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A weed is a plant considered undesirable. Weeds are commonly unwanted plants in human-controlled settings, such as farm fields, gardens, lawns, and parks. Weeds have no botanical classification value, since a plant that is a weed in one context is not a weed when growing where it is wanted. A number of plants that many consider weeds are often intentionally grown in gardens and other cultivated settings. It is applied to any plant that grows or reproduces aggressively, or is outside its native habitat.[1] The term is occasionally used to broadly describe species outside the plant kingdom that can live in diverse environments and reproduce quickly, and has even been applied to humans.[2]

weed: "A herbaceous plant not valued for use or beauty, growing wild and rank, and regarded as cumbering the ground or hindering the growth of superior vegetation... Applied to a shrub or tree, especially to a large tree, on account of its abundance in a district... An unprofitable, troublesome, or noxious growth."

-- The New shorter Oxford English dictionary on historical principles[3]

Ecological role

A dandelion is a common plant all over the world, especially in Europe, Asia, and the Americas. It is a well-known example of a plant that is considered a weed in some contexts (such as lawns) but not a weed in others (such as when it is used as a leaf vegetable or herbal medicine).

Weeds generally share similar adaptations that give them advantages and allow them to proliferate in disturbed environments where soil or natural vegetative cover has been damaged. Different types of habitat and disturbances will result in colonization by different communities of weed species.[4]

A number of plant species are adapted to naturally occurring disturbed environments include dunes and other windswept areas with shifting soils, alluvial flood plains, river banks and deltas, and areas that are often burned, and plant scientists call then ruderal species.[5] Since human agricultural practices often mimic these natural environments where weedy species have evolved, some weeds have adapted to grow and proliferate in human-disturbed areas such as agricultural fields, lawns, roadsides, and construction sites. 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.[6]

Some plants become dominant when introduced into new environments because the animals which specialize in feeding on them are absent; in what is sometimes called the “natural enemies hypothesis”, plants freed from these specialist consumers may increase their competitive ability. In locations where predation and mutual competitive relationships no longer exist, some plants are able to increase allocation of resources into growth or reproduction. The weediness of some species that are introduced into new environments may be caused by their production of allelopathic chemicals which indigenous plants are not yet adapted to, a scenario sometimes called the "novel weapons hypothesis". These chemicals may limit the growth of established plants or the germination and growth of seeds and seedlings.[7][8]


Many weed species have moved out of their natural geographic ranges and spread around the world in tandem with human migrations and commerce. Weed seeds are often collected and transported with crops after the harvesting of grains, so humans are a vector of transport as well as a producer of the disturbed environments to which weed species are well adapted, resulting in many weeds having a close association with human activities.[9][10]

Some weed species have been classified as noxious weeds by government authorities because, if left unchecked, they often compete with native or crop plants or cause harm to livestock.[11] They are often foreign species accidentally or imprudently imported into a region where there are few natural controls to limit their population and spread.[12]

Competition with cultivated and endemic plants

700 cattle that were killed overnight by a poisonous weed.[13]

A number of native or non-native plants are unwanted in a specific location for a number of reasons.[14] An important one is that they interfere with food and fiber production in agriculture, wherein they must be controlled in order to prevent lost or diminished crop yields. Other important reasons are that they interfere with other cosmetic, decorative, or recreational goals, such as in lawns, landscape architecture, playing fields, and golf courses. Similarly, they can be of concern for environmental reasons whereby introduced species out-compete for resources or space with desired endemic plants. For all these reasons; horticulture, both functional and cosmetic, and environmental, - weeds interfere by:

  • competing with the desired plants for the resources that a plant typically needs, namely, direct sunlight, soil nutrients, water, and (to a lesser extent) space for growth;
  • providing hosts and vectors for plant pathogens, giving them greater opportunity to infect and degrade the quality of the desired plants;
  • providing food or shelter for animal pests such as seed-eating birds and Tephritid fruit flies that otherwise could hardly survive seasonal shortages;[15]
  • offering irritation to the skin or digestive tracts of people or animals, either physical irritation via thorns, prickles, or burs, or chemical irritation via natural poisons or irritants in the weed (for example, the poisons found in Nerium species);[16]
  • causing root damage to engineering works such as drains, road surfaces, and foundations,[17] blocking streams and rivulets.[18]

In weed ecology some authorities speak of the relationship between "the three Ps": plant, place, perception. These have been very variously defined, but the weed traits listed by H.G. Baker are widely cited.[19][20]

Weeds have long been a concern, perhaps as long as humans have cultivated plants. They are mentioned in various historic texts, such as a Shakespearean sonnet:

"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."[21]

and the Bible:[22]

"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,"[23]

Benefits of weed species

"What would the world be, once bereft,
of wet and wildness? Let them be left.
O let them be left; wildness and wet;
Long live the weeds and the wilderness yet."

