Blue Mountains National Resources Institute

Wanted: DEAD!

Canada Thistle

Alias: Creeping thistle

• Stems: 1 to 4 feet tall, erect, rigid, branching.
• Leaves: Sessile, deeply lobed, edged with stiff yellow spines.
• Flowers: Many 1/2-inch flowering heads; male and female flowers borne on different plants. Flowers purple, lavender, or white, with overlapping, weakly spine-tipped bracts. Flowers in July and August.
• Seeds: 1/8-inch long, somewhat flattened, tan, with an apical circle of long hairs.

Canada thistle is a perennial that grows from a deep, complex root system. The roots spread horizontally and send up new stems, forming dense colonies. It grows in a wide range of soils and environmental conditions. It is aggressive, spreading by root and seed. The deep root system makes it difficult to control.

Control: An integrated system must be used for many years to control this pest.

• Biological: Competition with grasses and alfalfa is effective, as are seedhead weevils, and the painted lady butterfly. A 3-year intensive cattle grazing program has proved effective in southwestern Colorado.
• Chemical: Fall treatments, combined with mowing, are effective. Get in touch with you local weed control office for the latest information on controlling this pest.
• Mechanical: Mowing can be effective if combined with herbicide application. Plowing increases plant numbers and should be avoided.

Report all sightings to your local Weed Board