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Canada Thistle Stems: 1-4 ft 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: 1/8th-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 three-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. Source: USDA Forest Service