Canada Thistle

Canada thistle (Cirsium arvense)

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 U.S. states and the Canadian provinces. It is a detriment to the environment and economy of Colorado, negatively affecting agriculture and natural areas. See comprehensive information on the biology of this species.

Canada thistle is a common problem on roadsides, rangeland, pastures, farmland and natural areas in Larimer County. This aggressive perennial can develop an incredibly extensive root system in a short amount of time (figure 2.). The underground rhizomes aggressively compete for moisture and nutrients feeding above ground growth that can out-compete many plants for sunlight. Canada thistle is an expensive weed problem in corn, barley, wheat and other local crops, significantly reducing farm production.

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 2-3 years, but subsequent follow-up spot treatments may be necessary for several years. Best management recommendations:

Cultural control - 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 counter-productive. 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. The most effective herbicides for controlling Canada thistle:

Milestone - provides excellent control, has low toxicity, and can be applied to water's edge.
Transline and pre-mixes with Transline - Redeem and Curtail. These products have all shown excellent results for Canada thistle control in University field trials.
Tordon is very effective for control of Canada thistle but is a restricted use product requiring certification for purchase and use.
Telar alone provides fair control, but when tank-mixed with any of the above products produces excellent results.
Do not use Roundup or any other glyphosate products unless treating individual Canada thistle plants. Glyphosate is non-selective and can kill or injure surrounding grass or other desirable vegetation.

Other management methods:

Hand pulling/clipping - not effective for controlling Canada thistle infestations unless one is extremely persistent (see root box photos). 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. Experts with Colorado Department of Agriculture, Colorado State University and USDA do not recommend insect releases for control of Canada thistle even though insect agents are available.