Canada Thistle

Canada Thistle, courtesy of Al Schneider, Southwestern Colorado Wildflowers
© Al Schneider, www.swcoloradowildflowers.com 
Used by Permission
 
Common Name(s):

Canada Thistle

Scientific Name:

Cirsium arvense (L.) Scop.

Scientific Name Synonyms:

Carduus arvensis (L.) Robson

Symbol:

CIAR4

Description:

Life Span: Perennial

Origin: Introduced

Season: Cool

Growth Characteristics: A colony-forming weed, growing from deep and extensive horizontal roots, and forming dense clones. Stems are 1 to 6 ½ feet tall, rigid, and branching above. Flowering occurs during July and August. It reproduces mainly from rhizomes, but can also reproduce by seed.

Flowers/Inflorescence: Flowers are unisexual (found on separate plants). Flowers are purple, occasionally white, in head ½ to ¾ inch in diameter, clustered at the top of the stems. Bracts below flower are spineless, and not painful to touch.

Fruits/Seeds:Seeds are somewhat flattened, brownish, with a tuft of hairs at the top. They grow to 1/8 long.

Leaves: Alternate placement on stem, lacking petioles, oblong or lance-shaped, divided into spiny-tipped irregular lobes.

Ecological Adaptations:

Canada thistle establishes and develops best on open, moist, disturbed areas, including ditch banks, overgrazed pastures, meadows, tilled fields or open waste places, fence rows, roadsides, and campgrounds; and after logging, road building, fire and landslides in natural areas. It differs from other species of the true thistles in that there are male and female flower heads, and these are on separate plants.

Soils: Adapted to a broad range of soils.

Associated Species: Widespread

Uses and Management:

Canada thistle has little or no forage value for livestock. It can be a minor component of the winter and spring diet of mule deer, and is used somewhat by bears.

Canada thistle is a listed noxious weed in Utah. It is very aggressive and difficult to control. Breaking up the roots by plowing only increases the number of plants.

When reproducing asexually, it is possible that a colony of male plants could produce no fruit, but still maintain itself.