Fall is a great time to treat Canada thistle


Canada thistle, with its bright purple flower, is towering above most plants in many pastures, landscapes and along roadsides in Archuleta County this time of year. If you haven’t noticed it before now and been meticulous in its management, you might be fighting a tough battle.

Canada thistle is the most common noxious weed problem in Colorado and most states in the Rocky Mountain region. This exotic species was first introduced from Eurasia to the United States during colonial times, 400 years ago, and has since spread to 42 states and the Canadian provinces. It is a detriment to the environment and economy of Colorado, negatively affecting agriculture and natural areas.

Canada thistle is a common problem on roadsides, rangeland, pastures, farmland and natural areas. This aggressive perennial can develop an incredibly extensive root system in a short amount of time. The underground rhizomes aggressively compete for moisture and nutrients feeding above-ground growth that can out-compete many plants for sunlight. Extensive stands of Canada thistle in natural areas reduce native plant abundance and diversity and, in doing so, reduce wildlife habitat. Aesthetically, this noxious weed is a detriment to the beauty of our natural areas, often seen as the dominant plant along drainages.

Canada thistle can be controlled by diligent land managers and private landowners, but persistence is required. A combination of management efforts can remove the majority of a Canada thistle infestation in two to three years, but subsequent follow-up spot treatments may be necessary for several years.

Best management recommendations include:

• Cultural controls — establishment of competitive desirable vegetation is the bottom line for management of any noxious weed. Weeds do best when invading disturbed sites, so by maintaining a vigorous stand of competing vegetation, weed problems are minimized. Grasses provide best competition because of their tolerance to mowing and most herbicides used for control of Canada thistle. If grasses are not present in a problem area, then seeding is necessary. If grasses are present at all, the densities should increase as Canada thistle decreases.

• Mowing/grazing — removal of Canada thistle above-ground plant tissue will stimulate re-growth, which reduces carbohydrate reserves within the root system and weakens the plant. Canada thistle is palatable to livestock until the plants begin to dry down. Grazing needs to be monitored for adverse effects on desirable grasses; otherwise this management tool becomes counterproductive. Mowing Canada thistle at the early flower stage maximizes stress on the plant’s carbohydrate reserves. Mowing/grazing alone will not control Canada thistle, but used in conjunction with a fall herbicide application, provides excellent control.

• Herbicide application — provides the most effective control of any perennial weed, particularly when applied in the fall following summer mowing/grazing. The most beneficial time to apply is September or October, prior to a hard frost. For specific herbicide recommendations, contact the Archuleta Weed and Pest office at 264-6773.

Other management methods include:

• Hand pulling/clipping — not effective for controlling Canada thistle infestations unless one is extremely persistent. Above-ground growth is the “tip of the iceberg” and hand pulling, at least on larger areas, is seldom feasible.

• Insect bio-control — not effective at this point. Experts with Colorado Department of Agriculture, Colorado State University and U.S. Department of Agriculture do not recommend insect releases for control of Canada thistle even though insect agents are available. More research is being done in this area.