Weed Department
Idaho State Noxious 
Weed Law
Canada Thistle

   Interesting Facts

  • Contrary to the name, Canada thistle did not come into the U.S. from our northern neighbor, but rather was introduced in the 17th century from the Mediterranean region and southeast Europe.
  • Tea made from Canada thistle leaves has been used as a diuretic as well as for treatment of tuberculosis
  • It is the only thistle with male and female flowers on separate plants
  • Roots can branch out to 15 ft. wide and deep.
  • Seeds are winged and are easily dispersed by wind
  • New plants can develop from small broken pieces of the plant
  • Disperses a chemical that may inhibit growth of other plants
  • Seed may remain viable in soil up to 20 years


Canada thistle is an herbaceous perennial in the aster family with erect stems, prickly leaves, and an extensive creeping rootstock.


Canada thistle is found in open areas with a moderate amount of moisture but does poorly on wet soils lacking sufficient oxygen. It can grow on many different soil types but it does not grow well in shade and is rarely found within wooded sites, except in clearings. It is commonly found in abandoned fields or lots, abandoned gravel pits, pastures, right-of-ways, roadsides, railway embankments, lawns, gardens, and agricultural fields. It also invades wet areas with fluctuating water levels such as stream banks or irrigation ditches and sloughs.