Canada thistle flowers

Canada thistle (Cirsium arvense)

Herbaceous perennial, 2-6.5’ tall with upright, grooved stems that branch near top of plant. The stems are hairy.

Overview

Regulated areas of Canada thistle
Canada thistle is Restricted (Orange counties)

Other names for this plant include:

  • Common names: creeping thistle, field thistle, perennial thistle
  • Scientific names: Carduus arvensis; Cirsium incanum

Ecological threat:

  • Invades undisturbed areas such as prairies, savannas, glades, dunes, streambanks, sedge meadows, and forest openings. Also invades croplands, pastures, lawns, gardens, roadsides, ditches, and waste sites.
  • Once it has established it spreads quickly, forming monocultures.

Classification in Wisconsin: Restricted

Species Assessment Groups (SAG) were assembled to recommend a legal classification for each species considered for NR 40. The recommendation for Canada thistle was based upon this literature review developed by the department.

Identification

Leaves: Simple, alternate, lance-shaped, tapering, irregularly lobed, with spiny, toothed margins, stalkless. Green on both sides; smooth early but becoming pubescent with maturity.

Flowers: Numerous, small (0.5-0.75” wide), purple to pink (rarely white) terminal flower heads. Bracts have spineless tips. Blooms June-September.

Fruits & seeds: Small, light brown with a tuft of tan hairs loosely attached to the tip to enable wind dispersal. Seeds are often spread by mowing after flowering has begun.

Roots: Reproduces clonally by creeping roots that grow laterally in soil, up to 10-12’ per year. Also produces taproots that may grow more than 6’ deep. Readily regenerates from root fragments.

Similar species: Canada thistle is distinguished from all other thistles by creeping lateral roots, dense clonal growth; and small dioecious flower heads(meaning male and female flowers are produced on separate plants; however, it is difficult to distinguish the two flower types based on appearance).

Other invasive thistles include: European marsh thistle (Cirsium palustre) and Canada thistle (Cirsium arvense).

Distribution

Known county distribution of Canada thistle
Counties in WI where Canada thistle has been reported (as of July 2011). Both vouchered and unvouchered reports included.

Do you have Canada thistle in your county but it isn't shaded on the map? Send us a report.

Control

Mechanical: Repeated pulling and mowing (minimum 3 times per growing season) weakens roots; mow when flower buds are formed, but have not yet opened. Late spring (May/June) burns for 3 consecutive years stimulates seed germination and kills seedlings. Later season burns are needed because early season burning can stimulate plant growth and flowering.

Chemical: Foliar spray glyphosate during the early bolting phase when plants are 6-10” tall, during the bud to flower phase, or rosettes in the fall; foliar spray with clopyralid or metsulfuron-methyl.

Biological: Stem weevil (Ceutorhynchus litura), bud weevil (Larinus planus), stem gall fly (Urophora cardui), and foliage feeder (Cassida rubiginosa).

For more information on control techniques, visit the Canada thistle factsheet [exit DNR] by University of Wisconsin-Extension.

Photos

View Canada thistle pictures in our photo gallery!

Resources

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Last revised: Wednesday June 05 2013