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 harmful 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, though many localities define them as necessarily being non-native. Typically, they are plants that grow aggressively, multiply quickly without natural controls (such as native herbivores or soil chemistry), and cause adverse effects upon contact or ingestion. Noxious weeds are a major problem in many parts of the world, greatly affecting areas of agriculture, forest management, nature reserves, parks, and other open spaces. Many noxious weeds have come to new regions and countries through contaminated shipments of feed and crop seeds, or have been intentionally introduced as ornamental plants for horticultural use. Some "noxious weeds", such as ragwort, produce copious amounts of nectar, which is valuable for the survival of bees and other pollinators, as well as providing larval host foods and habitats. In the US, 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 benefits.
Types
Some noxious weeds 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. There are marine, terrestrial, and parasitic noxious weeds.
Control
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 cannot identify. For noxious weeds in flower or with seeds on plants, pulling 'gently' out and placing them 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, native plants, and 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 be used for eradication, invasion prevention, or native habitat and plant community restoration projects. Insects and fungi have long been used as biological controls of some noxious weeds and more recently nematodes have also been used.
Eradication According to control experts, there are chemical, physical, and environmental ways of eradicating noxious weeds. Those include pulling the entire weed out of the ground, spraying herbicide if it's a large area, and using machines to turn over the soil. According to farmers, using goats can prove a more ecological way of getting rid of noxious weeds, instead of using herbicides. Also, overplanting a native species is a long-term solution in eradicating noxious weeds.
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 in establishing bumblebee colonies. Thistles that are considered 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 nectar production studies in the UK, one of its native locations. These thistles also serve as a larval host plant for the painted lady butterfly. There can, therefore, be a conflict between agricultural policy and the point of view of conservationists or other groups.
By country
Australia
In Australia, the term "noxious weed" is used by state and territorial governments.
Some noxious weeds in Australia are alligator weeds, horsetails, and branched broomrape. The government of Victoria will get rid of all these plants for free. Alligator weeds are banned in all the states and territories of Australia. They can create large mats that can cause considerable blockages of waterways. Horsetails are poisonous to livestock. They are also extremely challenging to eradicate, as they can fragment and the fragments can grow new plants, much like succulents. Branched broomrapes are parasitic noxious weeds. They attach themselves to the roots of other plants and extract water and nutrients.
Canada In Canada, constitutional responsibility for the regulation of agriculture and the environment is 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. The CFIA has designated certain plant species 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.
European Union
Since 2016, following the EU Regulation 1143/2014 on Invasive Alien Species (IAS), the European Commission publishes updated lists of IAS of Union concern. Since 2022 August 2 there are 41 plant species of Union concern.
An example of highly invasive and phototoxic weeds are 3 related species of hogweed:
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