Canada Thistle (Cirsium arvense)

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Canada thistle is a member of the aster family with stems growing as tall as four feet, prickly leaves and a horizontal root system. Dark green leaves varying from oval-shaped to arrow-shaped alternate along the stem, which is branched, slightly hairy, and ridged. Flowers appear from June through October in umbrella-shaped clusters with colors ranging from rose-purple to lavender to sometimes white. Seeds are called achenes and vary from 1-4.5 inches, and have feathery structured base.

Canada thistle was originally from Eurasia around the Mediterranean and introduced into the United States in the 1600's. Today, it is distributed throughout the central region of North America, from northern California to the southern parts of Canada. Habitats of Canada thistle varies. Barrens, glades, meadows, prairies, fields, pastures, and waste places are just some of the places it may be found. Though it prefers disturbed upland areas, it sometimes will be found in areas such as streambank sedge meadows and wet prairies.

Wind is one of the major distributors of Canada thistle seeds. These seeds can stay viable for up to twenty years, but most germinate with a year. While its horizontal root system spreads across the ground, shoots will sprout from these roots producing new thistles. Also, new thistles can be regenerated from root fragments less than an inch in length.

Because of it extensive root system and ability to regenerate, Canada thistle will crowd out native plants in areas it has invaded. As it spreads, it takes away the sunlight of smaller plants as well as the nutrients and water from the soil. There is a possibility that it may also release a chemical toxin that is poisonous to other plants.

Canada thistle has been declared as a "noxious weed" since the 1950s and has cost millions of dollars in crop losses and additional millions for control.

Want to Help Us Better Understand the Park?

See our iNaturalist project, "The Life of the Mississippi National River and Recreation Area" and contribute to it by downloading the iNaturalist app and uploading your sightings of this species, and others, to the project. You can also upload your sightings from your computer.

Last updated: March 22, 2018

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