Noxious Weeds by the County Extension Service

A weed is defined as any plant that interferes with the management objectives for a particular site. An invasive or noxious weed is a plant that is not native to a particular ecosystem. Noxious weeds can have serious negative impacts on site quality and can rapidly invade into and take over growing sites, displacing native species.

Invasive weeds have been deemed a major problem in the western U.S. From 1985 to 1995, weeds have increased their range four-fold and now occupy about 17 million acres of public land.

The negative impacts of invasive weeds include a reduction in land use opportunities, watershed damage, increased soil erosion, the displacement of native vegetation, decreased recreational opportunities, and an increased need for intensified (and costly) management activities.

Three classes of invasive weeds have been delineated in New Mexico. Class A weeds are those which are not native to an ecosystem and have a limited distribution. Class A weeds receive the highest priority for attention, since their limited distribution provides potential for success in removing current infestations and preventing future spread. Weed species that have yet to invade New Mexico are also labeled as Class A weeds.

W1-HoaryCress_WEB.jpg (29135 bytes)Hoary Crest

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Spotted Knap Weed

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Leafy Spurge

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Canada Thistle

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Musk Thistle






Class A weeds listed for New Mexico include: Camelthorn (Alhagi pseudalhagi), Horary cress (Cardaria draba), Halogeton (Halogeton glomeratus), Black henbane (Hyoscyamus niger), Diffuse knapweed (Centaurea diffusa), Spotted knapweed (Centaurea maculosa), Purple loosestrife (Lythrum salicaria), Onionweed (Asphodelus fistulosus), Perennial pepperweed (Lepidium latifolium), Leafy spurge (Euphorbia esula), Purple starthistle (Centaurea calcitrapa), Yellow starthistle (Centaurea solstitialis), Canada thistle (Cirsium arvense), Scotch thistle (Onopordum acanthium), Dalmatian toadflax (Linaria genistifolia ssp. dalmatica), Yellow toadflax (Linaria vulgaris), and Dyer’s woad (Isatis tinctoria).

Class B weeds are not native to the ecosystem in which they occur and are limited to specific areas in New Mexico. Management emphasis is given to containing these weeds to their current range and keeping such plants from spreading into new areas. Class B weeds in New Mexico include Woollyleaf bursage Ambrosia grayi), Russian knapweed (Acroptilon repens), African rue (Peganum harmala), Malta starthistle (Centaurea melitensis), and must thistle (Carduus nutans).

Class C weeds are non native to the ecosystem in which they occur but are widespread throughout New Mexico. The development of long-term programs which deal with the management and suppression of these invaders are necessary to achieve any degree of success. Pervasive Class C weeds listed in New Mexico include Field bindweed (Convolvulus arvensis), jointed goatgrass (Aegilops cylindrica), bull thistle (Cirsium vulgare), russian olive (Eleagnus angustifolia), salt cedar (tamarisk spp.), and Siberian elm (Ulmus pumila).

Successful noxious weed management programs include an integrated approach of prevention and control. Preventive measures include:

-- Identifying all weeds that are unfamiliar in disturbed areas such as road sides, fields, pastures, and irrigation ditches. Plants determined to be noxious should be eradicated when feasible.

-- Ensure that seed, hay, straw and other ground cover used in management activities are certified as weed free. Be very careful when using hay and straw.

-- Minimize the amount of bare mineral soil that is exposed during activities such as logging, road construction, and ditch installation. Broadcast weed-free, competitive grass seed over areas with bare mineral soil.

-- Avoid overgrazing or trampling in grassland areas.

-- Control the noxious weeds that can be transported to a site by cattle. When receiving new shipments of livestock, contain the herd in a holding area for up to 96 hours to allow for weed seed to pass through their digestive systems. Eradicate any noxious weeds that appear in or near holding pens or pastures.

-- When noxious weeds are discovered on a land base, effective control measures are warranted. Control includes cultural, mechanical, biological, and chemical methods. Cultural methods such as prescribed fire, planting, and thinning can be employed to promote the growth and maintenance of desired species. Mechanical control can be implemented to physically disrupt weed growth and includes hand pulling, mowing, tilling, shredding, and mulching. Biological control involves the use of livestock (sheep, cows, goats, moufflon, etc.), in a managed grazing system or the introduction of known diseases or insects which kill or severely damage noxious plants. Chemical control methods involve the use of growth inhibitors (herbicides) to disrupt plant development.

The final key to successful noxious weed management is careful monitoring to ensure that control methods have been adequate and that the weeds do not persist or spread.

Class A weeds known to occur in Otero County include hoary cress, spotted knapweed, leafy spurge, and Canada thistle. Class B weeds found in Otero County include Russian knapweed, African rue, malta starthistle, and musk thistle. Class C weeds found in Otero County include field bindweed, jointed goatgrass, Russian olive, salt cedar, and Siberian elm. Purple loosestrife, although not noted in Otero County, is still sold in seed catalogs and is considered a noxious weed because of its competitive nature and ability to generate dense, ecosystem-altering populations.

If noxious weeds are found on the property, professional assistance should be sought immediately by contacting the Otero County Cooperative Extension Service in Alamogordo at (505) 437-0231 or the New Mexico Department of Agriculture at (505) 646-2642. A guide to New Mexico’s invasive weeds is available from NMDA.