Canada Thistle

 

Canada Thistle closeup.

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

Description

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

  • Stems are branched, often slightly hairy, and ridged
  • Leaves are lance-shaped, irregularly lobed with spiny, toothed margins and are borne singly and alternately along the stem
  • Flowers are rose-purple, lavender, or sometimes white flower and occur in rounded, umbrella-shaped clusters
  • Seeds are tan curved and has a papery covering

Habitat

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.

Ecological Impacts

Natural communities that are threatened by Canada thistle include non-forested plant communities such as prairies, barrens, savannas, glades, sand dunes, fields and meadows that have been impacted by disturbance. This highly invasive thistle prevents the coexistence of other plant species through shading, competition for soil resources and possibly through the release of chemical toxins poisonous to other plants.

Control

  • Mechanical: Hand cutting or mowing with a brush type mower can be an effective method for control. Special care must be considered to remove any fragments of cut plants since they can re-sprout in some situations.
  • Chemical: It can be effectively controlled using any of several herbicides that include aminopyralid, aminocyclopyrachlor, clopyralid, and picloram mixed alone or in combination with 2,4–D. Because of the difficulty in getting enough root uptake of herbicides, application timing is important.