Weed
A
weed is often an unwanted, usually wild or feral, plant. The notion of "weed" is of course entirely in the eye of the beholder. A weed in one situation might be a wildflower in another. For example,
dandelions are widely viewed as weeds, but some consider them attractive flowers. They are also edible, and can break up
hardpan for agriculture, making them
beneficial weeds. Indeed, there are
many beneficial "weeds". Children commonly enjoy blowing their puffball seed heads, and their leaves and roots may be utilized for food or
herbal medicine. Yet the caretaker of a
lawn will generally regard the dandelion as a troublesome weed. It is typically necessary for a plant to grow easily to be considered a weed because plants which need to be nurtured are easy to remove without returning.
Many plants have become weeds by being transferred by human action to locations where they have no natural grazing predators; and they will compete with other plants for space. 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.
The term
noxious weed is reserved for those that can be harmful to human or animal health. Under
United Kingdom law (Weeds Act 1959), a duty is placed on controllers of land to remove the following scheduled weeds from their land to prevent seeds contaminating their neighbours' land:
* broad leaf dock
* common ragwort
* creeping or field thistle
* curled dock
* spear thistle
In addition, in the UK, the Wildlife and Countryside Act 1981 specifies control of certain plants such as giant hogweed or
Japanese knotweed.
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. Today, chemical weed killers known as
herbicides are widely used. Modern herbicides such as
glyphosate are designed to leave no harmful residue in the soil, although the
surfactants can be determental to watercourses.
In domestic gardens, methods of weed control include covering an area of ground with several layers of wet newspaper or a black plastic sheet 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 germinating 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 planted later. In the case of using the black plastic sheet, the
greenhouse effect is used to kill the plants beneath the sheet. A 5-10 cm layer of wood chip
mulch on the ground will also prevent most weeds from sprouting. Also, gravel can be spread over the ground as an inorganic mulch.
In agriculture, irrigation is sometimes used as a weed control measure such as in the case of
paddy fields.
*
Herbicide*
List of beneficial weeds*
List of plants and fungi which damage buildings*
Pesticide*
Weedy species*
Weed Identification Guide from
Virginia Tech (Southeastern United States)
