A mature Scotch thistle
, an invasive weed in Australia (person for scale)
A noxious weed, harmful weed or injurious weed is a weed that has been designated by an agricultural authority as one 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 (native herbivores, soil chemistry, etc.), 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.
These weeds are typically agricultural pests, though many also have impacts on natural areas. Many noxious weeds have come to new regions and countries through contaminated shipments of feed and crop seeds or intentional introductions such as ornamental plants for horticultural use.
There is controversy about the definition of weed as well as the definition of noxious weed, particularly when it comes to how agricultural interests relate to conservationism. Some "noxious weeds", such as ragwort, produce copious amounts of nectar, valuable for the survival of bees and other pollinators, or other advantages like larval host foods and habitats. Wild parsnip, Pastinaca sativa, for instance, provides large tubular stems that some bee species hibernate in, larval food for two different swallowtail butterflies, and other beneficial qualities. One study of restoration meadows using commercial mixes found that several weeds greatly outperformed, in terms of nectar production, the top-performing annual flower planted for meadow restoration in the meadows studied (cornflower). The best performers also quite significantly outperformed the top-performing perennial that wasn't classified as a weed (rough hawkbit). The farming practice of using beetle banks may also complicate the nature of dubbing certain plants noxious weeds, due to their beneficial qualities in that role.