A dandelion is a common weed in North America.

Weeds may be unwanted because they are unsightly, or they limit the growth of other plants by blocking light or using up nutrients from the soil. They also can harbor and spread plant pathogens that can infect and degrade the quality of crop or horticultural plants. Weeds may be a nuisance because they have thorns or prickles, cause skin irritation when contacted, or parts of the plants might come off and attach to fur or clothes.

Definition

The term weed in its general sense is a subjective one, without any classification value, since a "weed" is not a weed when growing where it belongs or is wanted. Indeed, a number of "weeds" have been used in gardens or other cultivated-plant settings. An example is the corncockle, Agrostemma, which was a common field weed exported from Europe along with wheat, but now sometimes grown as a garden plant.^[2]

Distribution

Yellow starthistle, a thistle native to southern Europe and the Middle East that is an invasive weed in parts of North America.

The weedy nature of these species often gives them an advantage over more desirable crop species because they often grow quickly and reproduce quickly, have seeds that persist in the soil seed bank for many years, or have short lifespans with multiple generations in the same growing season. Perennial weeds often have underground stems that spread out under the soil surface or, like ground ivy (Glechoma hederacea), have creeping stems that root and spread out over the ground.^[3] A number of weedy species have developed allelopathy, chemical means to prevent the germination or growth of neighboring plants.

Relation to humans

As long as humans have cultivated plants, weeds have been a problem. Weeds have even been mentioned in religious and literature texts like the following quotes from Genesis and a Shakespearean sonnet:

"Cursed is the ground because of you; through painful toil you will eat of it all the days of your life. It will produce thorns and thistles for you, and you will eat the plants of the field. By the sweat of your brow you will eat your food until you return to the ground,"^[4]

"To thy fair flower add the rank smell of weeds: But why thy odour matcheth not thy show, The soil is this, that thou dost common grow."^[5]

Weed seeds are often collected and transported with crops after the harvesting of grains, and so many weed species have moved out of their natural geographic locations and have spread around the world with humans. (See Invasive species.) Not all weeds have the same ability to damage crops and horticultural plants. Some have been classified as noxious weeds because if left unchecked, they often dominate the environment where crop plants are to be grown.

700 cattle that were killed overnight by a poisonous weed.^[6]

A number of weeds, such as the dandelion Taraxacum, are edible, and their leaves and roots may be used for food or herbal medicine. Burdock is common weed over much of the world, and is sometimes used to make soup and other medicine in East Asia. These so-called "beneficial weeds" may have other beneficial effects, such as drawing away the attacks of crop-destroying insects, but often are breeding grounds for insects and pathogens that attack other plants. Dandelions are one of several species which break up hardpan in overly cultivated fields, helping crops grow deeper root systems. Some modern species of domesticated flower actually originated as weeds in cultivated fields and have been bred by people into garden plants for their flowers or foliage.

Examples

• Bindweed
• Broadleaf plantain - perennial, spreads by seeds that persist in the soil for many years
• Burdock - biennial
• Creeping Charlie - perennial, fast-spreading plants with long creeping stems
• Dandelion - perennial, wind-spread, fast-growing, and drought-tolerant
• Goldenrod - perennial
• Japanese Knotweed
• Kudzu - perennial
• Leafy spurge - perennial, with underground stems
• Milk thistle - annual or biennial
• Poison ivy - perennial
• Ragweed - annual
• Sorrel - annual
• Sumac - woody perennial
• Wild carrot - biennial
• Wood sorrel - perennial

The five plants designated "injurious weeds" under UK law are:^[7]

• common ragwort
• spear thistle
• creeping or field thistle
• curled dock
• broad leaved dock

See also

• Herbicide
• Introduced species
• Invasive species
• List of beneficial weeds
• Pesticide
• R-selection
• Vavilovian mimicry
• Weed control
• Weedy species

References

External links

• Weed Identification Guide from Virginia Tech (Southeastern United States)
• Common weeds of the northern United States and Canada from Canadian Weed Science Society

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