A noxious weed, harmful weed or injurious weed is a weed that has been designated by an agricultural or other governing authority as a plant that is injurious to agricultural or horticultural crops, natural habitats or ecosystems, or humans or livestock. Most noxious weeds have been introduced into an ecosystem by ignorance, mismanagement, or accident. Some noxious weeds are native. Typically they are plants that grow aggressively, multiply quickly without natural controls, and display adverse effects through contact or ingestion. Noxious weeds are a large problem in many parts of the world, greatly affecting areas of agriculture, forest management, nature reserves, parks and other open space.
A mature Scotch thistle, an invasive weed in Australia.
The algae Caulerpa Taxifolia, is a noxious marine weed nicknamed killer algae. This is a picture of it invading a native plant bed.
This is a parasitic noxious weed, Striga gesnerioides, commonly called Witchweed.
This is a terrestrial noxious weed, Drymaria arenarioides commonly called Lightning weed. It is sometimes confused with Daisies.
A weed is a plant considered undesirable in a particular situation, growing where it conflicts with human preferences, needs, or goals. Plants with characteristics that make them hazardous, aesthetically unappealing, difficult to control in managed environments, or otherwise unwanted in farm land, orchards, gardens, lawns, parks, recreational spaces, residential and industrial areas, may all be considered weeds. The concept of weeds is particularly significant in agriculture, where the presence of weeds in fields used to grow crops may cause major losses in yields. Invasive species, plants introduced to an environment where their presence negatively impacts the overall functioning and biodiversity of the ecosystem, may also sometimes be considered weeds.
Weeds growing in the cracks of a concrete staircase (Epilobium roseum, Chelidonium majus, Oxalis corniculata, Plantago major)
A dandelion is a common plant all over the world, especially in Europe, Asia, and the Americas. It is considered a weed in some contexts (such as lawns) but not others (such as when it is grown as a vegetable or herbal medicine).
Invasive Canada Goldenrod as a roadside weed in Poland
Australia, 1907: Cattlemen survey 700 carcasses of cattle that were killed overnight by a poisonous plant.
