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.
There are types of noxious weeds that are harmful or poisonous to humans, domesticated grazing animals, and wildlife. Open fields and grazing pastures with disturbed soils and open sunlight are often more susceptible. Protecting grazing animals from toxic weeds in their primary feeding areas is therefore important.
Some guidelines to prevent the spread of noxious weeds are:
- Avoid driving through noxious weed-infested areas.
- Avoid transporting or planting seeds and plants that one can't identify.
- For noxious weeds in flower or with seeds on plants, pulling 'gently' out and placing in a secure closable bag is recommended. Disposal such as hot composting or contained burning is done when safe and practical for the specific plant. Burning poison ivy can be fatal to humans.
- Using only certified weed-free seeds for crops or gardens.
Maintaining control of noxious weeds is important for the health of habitats, livestock, wildlife and native plants, and of humans of all ages. How to control noxious weeds depends on the surrounding environment and habitats, the weed species, the availability of equipment, labor, supplies, and financial resources. Laws often require that noxious weed control funding from governmental agencies must be used for eradication, invasion prevention, or native habitat and plant community restoration project scopes.
Controversy and biases
Agricultural needs, desires, and concerns do not always mesh with those of other areas, such as pollinator nectar provision. Ragwort, for instance, was rated as the top flower meadow nectar source in a UK study, and in the top ten in another. Its early blooming period is also particularly helpful for the establishment of bumblebee colonies. Thistles that have been dubbed noxious weeds in the US and elsewhere, such as Cirsium arvense and Cirsium vulgare, have also rated at or near the top of the charts in multiple UK studies for nectar production. These thistles also serve as a larval host plant for the Painted lady butterfly. There can be, therefore, a conflict between agricultural policy and point of view and the point of view of conservationists or other groups.
Another example is seen in how the USDA's Animal and Plant Health Inspection Service (APHIS) Plants for Planting Manual bans all parasitic plants from being sold or planted in North America. Yet, many conservation seed companies offer Indian paintbrush (Castilleja species) seeds and plants. The 200+ species of Castilleja in North America is typically seen by conservationists as an important part of a high-quality natural meadow or similar landscape, along with some other members of the Orobanchaceae family, a family that includes 90 genera and over 2000 species. Castilleja plants often have showy flowers that attract hummingbirds, so they are considered valuable from an aesthetic point of view as well. Many of North America's species in this family are not threats to agriculture, yet they are banned by the USDA's sweeping prohibition on anything parasitic.
By contrast, Rhinanthus minor ("Yellow Rattle"), native to North America according to the USDA, is very frequently cited by United Kingdom authorities as being a good plant to use to establish wildflower meadows, praised for its ability to control grass without herbicides. These UK writers say it can be easily controlled, by mowing it once before it sets seed. US writers, from an agricultural perspective, use dramatic language about the same plant, claiming that it's very noxious and threatening. It is typical to find the plant referred to as an infestation in these sources, while such language is never present in the UK sources. The US sources see the plant as invasive and describe only methods for destroying it.
A third example is how garlic mustard, Alliaria petiolata, not native to North America, is seen by North American conservationism as extremely noxious, particularly for woodlands and woodland edges. Despite that, it is not included in the USDA's Federal Noxious Weeds List. It does not cause issues with agricultural fields, instead causing major damage to diverse plant communities. There has also been no approval for biological control agents for that species, the approval requests for even the monophagous weevil, Ceutorhynchus scrobicollis, being blocked many times by the USDA's Technical Advisory Group, TAG, even though experts had expected that it would be approved as early as 2004.
A fourth example is seen in the usefulness of beetle banks. Bull thistle plants, as an example thistle, produce Painted lady caterpillars which feed insect-eating birds. These birds, in turn, can feed upon pest insects in neighboring farm fields. The thistle plants also produce a great deal of nectar while they are in bloom. American goldfinches also eat the thistle seeds and use the down from seed pods to build nests. Blooming bull thistle flowers are used by large butterflies, such as the monarch butterfly, as well as by bumblebees and small butterflies. The limited land area given to beetle banks, in comparison with farm fields, also places a premium on the higher nectar production of certain weeds such as ragwort and thistles such as bull thistle.
The examples and perspective in this article deal primarily with the English-speaking world and do not represent a worldwide view of the subject. (February 2013) (Learn how and when to remove this template message)
In Canada, constitutional responsibility for the regulation of agriculture and the environment are shared between the federal and provincial governments. The federal government through the Canadian Food Inspection Agency (CFIA) regulates invasive plants under the authority of the Plant Protection Act, the Seeds Act and statutory regulations. Certain plant species have been designated by the CFIA as noxious weeds in the Weed Seeds Order.
Each province also produces its own list of prohibited weeds. In Alberta, for example, a new Weed Control Act was proclaimed in 2010 with two weed designations: "prohibited noxious" (46 species) which are banned across Alberta, and "noxious" (29 species) which can be restricted at the discretion of local authorities.
New Zealand has had a series of Acts of Parliament relating to noxious weeds: the Noxious Weeds Act 1908, Noxious Weeds Act 1950, and the Noxious Plants Act 1978. The last was repealed by the Biosecurity Act 1993, which used words such as "pest", "organism" and "species", rather than "noxious". Consequently, the term "noxious weed" is no longer used in official publications in New Zealand.
The Weeds Act, 1959 covers Great Britain, and is described as "preventing the spread of harmful or injurious weeds". It is mainly relevant to farmers and other rural settings rather than the allotment or garden-scale growers. Five "injurious" (that is, likely to be harmful to agricultural production) weeds are covered by the provisions of the Weeds Act. These are:
- Spear thistle (Cirsium vulgare)
- Creeping, or field, thistle (Cirsium arvense)
- Curled dock (Rumex crispus)
- Broad-leaved dock (Rumex obtusifolius)
- Common ragwort (Senecio jacobaea)
The Department for Environment, Food and Rural Affairs (DEFRA) provides guidance for the removal of these weeds from infested land. Much of this is oriented towards the use of herbicides.
The Act does not place any automatic legal responsibility on landowners to control the weeds, but they may be ordered to control them. Most common farmland weeds are not "injurious" within the meaning of the Weeds Act and many such plant species have conservation and environmental value. DEFRA has a duty to try to achieve reasonable balance among different interests. These include agriculture, countryside conservation and the general public.
Section 14 of the Wildlife and Countryside Act 1981 makes it an offence to plant or grow certain specified plants in the wild, listed in Schedule 9 of the Act, including giant hogweed and Japanese knotweed. Some local authorities have by-laws controlling these plants. There is no statutory requirement for landowners to remove these plants from their property.
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- Weeds Act 1959
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- New Zealand
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- United States