Weeds

A weed in a general sense is a plant , usually wild or feral, that is considered by the speaker to be a nuisance in a garden, lawn, or other agricultural development.

More specifically, the term is often used to describe plants that grow and reproduce aggressively. Weeds may be unwanted because they are unsightly, or because they limit the growth of other plants by blocking light or using up nutrients from the soil. They also can harbor and spread plant pathogens that can infect and degrade the quality of crop or horticultural plants.

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 is a garden plant.

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 the natural environments in which 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 tend to 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. A number of weedy species have developed chemical means to prevent the germination or growth of neighboring plants.

Weeds and human civilization have a long history. Often their seeds are collected and transported with crops after the harvesting of grains , so that many of these weed species have moved out of their natural geographic locations and have spread around the world with humans. (See Invasive species .)

Many modern species of domesticated flower actually originated as weeds in cultivated fields. The most attractive weeds were the ones least likely to be disturbed by the farmers, and so this selection process produced, over thousands of years, increasingly attractive plants, until they finally attracted conscious domestication.

Not all weeds have the same ability to damage crops and horticultural plants. Some have been classified as noxious weeds because if left unchecked, they often dominate the environment where crop plants are to be grown. They are often foreign species mistakenly or accidentally imported into a region where there are few animals or plants to control their population and spread. With the conversion of land to agriculture and distribution of food crops from other parts of the world, these weeds have ideal areas for growth and reproduction, with humans being the vector of transport and the producer of disturbed environments for their growth.

A number of weeds, such as the dandelion , are edible, and their leaves and roots may be used for food or herbal medicine . So-called ” beneficial weeds ” may have other beneficial effects, such as drawing away the attacks of crop-destroying insects. Indeed, dandelions are one of several species which break up hardpan in overly cultivated fields, helping crops grow deeper root systems.

Examples

• Broadleaf plantain - perennial, spreads by seeds that persist in the soil for many years
• Burdock - biennial
• Creeping Charlie - perennial, fast-spreading plants with long creeping stems
• Dandelion - biennial, wind-spread, fast-growing, and drought-tolerant
• Goldenrod - perennial
• Kudzu - perennial
• Leafy spurge - perennial, with underground stems
• Milk thistle - annual or biennial
• Poison ivy - perennial
• Ragweed - annual
• Sorrel - annual
• Sumac - woody perennial
• Wild carrot - biennial
• Wood sorrel - perennial