Common name: Canada thistle
Scientific name: Cirsium arvense L. Scop.

Life Cycle: Perennial
Origin: Eurasia
Poisonous: No

Canada thistle

Canada thistle

Canada thistle is distributed across the northern United States and Canada. It's listed as noxious in many states and classified as a prohibited noxious weed seed in many state seed laws.

This invasive species can grow up to 4 feet tall and is frequently found in pastures, rangeland, and along roadsides. This perennial thistle reproduces from seeds, on underground crown buds of the parent plant, and on adventitious buds on creeping roots. Purple- to rose-colored flowers form at the end of stems and are about one inch in diameter. Flowering occurs from July through August. Unlike many other thistles, Canada thistle is dioecious, which means one plant has either female or male flowers but not both.

Canada thistle is native to Europe, western Asia, and northern Africa. It was first introduced into North America as an impurity in seeds imported into Canadian provinces, hence its name. The weed was first known to be in the United States in the early 1700s. Canada thistle grows rapidly and spreads readily in pastures. It decreases available forage, because it inhibits grazing due to sharp spines on leaves and stems.

Controlling Canada thistle in pastures is very difficult. Mowing and tillage are ineffective, and treatment with herbicides requires multiple applications. Consult your local Cooperative Extension Service personnel for a list of herbicidal controls in your area.

William W. Witt, PhD, a researcher in Plant and Soil Sciences, provided this information.

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