Posted on September 19, 2009 - by mckenzie

Noxious Weeds Part III: Canada Thistle

Canada Thistle

Canada thistle is found in the northern U.S. and southern Canada. Canada thistle is present is every county in Idaho. It is one of Idaho’s most widespread and damaging noxious weeds.

Canada thistle is a perennialPlant that lives for more than 2 growing seasons plant up to 5 feet tall. Leaves are wavy marginedmargin: edge of the leaf to lobedA cut into a leaf from the edge toward the center; greater than toothed, but not quite compound, up to 6 inches long, and armed with yellowish spines. Stems are single, branched near the top, ridged, and hollow. Flower headsA group of flowers borne tightly together are borne in midsummer, ½ inch in diameter, and are not particularly spiny. Flowers are purple to lavender, occasionally white, with male and female flowers borne on separate plants. Seeds are slender, tan, 1/8 inch long, and bear fine plumesA hairlike or featherlike structure, often on a seed.t brown comblike marginThe edge of the leaf. The upper part of each bract narrows into a short, stiff spine. Seeds are brown to gray in color and are tipped by plumesA hairlike or featherlike structure, often on a seed that fall off at maturity.

Natural communities that are threatened by Canada thistle include non-forested plant communities such as prairies, barrens, savannas, glades, sand dunes, fields and meadows that have been impacted by disturbance. As it establishes itself in an area, Canada thistle crowds out and replaces native plants, changes the structure and species composition of natural plant communities and reduces plant and animal diversity. This highly invasive thistle prevents the coexistence of other plant species through shading, competition for soil resources and possibly through the release of chemical toxins poisonous to other plants. 

Canada thistle is declared a “noxious weed” throughout the U.S. and has long been recognized as a major agricultural pest, costing tens of millions of dollars in direct crop losses annually and additional millions costs for control. Only recently have the harmful impacts of Canada thistle to native species and natural ecosystems received notable attention.

Canada thistle was introduced to the United States, probably by accident, in the early 1600s and, by 1954, had been declared a noxious weed in forty three states. In Canada and the U.S., it is considered one of the most tenacious and economically important agricultural weeds, but only in recent years has it been recognized as a problem in natural areas.

Management of Canada thistle can be achieved through hand-cutting, mowing, controlled burning, and chemical means, depending on the level of infestation and the type of area being managed. Due to its perennial nature, entire plants must be killed in order to prevent regrowth from rootstock. Hand-cutting of individual plants or mowing of larger infestations should be conducted prior to seed set and must be repeated until the starch reserves in the roots are exhausted. Because early season burning of Canada thistle can stimulate its growth and flowering, controlled burns should be carried out late in the growing season for best effect.

In natural areas where Canada thistle is interspersed with desirable native plants, targeted application of a systemic herbicide such as glyphosate (e.g., Roundup® or Rodeo®), which carries plant toxins to the roots, may be effective. For extensive infestations in disturbed areas with little desirable vegetation, broad application of this type herbicide may be the most effective method. Repeated applications are usually necessary due to the long life of seeds stored in the soil.