How noxious weeds impact native flora

By Jan Martenson
Posted 7/11/19

What is a weed anyway? To many it's simply a plant growing where you don't want it, whether it's native or "exotic," originating outside of North America. But one class of weeds is …

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How noxious weeds impact native flora


What is a weed anyway? To many it's simply a plant growing where you don't want it, whether it's native or "exotic," originating outside of North America. But one class of weeds is particularly damaging, and that class is the noxious weeds. The United States Department of Agriculture defines a noxious weed as "any plant or plant product that can directly or indirectly injure or cause damage to crops (including nursery stock or plant products), livestock, poultry or other interests of agriculture, irrigation, navigation, the natural resources of the United States, the public health or the environment."

Whew - that's quite a definition.

On Wednesday (July 17), the Taos Chapter of the Native Plant Society of New Mexico will welcome Jim Wanstall, who will speak on the subject "Noxious Weeds of Northern New Mexico." Wanstall will discuss the impacts of noxious weeds on native plant communities in our region and the ecosystems they depend on. The talk is at 6 p.m. in the Kit Carson Electric Co-op boardroom, 118 Cruz Alta Road, Taos.

Wanstall is a natural resources specialist for the New Mexico Department of Agriculture. A graduate of New Mexico State University, he is the state noxious weed coordinator, assisting local, state, federal and private entities to coordinate efforts and form partnerships to address noxious and invasive species concerns statewide. He has assisted Soil and Water Conservation Districts in New Mexico for 21 years and, in that time, he has worked on a wide variety of riparian, upland and agricultural projects.

Speaking of Weeds

Along with the bountiful growth of our native wildflowers, weeds are enjoying a stellar year as well.

But what about the introduced species that are not "noxious" as defined above, but simply annoying to home gardeners and landscapers? Recently I identified one that appeared for the first time this year on my property as cutleaf vipergrass - Scorzonera laciniata. I first became aware of it while reading a discussion in the New Mexico Native Plants Google group. According to one participant, it is becoming annoyingly invasive in Santa Fe, to the point where it "makes the dandelion look like a complete wimp of a weed."

A native of Europe, Africa and Asia, vipergrass (which is not really a grass) was planted in the United States for erosion control and can be found in disturbed areas such as old fields and along roadsides. I believe the seeds for mine were lurking in a load of gravel we used for mulching a rock garden.

Vipergrass was first identified in New Mexico in 1987 and by now has been reported in every New Mexico county. It is in the sunflower family, grows to one to three feet tall, and has small yellow flowers (one-two inches across) that resemble those of dandelions but only open in full sun for a few hours in the morning. The bud is quite distinctive. If you see one, do us all a favor by pulling it and discarding it. It has a taproot that comes up easily, but beware of its milky sap which can stain hands and gloves.

What about those thistles?

For the first time I recognized the native New Mexico thistle (Cirsium neomexicanum) in late May near Silver City. I know they are found in Taos County but I've never yet identified one here. The flowers appear earlier and are a paler pink than the nonnative musk (also called nodding thistle because of the tendency of the flower heads to nod downward) and Scotch thistles we see around here starting in late June.

But there are two features for distinguishing the native versus the musk thistles. First, even though the leaves are spiny, the lower stems of the New Mexico thistle are almost devoid of spines, whereas the nonnatives are horrifically spiny both on leaves and stems. Second, compare the appearance of the phyllaries (spiky bracts) located below the floret of many small flowers. The phyllaries of the New Mexico thistle are narrow, widely spaced, end in spikes and have cobwebby filaments between the bracts, whereas the phyllaries of the musk thistle are quite broad and spiky with no filaments.

Want to learn more? The best guide for identifying thistles is available on the Native Plant Society of New Mexico website, as a PDF file that can be downloaded:

Do be aware of our local native thistles and protect them, especially those seen along our hiking trails: the yellow-flowered Parry's thistle, the white-flowered meadow thistle and the pinkish alpine thistle found above 10,000 ft. elevation. Our native thistles support a wide variety of native pollinators.

Weed Reference Guides

Scott Canning, director of horticulture and special projects at the Santa Fe Botanical Gardens, recently advised me of an online update (2011) of a widespread reference book, "Weeds of the West," from the Western Society of Weed Science. "For a free reference it's rather good, but it does call some of our aggressive natives, like yuccas, milkweeds and even sunflowers, 'weeds,' so reader beware. It is an identification guide; you will need to research control methods elsewhere." Hard copies of earlier editions can still be found for purchase, but this 11th edition can found here:

An online source of invasive plants commonly found in the Jemez region (and likely in Taos County as well) can be found at the Pajarito Environmental Education Center website, Written by Terry Foxx of Los Alamos, It contains good descriptions, photos, uses and control methods.

The take-home lesson, especially for home gardeners, is that the best thing we can do to prevent an infestation of weeds is maintaining a healthy native plant community.

Jan Martenson is the president of the Taos chapter of the Native Plant Society of New Mexico and a member of the board of NPSNM.


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