Canada Thistle (Cirsium arvense) is found in most regions of Minnesota and is listed on the Minnesota Department of Agriculture Prohibited Noxious Weed Control List. Any species on this list must be controlled to prevent the spread of its seeds and other propagating parts. It also means that there is no propagation, transportation, or sale of these species allowed.
Introduced to North America during the 1600–1700s when agricultural seed shipments arrived from Europe, Canada Thistle now grows throughout North America in disturbed landscapes. These landscapes can include trails, roadsides, pastures, natural areas, and unmaintained gravel pits.
Canada Thistle can grow two to five feet in height. Small rosettes grow in the fall or early spring and develop into an erect flowering stem. The leaves alternate, with deep divisions and spines along the edges.
It develops pink to purple-colored flowers from mid-June to August that turn into white, fluffy tufts when the plant goes into seeding. These seeds travel by air and can survive in the soil for 20 years or more. This plant has a fibrous taproot with horizontal spreading roots. A new plant can develop from these horizontal roots, which allows the plant to spread vegetatively.
Similar-looking species to Canada Thistle include Spotted Knapweed, Marsh Thistle, Bull Thistle, and Swamp thistle. Canada thistle can be identified from these other species by the size of its flower, the texture of the stem, and the size, shape, and presence of spines of the leaves. It is easily identifiable during the summer months when it is in bloom.
Prevention and management are the means to help eradicate this species. A management plan can include mechanical and chemical controls. Mechanical controls include hand pulling, digging out, or mowing the plant or affected area. If hand pulling or digging out the plant, leave the plant on-site to decompose or follow the Minnesota Department of Agriculture guidance for plant disposal. Try to remove as much of the root as possible of this plant. If mowing, do not mow when seed is present as it will help spread the seeds into new areas.
Chemical controls include applying a foliar herbicide to the plants in the fall and spring. Treatment may take a few years to eradicate Canada Thistle. It is important to monitor the affected areas for any sign of Canada Thistle regrowth.
The presence of Canada Thistle in the landscape can decrease the diversity of the plants in an area and cause challenges in restoration projects. It can also decrease the amount of forage available for livestock in pastures. Due to its growth and ability to spread quickly, it can be hard to manage disturbed areas. It can replace native plants and become a dominant species if not controlled. With a sound management plan in place, native vegetation will grow back in the areas and replace the Canada Thistle.
If you need help to identify the species, contact our office at 507-332-5408. To learn more about how to control Canada Thistle and other invasive species, visit our website at riceswcd.org/invasive-species-and-noxious-weeds.