Invasive Plants in Northern Virginia: Canada Thistle

By Elaine Mills, Extension Master Gardener

Flowers are pink and bristly.
Photo: Oregon State University

Cirsium arvense, (Canada thistle), a native of southeastern Europe and the Mediterranean, was brought to North America in early 1700s, probably as a contaminant of crop seed.  By the end of the century is was already recognized as a noxious weed in crops, and it is now recognized as invasive in open natural areas, such as fields, meadows, wet prairies, and even inhospitable sand dunes, through most of the United States.

This member of the sunflower family is an herbaceous perennial with erect stems growing 2 to 5 feet tall. It has spiny, irregularly-lobed leaves and numerous tubular lavender flowers clustered together into a head that bloom from June to August.

Canada thistle is introduced to new areas via prolific seed production. In sunny locations, a single plant can produce over 40 flower heads and 1,500 seeds in a season. These abundant seeds have plumes of silky hairs that facilitate their dispersal by wind. The seeds can also be dispersed by water, withstanding months of immersion. They remain viable in the soil for up to 20 years.

Noxious weed Canada thistle.
Photo @ 2014 Jim Kennedy

Once it establishes a foothold, Canada thistle spreads rapidly by cloning. Its vigorous root system, measuring 3 feet deep and wide, sends up new shoots at 2- to 6-inch intervals from lateral roots. The resulting monocultural stand crowds out and displaces native grasses and forbs, reducing important pollinator habitat. The plant also readily regenerates from root fragments less than 1 inch in length, even when fully buried. In addition, Canada thistle can change morphology in response to environmental conditions and produce toxins that inhibit the growth of neighboring plants.

Canada thistle is best controlled by combining a variety of strategies. A number of insect pests and diseases are being studied as possible biological controls. For details, see Canada thistle (Cirsium arvense) from the Introduced Species Summary Project at Columbia University.


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