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.[1] 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, many "weeds" are beneficial, even in gardens or other cultivated-plant settings.
In its more specific use, weeds are naturally occurring plants, that due to their aggressive growth, are colonizers and may damage plants and landscapes that are desirable to people. Weedy plants generally share similar adaptive speciation that gives them advantages and allows them to proliferate in disturbed environments such as agricultural fields or areas with disturbed soils like roadsides, construction sites and other areas that have had the soil and/or natural vegetative cover damaged.
These plants have naturally evolved to colonize disturbed environments. These naturally occurring environments include dunes and other windswept areas with shifting soils, alluvial flood plains, river banks and deltas, areas that are often burned plus others. The weedy nature of these species tends to give them an advantage over more desirable crop species because they tend to grow quickly and reproduce quickly, have heavy seed set with seeds that persist in the seed bank for many years or short lifespans with multiple generations in the same growing season. Perennial weeds often have underground stems that spread out under the soil surface. A theory has been developed to express the interrelationship of these plants with the environment, called r/K selection theory. Invasive species are also considered weeds.
Weeds and human civilization have a long history. Since human agricultural practices often mimic the natural environments that weedy species have evolved in, they are adapted to grow and proliferate in fields. 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 to become cosmopolitan 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, often because they are foreign species mistakenly or accidentally imported into a region where they have no natural enemies. Most noxious weeds in North America are not native to North America but have come from other areas of the world. 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 transpor t and the producer of disturbed environments for their growth.
Some 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 types which break up hardpan in overly cultivated fields, helping crops grow deeper root systems.
Examples
Some plants that are commonly considered weeds include:
See also
References
- ↑ ISBN 0-7167-1031-5 Janick, Jules. Horticultural science. San Francisco: W.H. Freeman, 1979. Page 308.
External links
- Weed Identification Guide from Virginia Tech (Southeastern United States)
- Common weeds of the northern United States and Canada from Canadian Weed Science Society