page_title: Pest (organism)
section_title: Weeds
hierarchical_section_title: Pest (organism) / Economic impact / Weeds
caption_attribution_description: A noxious weed in NSW; here growing on the Williams River at Clarence Town; furthest up the river it has been recorded. Introduced, warm-season, perennial terrestrial to aquatic, hairless herb; with stolons to 10 m long. Leaves are opposite, sessile and obovate. Flowerheads are axillary to terminal, globose clusters of 1 to several flowers on peduncles to 4 cm long. Perianth segments are white and scarious. Bracts are shorter than the perianth. Flowering is from late spring to early autumn. A native of South America, it forms dense floating or rooted mats in aquatic and seasonally inundated land. A noxious weed that is already having severe impacts on irrigated agriculture, turf industry, extraction industry and vegetable industry; also impacting native areas. Control? Good luck!
context_page_description: A pest is any animal or plant deleterious to humans or human concerns. The term is particularly used for creatures that damage crops, livestock and forestry, or cause a nuisance to people, especially in their homes. Humans have modified the environment for their own purposes and are intolerant of other creatures occupying the same space when their activities impact adversely on human objectives. Thus, an elephant is unobjectionable in its natural habitat but a pest if it tramples crops.
  Some animals are disliked because they bite or sting; snakes, wasps, ants, bed bugs, fleas and ticks belong in this category. Others enter the home thus invading our own private space; these include houseflies, which land on and contaminate food, beetles which tunnel into the woodwork, and other animals that scuttle about on the floor at night, like cockroaches, rats and mice, which are often associated with insanitary conditions.
  Agricultural and horticultural crops are attacked by a wide variety of pests, the most important being insects, mites, nematodes and gastropod molluscs.
context_section_description: A weed is a plant considered undesirable in a particular situation; the term has no botanical significance. Often, weeds are simply those native plants that are adapted to grow in disturbed ground, the disturbance caused by ploughing and cultivation favouring them over other species. Any plant is a weed if it appears in a location where it is unwanted; Bermuda grass makes a good lawn plant under hot dry conditions but become a bad weed when it out-competes cultivated plants.  
  A different group of weeds consists of those that are invasive, introduced, often unintentionally, to habitats to which they are not native but in which they thrive. Without their original competitors, herbivores, and diseases, they may increase and become a serious nuisance. One such plant is purple loosestrife, a native of Europe and Asia where it occurs in ditches, wet meadows and marshes; introduced into North America, it has no natural enemies to keep it in check, and has taken over vast tracts of wetlands to the exclusion of native species.
caption: Alligator weed, a native of South America, is an invasive species in many other countries and is considered a noxious weed as it is harmful to aquatic ecosystems, recreational activities, and favours the spread of mosquitoes. Control is difficult.[31]
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