Canada Thistle (Cirsium arvense)

Canada thistle gone to seed, Elm Rock Farm, August 2017

Other Common Names:

  • Creeping thistle, field thistle, perennial thistle, prickly thistle.

Characteristics:

  • Herbaceous perennial plant, up to 1.5m tall.
  • Stem is hollow, slightly ridged and glabrous (hairless), or sometimes with scattered hairs.
  • Leaves are sessile (attached directly to stem), clasping, simple, alternate, lobed, with spiny, wavy margins, and up to 20cm long. Upper leaves are narrower, and usually more toothed than lobed. Leaves can be somewhat hairy on the underside.
  • Bracts (leaf-like structures directly beneath flowerhead) are spineless.

Reproduction:

  • Flowerhead is pink-purple, up to 2cm in diameter.
  • Usually dioecious (male and female flowers are on separate plants), but some plants bear hermaphroditic flowers. Male and female flowers look very similar.
  • Primarily insect-pollinated.
  • Seeds are up to 5mm long, with feathery, cottony hairs that aid in wind dispersal.
  • Spreads primarily by its creeping root system, forming large clonal colonies.

Similar Species:

  • The flower heads of other Cirsium have bracts with spines, but the bracts of Canada thistle lack these spines.
  • Canada thistle and Flodman’s thistle are the only two local Cirsiums that form large colonies via creeping roots. Flodman’s thistle is differentiated from Canada thistle by its wooly stem.
  • The stem of bull thistle has spiny wings, absent in Canada thistle. Also, the leaves of bull thistle are covered with soft hair, while the top of Canada thistle’s leaves are hairless. The upper surface of bull thistle is spiny. Bull thistle flower heads are larger, 2-4 cm in diameter.
  • Gray (wavyleaf) thistle has a woolly stem, and leaves with a lightly wooly upper surface, as opposed to the glabrous stems and upper leaf surfaces of Canada thistle.
  • Burdock has similar flower heads to thistle, but is differentiated by its large, spineless leaves.
  • Sowthistle and wild lettuce both have spiny leaves like the Cirsium species, but the flowers are yellow. Also, the leaves and stems of sowthistle and wild lettuce exude a white sap when damaged, unlike thistle.

Ecological Role:

  • Seeds are an important food source for many birds
  • Leaves are eaten by many insects.
  • Flowers provide nectar and pollen for many insects.

Edibility:

  • Taproot
  • Young leaves
  • Stalks

Other Uses:

  • Cottony seeds can be used as a fire starter
  • Has been used to coagulate milk

Etymology:

  • “Cirsium” is Latin for Greek “kirsion,” likely from “kirsos,” meaning “swollen veins.” The plant was apparently used as a remedy for swollen veins.
  • “Arvensis” is Latin for “from the field, meadow, or grassland.”

Other:

  • National flower of Scotland
  • “Order of the Thistle” is a Scottish order of chivalry.
  • Brought to North America in the 17th century.
  • The common name is misleading, as the species is native in Europe and northern Asia, but not in Canada.