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About|plants specifically called weedsSee also|Invasive species|Volunteer (botany)TOCrightThe term weed is used in both technical and general word sense|senses . In the general sense, a typical definition is "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."cite book |author=Brown, Lesley |title=The New shorter Oxford English dictionary on historical principles |publisher=Clarendon |location=Oxford Eng. |year=1993 |pages= |isbn=0-19-861271-0 |oclc= |doi= |accessdate= In general, therefore, a weed is a plant that is considered by the user of the term to be a nuisance. The word commonly is applied to unwanted plants in human-controlled settings, especially agriculture|farm fields and garden s, but also lawn s, park s, Forest|woods , and other areas. More vaguely, "weed" is applied to any plants that grow and reproduce aggressively and Invasive species|invasively .cite book | isbn = 0-7167-1031-5 | last = Janick | first = Jules | title = Horticultural Science | location = San Francisco | publisher = W.H. Freeman | year = 1979 | page = 308 | edition = 3rd
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.Baker, H.G. The Evolution of Weeds. Annual Review of Ecology and Systematics, Vol. 5: 1-24 November 1974 DOI: 10.1146/annurev.es.05.110174.000245Baker 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
Weeds may be unwanted for a number of reasons. The most 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. The next most important reason is that they interfere with other cosmetic, decorative, or recreational goals, such as in lawn s, landscape architecture , playing field s, and golf course s. In all of these forms of horticulture , functional and cosmetic, 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 pathogen s, giving them greater opportunity to infect and degrade the quality of the desired plants;
offering irritation to the skin or digestive tracts of people or animals, either physical irritation via thorns, spines, and prickles|thorns, prickles , or bur s, or chemical irritation via natural poisons or irritants in the weed (for example, the poisons found in Nerium species)Watt, John Mitchell; Breyer-Brandwijk, Maria Gerdina: The Medicinal and Poisonous Plants of Southern and Eastern Africa 2nd ed Pub. E & S Livingstone 1962
The term weed in its general sense is a subjective one, without any classification value, since a plant that is a weed in one context 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. 'Volunteer weeds' are crop plants from one year which are growing in the subsequent crop. An example of a crop weed that is grown in gardens is the corncockle , Agrostemma , which was a common field weed exported from Europe along with wheat , but now sometimes grown as a garden plant.cite web | url = http://davesgarden.com/pf/go/1341/ | work = PlantFiles | title = Detailed information on Corn Cockle (Agrostemma githago) | accessdate = February 15, 2009
Distribution
Weeds generally share similar adaptation s that give them advantages and allow them to proliferate in disturbed environments whose soil or natural vegetative cover has been damaged. Naturally occurring disturbed environments include dune s and other windswept areas with shifting soils, alluvial flood plains, river bank s and river delta|deltas , and areas that are often burned. Since human agricultural practices often mimic these natural environments where weedy species have evolved, 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.cite web | url = http://employees.csbsju.edu/ssaupe/biol327/Lecture/Plant_Foraging_case_study.pdf | title = Plant Foraging: Two Case Studies | first = Stephen G. | last = Saupe | accessdate = February 15, 2009
Some plants become dominant when introduced into new environments because they are freed from specialist consumers; in what is sometimes called the “natural enemies hypothesis,” plants freed from these specialist consumers may increase their competitive ability. In locations were 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 can be caused by the introduction of new chemicals; sometimes called the "novel weapons hypothesis," these introduced allelopathy ic chemicals, which indigenous plants are not yet adapted to, may limit the growth of established plants or the germination and growth of seeds and seedlings.Cite book| title = The History of Allelopathy| url = http://www.google.com/books? id=C-nPBYjDAjYC& pg=PA3& | year = 2007| author = Willis, Rick J.| journal = Springer| page = 8| isbn = 1-4020-4092-X| accessdate = 2009-08-17cite web|url= http://plantecology.dbs.umt.edu/Full%20text%20papers%20and%20abstracts/2004%20papers/Callaway& Ridenour2004NOVEL.pdf |title=Callaway.qxd |format=PDF |date= |accessdate=2010-03-20
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 Book of Genesis|Genesis and a William Shakespeare|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,"Genesis 3:17-19 New International Version
"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."cite book | authorlink = William Shakespeare | first = William | last = Shakespeare | url = http://www.infoplease.com/t/lit/shakespeare-sonnets/69.html | title = Those parts of thee that the world's eye doth view | publisher = Infoplease | accessdate = February 15, 2009
Weed seeds are often collected and transported with crops after the harvesting of grain s. 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 or cause harm to animals. Some have been classified as noxious weed s by governmental authorities because if left unchecked, they often dominate the environment where crop plants are to be grown or cause harm to livestock . They are often foreign species mistakenly or accidentally imported into a region where there are few natural controls to limit their population and spread. Many weeds have ideal locations for growth and reproduction because of the large areas of open soil created by the conversion of land to field agriculture. Farming practices that produce unvegetated soils part of the year and human distribution of food crops mixed with seeds of weeds from other parts of the world have facilitated the colonization of vast new areas for many weedy species; humans are the vector of transport and the producer of disturbed environments, thus many weedy species have an ideal association with humans.
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 weed s" may have other beneficial effects, such as drawing away the attacks of crop-destroying insect s, 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.
Some people have appreciated weeds for their tenacity, their wildness and even the work and connection to nature they provide. As Christopher Lloyd (gardener)|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."Christopher Lloyd, The Well-Tempered Garden, 1973
Shunryu Suzuki, the Zen master, is credited with proclaiming, "For Zen students, a weed is a treasure."
Perhaps the greatest defense of weeds is contained in the last stanza of Gerard Manley Hopkins' poem Inversnaid:
"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."
Plants considered to be weeds
A short list of some plants that are considered to be weeds is listed below:
