Weed is a subjective term used in a variety of senses, usually to describe a plant considered undesirable within a certain context. The word—commonly applied to unwanted plants in human-controlled settings, such as farm fields, gardens, lawns, and parks—carries no botanical classification value, since a plant that is a weed in one context is not a weed when growing where it is wanted. Indeed, a number of plants that many consider weeds are often intentionally grown in gardens and other cultivated settings. Less commonly, the term is applied to any plant that grows and reproduces aggressively 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]

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 whose 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]

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.^[5]

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 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 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.^[6][7]

Dispersal

This section does not cite any references or sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (May 2013)

Weed seeds are often collected and transported with crops after the harvesting of grains. Many weed species have moved out of their natural geographic locations and spread around the world in tandem with human migrations and commerce. (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 weeds by governmental authorities because, if left unchecked, they often compete with native or crop plants or cause harm to livestock. They are often foreign species accidentally or imprudently 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 destruction of natural habitats by development. Farming practices that produce unvegetated soils part of the year and human distribution of food crops mixed with non-native weed seeds 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.^{[citation needed]}

Competition with cultivated and endemic plants

700 cattle that were killed overnight by a poisonous weed.^[8]

Weeds may be unwanted for a number of reasons. 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;^[9]
• 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);^[10]
• causing root damage to engineering works such as drains, road surfaces, and foundations,^[11] blocking streams and rivulets.^[12]

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.^[13][14]

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."^[15]

Benefits of weed species

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 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. The dandelion is one of several species which break up hardpan in overly cultivated fields, helping crops grow deeper root systems. Some modern species of domesticated flower originated as weeds in cultivated fields and have been bred into garden plants for their flowers or foliage. 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 is now sometimes grown as a garden plant.^[16]

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."^[17]

Shunryu Suzuki, the Zen master, is credited with proclaiming, "For Zen students, a weed is a treasure."

Weeds as adaptable species

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.^[18][19]

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

• Bermuda grass - perennial, spreading by runners, rhizomes and seeds.
• Bindweed
• Broadleaf plantain – perennial, spreads by seeds that persist in the soil for many years
• Burdock – biennial
• Common lambsquarters - annual
• 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
• St John's wort - perennial
• Sumac – woody perennial
• Tree of heaven - woody perennial
• Wild carrot – biennial
• Wood sorrel – perennial
• Yellow nutsedge - perennial

See also

Plants portal

References

External links

Wikimedia Commons has media related to: Weeds (plants)

• IUCN Invasive Species Specialist Group
• Global Invasive Species Database