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A weed is an unwanted plant. 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. For example, 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 an herbal medicine. In some areas, dandelion leaves, which are edible, are sold in certain restaurants or grocery stores. 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 difficult to grow plants need to be nurtured and are easy to remove without returning.

Invasive species

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.

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 organic gardening).

Other methods of eliminating weeds 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.

These methods of weed elimination are best for small areas at a time, as this reduces neighbors' tendencies to complain about yard messiness.

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.

Plants that are often considered weeds

  • Cleaver
  • Crabgrass
  • Dandelion
  • Daucus
  • Diffuse knapweed
  • Dock
  • Eucalyptus
  • Giant salvinia
  • Gorse
  • Goutweed
  • Hairy Vetch
  • Henbit
  • Japanese knotweed
  • Johnson Grass
  • Kudzu
  • Milkweed
  • Mullien
  • Plantain
  • Quackgrass
  • Ragweed
  • Red deadnettle
  • Rubber vine
  • Shepherd's purse
  • Sicklepod
  • Sunflower
  • Thistle
  • Tufted Vetch
  • Tumbleweed
  • Water hyacinth
  • White ginger
  • Wild onion
  • and many more

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