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Wyoming Invasive Weed Awareness Highlighted

by Wyoming Livestock Roundup

By UW Extension Weed Specialist Brian Mealor

May 8-14 was Wyoming Invasive Weed Awareness Week, and many weed managers around the state hosted workshops, weed walks and other events to share important information about the impacts of invasive weeds in our state.

Sometimes we become so closely involved in details of our professions that we may not remember to look at how our efforts fit into other parts of society, or we assume that other people are aware of the same information we have come to accept as axiomatic.

Increased awareness of an issue includes understanding the scope and severity of the issue, and fitting the issue into the local context of how it “affects me.” For this article, we will step away from tactics and techniques for managing weeds on the landscape and briefly discuss the current state of weed invasion in Wyoming and surrounding areas, and the impacts of invasive weeds.

The scope of the invasive species problem is vast. By some estimates, well over 50 million acres of rangeland, pasture, forest and natural areas are infested by a subset of the many invasive weed species in the western United States. An estimate of infested acres in Wyoming may range from thousands to hundreds of thousands of acres. Environmental impacts of the worst invasive weeds include reducing the quantity and quality of forage for livestock and wildlife, reducing the abundance of native species, altering fire cycles to the detriment of native shrubs and changing soil nutrient cycling. These impacts can have broad-reaching ripple effects, which further change the goods and services ecosystems provide.

One way to evaluate changes induced by invasive weeds on ecosystem goods and services is through economic impacts. Large-scale economic analyses have been conducted for only a few weed species. In the mid-1990s, leafy spurge infestation was estimated to have a $130 million impact on rangelands in Montana, South Dakota, North Dakota and Wyoming, and the economic impacts of three knapweed species in Montana were estimated at $42 million annually.

Some indirect economic impacts, such as costs of controlling cheatgrass-driven wildfires, have been estimated at around $20 million per year – another significant, but sometimes overlooked effect of weed invasion. These regional impact estimates are impressive, and the local-scale impacts of decreased production and cost of control often become significant burdens on agricultural producers in Wyoming.

Wyoming has a relatively unique situation regarding invasive weeds when compared to other states in the West. Although the state has large populations of long-established weeds, such as leafy spurge, Canada thistle, Russian knapweed and others, many weed species that cause problems in other states are not present in Wyoming. This absence of self-sustaining populations of weed species like medusahead, yellow starthistle, rush skeletonweed and goatsrue may be attributed to several different causes.

Although some people may not always see it as a positive characteristic, Wyoming’s high-elevation, semi-arid climate with long winters may provide some insulation against the quick establishment and spread of some weed species. The relative remoteness of many areas of the state may also serve to limit introduction and establishment of weeds which are moved around by human activity. Another factor which has limited, and will continue to limit, new weeds entering the state is human intervention by county weed and pest programs, other natural resource professionals and interested residents who keep a vigilant watch for new weed populations in their areas.

Wyoming’s elevation and geography make weed management important for our neighboring states as well. The headwaters of major river systems like the Missouri-Mississippi, the Green-Colorado and the Snake-Columbia are situated within Wyoming. While this headwater nature of the state may also contribute to our reduced diversity of weeds in the state, it emphasizes the importance of controlling weed populations along river corridors where seeds may spread downstream into other states and exacerbate control efforts there.

Wyoming’s rich natural resources are enjoyed by many different groups for business, personal and recreational purposes – all of which can be negatively affected by weed invasion. In my opinion, the residents of Wyoming have a high degree of knowledge regarding many issues related to natural systems. Capitalize on this knowledge and help to reduce the impacts of invasive weeds by working with the Cooperative Extension Service, Weed and Pest Control Districts, conservation districts and other partners.

For more information on the impacts of invasive weeds in Wyoming and how you can assist in their management contact your local natural resource professional or email me at

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