Description:

CANADA THISTLE — Cirsium arvense (L.) Scop.

Asteraceae — (Sunflower family)

Canada thistle is a native of Southeastern Eurasia. It was introduced to Canada as a contaminant of crop seed in the late 18th century. An aggressive weed that spreads both by seed and extensive root systems, Canada Thistle first establishes itself in disturbed soils. A colony-forming perennial whose roots grow horizontally sending up shoots along their length, Canada Thistle forms dense stands with erect stems which are one to four feet tall, ridged, and branching above. Leaves are alternate, lacking petioles, oblong or lance-shaped, divided into spiny-tipped irregular lobes.

The base of the flower head lacks spines. Canada thistle differs from other species of the true thistle in that there are male and female flower heads, and these are on separate plants. Asexual reproduction makes it possible for a colony of male plants to maintain itself. Flowers are purple (occasionally white) in heads ½ to ¾ inch in diameter; involucral bracts spineless. Fruits are about 1/8 inch long, somewhat flattened, brownish, with a tuft of hairs at the top. Flowering occurs during June and August.

Growth Habit: Perennial, erect, up to 4` tall.

Leaves: Varies from light to dark green, oblong or lance shaped, deeply cut, spiny toothed margins (some may be smooth); slightly hairy below. Tremendous leaf variability.

Stems: Smooth to slightly hairy, branched at top.

Flower: Small bristly clusters, 3/8 to 5/8 " in diameter, light lavender to deep rose purple. Plants are male or female.

Roots: Extensive, fleshy, creeping rootstocks.

Seeds: Smooth, light to dark brown, tipped by a cupped conical point, approximately 1/8" long.

Other: Reproduces by seed and creeping rootstocks.

Control Strategies:

This aggressive weed is difficult to control due to its complex root system.  Continual cropping (2-3 times per year for three to four years) can weaken the plant. Mechanical control is not effective, i.e. breaking up roots by tillage generally serves only to spread the plant.  Reseeding of desired grasses after disturbing an area may reduce infestations of Canada Thistle.  Canada thistle is usually voted North America's number one weed as a result of aggressive campaigning by weed control authorities over the last 50 years. Actually, Canada thistle is seldom economic on rangeland. By that we mean that it does not interfere with grazing operations even though it is present. It is a different story on crop land where the ground is plowed every year. With annual tillage Canada thistle can totally dominate a field. There are many effective herbicides. Glyphosate treatments in the fall are very effective. Two quarts of 41% glyphosate per acre in the fall, followed by plowing in two weeks usually provide 90% control. If you can't plow, increase the rate to three quarts per acre. For spots use a 2% solution. That is two gallons of glyphosate in 100 gallons of water and spray to wet.  For use on pastures and rangeland, broadleaf herbicides take out the thistle and leave the grass.  Transline and Stinger at a pint per acre, Redeem at two quarts per acre, and Curtail at three to four quarts per acre contain the same active ingredient and will provide 90% control with a single treatment. A new herbicide, Milestone will do at least  that good at five ounces per acre. For crop recommendations please call the Weed and Pest District, read the product label, or consult the University of Wyoming Herbicide Manual.

Biological Control:

A number of biological control agents have been released on Canada thistle, but to date none actually result in measurable control with a single exception where Ceutorhyncus litura seems to be responsible for a decline of Canada thistle growing on wet sites. Since it does not reduce thistle on drier ground it is speculated that diseases associated with the wet soils are introduced by the insect's mining of the stem and roots of the plant. While the search continues for a more effective biological control agent, prospects are dim because Canada thistle is weedy in all of the temperate zones of both the Northern and Southern hemispheres.

To learn more on biological control for Canada Thistle visit this link.