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Fall is a Good Time to Manage Noxious Weeds

By George Beck, Colorado State University Cooperative Extension, Weed Specialist

Noxious weeds, AKA invasive plants, are an insidious problem in Colorado and throughout western U.S.

Noxious weeds displace native plants and disrupt evolved ecosystem processes. Infestations of noxious weeds in pastures, rangelands, and other natural areas readily disperse onto adjacent land causing further problems and eventually disperse into agronomic fields where they decrease crop quality and yield. Weeds will spread – that is their nature.

Prevention is the most powerful form of weed management and the cheapest and easiest weed to manage is the one you do not have. One of the best ways to prevent weeds from spreading is to control existing infestations. Fall is a good time to exert noxious weed control. Herbicides are the primary tool of choice in the fall. Biological control agents generally have finished preying on weeds by fall and most grazing livestock will not consume weeds this time of year.

Mowing and hand pulling are most effective when practiced before weeds go to seed. Biennial weeds, such as musk thistle and diffuse knapweed, are in the rosette growth stage (except those setting seed) and can be readily controlled with herbicides.

Perennial weeds, such as Canada thistle, Russian knapweed, and leafy spurge, also are very susceptible to fall-applied herbicides. The physiology of these latter weeds changes as the day length continues to shorten and temperatures decrease. It is not fully understood how these changes increase their susceptibility to herbicides applied in fall, but gardeners can seize the opportunity and take advantage of this fact.

The growing season is not finished, so make your fall noxious weed management plans and take advantage of their susceptibility to control methods this time of year.

Photo: Judy Sedbrook

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Date last revised: 01/05/2010