This is an old revision of this page, as edited by 173.27.112.111 (talk) at 02:11, 19 April 2010 (→Examples). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.
Revision as of 02:11, 19 April 2010 by 173.27.112.111 (talk) (→Examples)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)This article is about plants specifically called weeds. For other uses, see Weed (disambiguation). See also: Invasive species
A weed in a general sense is a plant that is considered by the user of the term to be a nuisance, and normally applied to unwanted plants in human-made settings such as gardens, lawns or agricultural areas, but also in parks, woods and other natural areas. More specifically, the term is often used to describe native or nonnative plants that grow and reproduce aggressively. Generally, a weed is a plant in an undesired place.
Weeds may be unwanted for a number of reasons: they might be unsightly, or crowd out or restrict light to more desirable plants or use limited nutrients from the soil. They can harbor and spread plant pathogens that infect and degrade the quality of crop or horticultural plants. Some weeds are a nuisance because they have thorns or prickles, some have chemicals that cause skin irritation or are hazardous if eaten, or have parts that come off and attach to fur or clothes.
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.
Professor Richard C. Lewontin of Harvard University defines weeds as plants that create environmental conditions in which it cannot reproduce. He takes the example of pine trees that crowd out sunlight such that its own offspring cannot grow. Weeds continue to exist, because the environment is continually being disturbed to create open conditions for new generations, such as forest fires and human activity.
Distribution
Weedy plants generally share similar adaptations 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 dunes and other windswept areas with shifting soils, alluvial flood plains, river banks and 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.
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 allelopathyic 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.
i like it
i like ie
See also
- Crop weeds
- Herbicide
- Introduced species
- Invasive species
- List of beneficial weeds
- Pesticide
- R-selection
- Vavilovian mimicry
- Weed control
- Weed of cultivation
References
- Janick, Jules (1979). Horticultural Science (3rd ed.). San Francisco: W.H. Freeman. p. 308. ISBN 0-7167-1031-5.
- "Detailed information on Corn Cockle (Agrostemma githago)". PlantFiles. Retrieved February 15, 2009.
- A speech given on the radio program Big Ideas, (5/11/2003): "A weed is literally a plant... which comes into a disturbed habitat, which then changes the nature of the soil, the shading, and everything, and the moisture, ectcetera, in such a way that it cannot reproduce itself in that habitat."
- Saupe, Stephen G. "Plant Foraging: Two Case Studies" (PDF). Retrieved February 15, 2009.
- Willis, Rick J. (2007). The History of Allelopathy. p. 8. ISBN 140204092X. Retrieved 2009-08-17.
{{cite book}}:|journal=ignored (help) - "Callaway.qxd" (PDF). Retrieved 2010-03-20.