-- Gerard Manley Hopkins' poem Inversnaid

While the term "weed" generally has a negative connotation, many plants known as weeds can have beneficial properties. A number of weeds, such as the dandelion (Taraxacum), are edible, and their leaves or roots may be used for food or herbal medicine. Burdock is common over much of the world, and is sometimes used to make soup and medicine in East Asia.[citation needed] Some weeds attract beneficial insects, which in turn can protect crops from harmful pests. Weeds can also prevent pest insects from finding a crop, because their presence disrupts the incidence of positive cues which pests use to locate their food. Weeds may also act as a "living mulch", providing ground cover that reduces moisture loss and prevents erosion. Weeds may also improve soil fertility; dandelions, for example, bring up nutrients like calcium and nitrogen from deep in the soil with their tap root, and clover hosts nitrogen-fixing bacteria in its roots, fertilizing the soil directly. The dandelion is also one of several species which break up hardpan in overly cultivated fields, helping crops grow deeper root systems. Some garden flowers originated as weeds in cultivated fields and have been selectively bred for their garden-worthy flowers or foliage. An example of a crop weed that is grown in gardens is the corncockle, (Agrostemma githago), which was a common weed in European wheat fields, but is now sometimes grown as a garden plant.[24]

Some people have appreciated weeds for their tenacity, their wildness and even the work and connection to nature they provide. As Christopher Lloyd wrote in The Well-Tempered Garden

"Many gardeners will agree that hand-weeding is not the terrible drudgery that it is often made out to be. Some people find in it a kind of soothing monotony. It leaves their minds free to develop the plot for their next novel or to perfect the brilliant repartee with which they should have encountered a relative's latest example of unreasonableness."[25]

Weeds as adaptable species

"We've got to be one of the most bomb-proof species on the planet."

paleontologist David Jablonsky[2]

An alternate definition often used by biologists is any species, not just plants, that can quickly adapt to any environment.[2] Some traits of weedy species are the ability to reproduce quickly, disperse widely, live in a variety of habitats, establish a population in strange places, succeed in disturbed ecosystems and resist eradication once established. Such species often do well in human-dominated environments as other species are not able to adapt. Common examples include the common pigeon, brown rat and the raccoon. Other weedy species have been able to expand their range without actually living in human environments, as human activity has damaged the ecosystems of other species. These include the coyote, the white-tailed deer and the brown headed cowbird.[2]

In response to the idea that humans may face extinction due to environmental degradation, paleontologist David Jablonsky counters by arguing that humans are a weed species. Like other weedy species, humans are widely dispersed in a wide variety of environments, and are highly unlikely to go extinct no matter how much damage the environment faces.[2]

Role in mass extinctions

A mass extinction is generally caused by some abrupt disruption to the entire planet's environment. This results in major changes in habitat worldwide, and most endemic species, specially adapted to a single habitat, cannot survive in the new habitats. Thus only weedy species survive, and they dominate the planet in the immediate aftermath. Cockroaches, for example, have survived several mass extinctions. The current Holocene extinction event, then, could lead to a planet inhabited entirely by what are known today as weeds. The fossil record indicates that after mass extinctions, a weed-dominated planet persists for five to ten million years before life re-diversifies.[2]

Plants often considered to be weeds

White clover is considered by some to be a weed in lawns, but in many other situations is a desirable source of fodder, honey and soil nitrogen.[26][27]

A short list of some plants that often are considered to be weeds follows:

See also


  1. ^ Janick, Jules (1979). Horticultural Science (3rd ed.). San Francisco: W.H. Freeman. p. 308. ISBN 0-7167-1031-5. 
  2. ^ a b c d e f David Quammen (October 1998), "Planet of Weeds", Harper's Magazine, retrieved November 15, 2012 
  3. ^ Brown, Lesley (1993). The New shorter Oxford English dictionary on historical principles. Oxford [Eng.]: Clarendon. ISBN 0-19-861271-0. 
  4. ^ Bell, Graham (2005). The Permaculture Garden. Chelsea Green Publishing. pp. 63–64. ISBN 9781856230278. 
  5. ^ Hans Lambers; F Stuart Chapin III; Thijs L. Pons (8 October 2008). Plant Physiological Ecology. Springer. pp. 507–. ISBN 978-0-387-78341-3. 
  6. ^ Saupe, Stephen G. "Plant Foraging: Two Case Studies". Retrieved February 15, 2009. 
  7. ^ Willis, Rick J. (2007). "The History of Allelopathy". Springer. p. 8. ISBN 1-4020-4092-X. Retrieved 2009-08-17. 
  8. ^ "Callaway.qxd" (PDF). Retrieved 2010-03-20. [dead link]
  9. ^ Rashid M. Hassan; Robert Scholes; Neville Ash (14 December 2005). Ecosystems and Human Well-Being: Current State and Trends: Findings of the Condition and Trends Working Group. Island Press. pp. 570–. ISBN 978-1-55963-228-7. 
  10. ^ National Geographic (2011). National Geographic Answer Book: 10,001 Fast Facts about Our World. National Geographic Society. pp. 175–. ISBN 978-1-4262-0892-8. 
  11. ^ United States. Bureau of Land Management. Oregon State Office (1985). Northwest area noxious weed control program: environmental impact statement : final. U.S. Dept. of the Interior, Bureau of Land Management, Oregon State Office. pp. 2–. 
  12. ^ House (U S ) Office of the Law Revision (25 April 2008). United States Code, 2006, V. 3, Title 7, Sections 701-End. Government Printing Office. pp. 1230–. ISBN 978-0-16-079998-3. 
  13. ^ Coupe, Sheena, ed. (1989). Frontier country: Australia's outback heritage. Vol. 1. Willougby: Weldon Russell. p. 298. 
  14. ^ Muhammad Ashraf; Münir Öztürk; Muhammad Sajid Aqeel Ahmad; Ahmet Aksoy (2 June 2012). Crop Production for Agricultural Improvement. Springer. pp. 525–. ISBN 978-94-007-4116-4. 
  15. ^ Annecke, D. R., Moran, V. C. (1982). Insects and mites of cultivated plants in South Africa. London: Butterworths. ISBN 0-409-08398-4. 
  16. ^ Watt, John Mitchell; Breyer-Brandwijk, Maria Gerdina: The Medicinal and Poisonous Plants of Southern and Eastern Africa 2nd ed Pub. E & S Livingstone 1962
  17. ^ Roberts, John; Jackson, Nick; Smith, Mark. Tree Roots in the Built Environment. 2006. ISBN 978-0117536203
  18. ^ Weeds Australia Black Willow
  19. ^ Baker, H.G. The Evolution of Weeds. Annual Review of Ecology and Systematics, Vol. 5: 1–24 November 1974 DOI: 10.1146/annurev.es.05.110174.000245
  20. ^ Baker H. G. "Characteristics and modes of origin of weeds". In The Genetics of Colonizing Species. H. G. Baker, G. L. Stebbins. eds. New York, Academic Press, 1965, pp. 147-172
  21. ^ Shakespeare, William. Those parts of thee that the world's eye doth view. Infoplease. Retrieved February 15, 2009. 
  22. ^ Janick, Jules (1979). Horticultural Science (3rd ed.). San Francisco: W.H. Freeman. p. 308. ISBN 0-7167-1031-5. 
  23. ^ Genesis 3:17-19 New International Version
  24. ^ Preston, Pearman & Dines. (2002). New Atlas of the British Flora. Oxford University Press.
  25. ^ Christopher Lloyd, The Well-Tempered Garden, 1973
  26. ^ Voisin, Andre. Grass Productivity. Publisher: Island Press 1988. ISBN 978-0933280649
  27. ^ Woodfield, Derek R. White clover, New Zealand's competitive edge. Symposium NZ Agronomy Society and Grassland Association at Lincoln University, New Zealand, November, 1995

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