Canada Thistle

Cirsium arvense

Species distinguishing characteristics: 

  • Prickly stems and stalkless leaves
  • Extensive creeping rhizomes
  • Purplish-pink flowers smaller than 2-5 cm across
  • Bracts below the petals lack sharp spines

Family Characteristics: 

  • Multiple layers of bracts beneath the flowers
  • Flower heads composed of many smaller (often tiny) flowers, each of which produces an individual seed
  • Flowers may contain disk florets, such as those in the yellow center of a daisy, and/or petal-like ray florets
  • Undivided leaves
  • Includes the food plants lettuce, artichoke and endive

Growth habitat: 

Non-native, noxious weed.  Perennial up to 4 feet (1.2 m) tall that often forms colonies from deep, spreading roots.  Produces a non-flowering rosette the first year.

Leaves and stems: 

Stalkless, alternate leaves, 5 to 15 cm long.  Leaves are prickly and spine-tipped with a wavy surface, toothed margins, and irregularly shaped lobes.  Soft woolly hairs often cover underside of leaf.  Tremendous leaf variability - some varieties have entire, spineless leaves.  Hollow, leafy stems lack wings, branch near the top, and become hairy with age.

Flowers: 

Clusters of pinkish purple (or occasionally white) flowers attach to top of branches or emerge from the joint where leaves attach to the stem.   Directly below the flower petals are several rows of overlapping floral bracts, or tiny leaf-like structures, that lack sharp spines.  The Canada thistle is the only thistle with female and male flowers appearing on separate plants.  Female flowers have a pleasant vanilla-like aroma.

Roots: 

Deep, creeping underground stems, or rhizomes, allow the plant to survive below the cultivation zone.  The rhizomes bear buds that will produce new shoots in future growing seasons.  A one-year old plant may have as many as 200 buds.

Seeds: 

Light brown seeds, 3 to 4 mm long, with feathery white plume attached.  Each plant may produce over 40,000 seeds that remain viable for up to 21 years.\

Interesting facts: 

Introduced from Southeastern Eurasia in contaminated crop seed around the late 18th century.  Provides a good source of pollen and nectar for honeybees.  The downy seed fluff makes good tinder for starting fires.

Biological Classification: