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Canada thistle

Quick facts

Canada thistle is an invasive species. Canada thistle is on Control noxious weed list meaning you must prevent the spread of this plant.

  • Canada thistle is found in natural areas such as prairies, savannas, glades and dunes where there has been disturbance.

  • It also prefers wet areas with fluctuating water levels, such as streambanks, sedge meadows and wet prairies.

  • Once established, it spreads quickly and can diminish native plant diversity.

Canada thistle should be reported. The Minnesota Department of Natural Resources provides detailed recommendations for reporting invasive species.

How to identify Canada thistle

  • Canada thistle (Cirsium arvense) is a perennial herbaceous plant, one to five feet tall.

Stem

  • Slender, smooth, grooved stems that sometimes have short hairs.
  • Stems do not have conspicuous spines.

Leaves

  • Alternate, smooth, dark green, irregularly lance shaped, tapering and deeply divided.
  • The leaves have prickly wavy margins; directly attached to the stem.
  • Leaf shape varies across different varieties and subspecies.

Flowers

  • Small (four tenths to six tenths of an inch diameter) flask-shaped purple-pink-white female flowers appear on top of the upper branched stems between June and September.
  • Male flowers are smaller and more globe shaped than female flowers.
  • Female flowers are fragrant, male flowers are not.

Seeds

  • Tiny (a tenth of an inch) light brown, slightly tapered seeds are tufted for dispersal by the wind.
  • Seeds survive in the soil for over 20 years.

Roots

  • Each plant has a fibrous taproot, up to six feet deep, with wide spreading horizontal roots.
  • It grows in circular patches, spreading vegetatively.
  • Roots that can grow 10–12 feet per year.
  • Each small section of root can form a new plant.
leaves on Canada thistle
seeds on Canada thistle

 

 

Angela Gupta, Extension educator; Amy Rager, Extension educator; Megan M. Weber, Extension educator

Reviewed in 2019

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