Paul Drobot, University of Wisconsin
Canada thistle (Cirsium arvense) is a tall (two-four feet) perennial herbaceous plant. Leaves are large with sharp, spiny edges. Flowers are bright or deep purple, bristly clusters and bloom continuously through the summer. Mature flower heads produce thousands of small, feathery seeds and are easily dispersed by wind.
This plant is native to Eurasia and North Africa. In the 1600s it was introduced in the United States, most likely by accident, in a seed or crop mixture. Canada thistle easily establishes itself in disturbed areas with exposed soil. These areas include old building sites, gravel pits, and roadsides. Beaver ponds are also excellent breeding grounds for this species.
Many biological factors make Canada thistle a successful invader. Sharp, spiny leaves discourage animals from eating.Deep, horizontal roots, up to 12’ long, can easily out-compete native plants for water. One plant can produce thousands of seeds annually, which are easily spread by birds and wind. Seeds can also remain dormant in the soil for 20 years.
Voyageurs National Park recognizes the negative effects canada thistle has on the ecosystem.Resource managers are taking steps to eliminate this plant from the park.The most effective removal method is herbicide application.
How you can help:
Please help us prevent the spread of canada thistle in Voyageurs National Park. Inform a park naturalist if you find this plant within the park.
For additional information on canada thistle, click on the link below to visit the Minnesota Department of Natural Resources website: MN DNR – Canada Thistle