natural methods of eliminating weeds include that of
- covering an area of ground with several layers of wet newspaper for several weeks, or
- covering the area of ground with a clear plastic tarp for several weeks.
In the case of using wet newspaper, the multiple layers prevent light from reaching all plants beneath, which kills them. Saturating the newspaper with water daily speeds the decomposition of the dead plants. Any weed seeds that start to sprout because of the water will also be deprived of sunlight, be killed, and decompose. After several weeks, all weed seeds present in the ground should be dead. Then the newspaper can be removed and the ground can be planted. The decomposed plants will help fertilize the plants or seeds you introduce.
In the case of using the clear plastic tarp, the green-house effect is used to fry the plants beneath the tarp. (More particulars and directions are needed, as I forget the rest of the way to do this.)
These methods of weed elimination are best for small areas at a time, as this reduces tendency neighbors' tendencies to complain about yard messiness.
A 2 inch to 4 inch layer of wood mulch can be applied to prevent weeds from sprouting. Also, gravel can be spread over the ground as an inorganic mulch.
Plants that are often considered weeds include: