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Noxious Weeds

Canada Thistle

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.

Growth Habit: Perennial, erect, up to 4 feet tall. Forms dense colonies with roots that grow horizontally sending up shoots along their length.

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.

Flower: Small bristly clusters, 3/8 to 5/8 inches in diameter, light lavender to deep rose purple. Plants are male or female. Asexual reproduction makes it possible for a colony of male plants to maintain itself. Flowering occurs June to August.

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

Roots: Extensive, fleshy, creeping rootstocks. The roots produce numerous new plants each spring. Broken roots can grow new plants so plowing or roto-tilling will increase the population. Although the plants may appear to be separate they are typically connected by an underground root system.

Status: State Designated Noxious Weed, Category 4 Fremont County. Widespread infestation, control and maintenance of current weed infestations.

Control: Mechanical controls are mostly ineffective. Herbicides are usually the recommended treatment. The Canada thistle stem gall fly is a biological control that has shown some effectiveness in Fremont County at suppressing this weed. FCWP offers a 40 percent Cost Share on chemicals for controlling this species and may have biological controls available. Contact your local weed and pest office for further treatment recommendations.

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Mary Ellen (Mel) Harte, Bugwood.org

Bonsak Hammeraas, NIBIO, Bugwood.org

GYCC/FCWP