A weed in a general sense is a plant that is considered by the user of the term to be a nuisance. The word is normally applied to unwanted plants in human-controlled settings, especially farm fields and gardens, but also lawns, parks, woods, and other areas. Generally, a weed is a plant in an undesired place. More specifically, the term is often used to describe any plants that grow and reproduce aggressively. 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 lawns, landscape architecture, playing fields, and golf courses. In all of these forms of horticulture, functional and cosmetic, weeds interfere by (1) 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; (2) providing hosts and vectors for plant pathogens, giving them greater opportunity to infect and degrade the quality of the desired plants; or (3) 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. 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. Distribution
Yellow starthistle, a thistle native to southern Europe and the Middle East that is an invasive weed in parts of North America. 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. 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. 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," "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."
700 cattle that were killed overnight by a poisonous weed. 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 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 weeds 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 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. 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--Sabithamanas (talk) 15:41, 30 April 2012 (UTC) "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." 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."