It was introduced to Canada probably as a crop seed contaminant before 1800. It is an aggressive weed that spreads both by seed and extensive root systems. Leaves are wavy margined to lobed , up to 6 inches long, and armed with yellowish spines.
Stems are single, branched near the top, ridged, and hollow.
Flower heads are borne in midsummer, inch in diameter, and are not particularly spiny. Flowers are purple to lavender, occasionally white, with male and female flowers borne on separate plants. The upper part of each bract narrows into a short, stiff spine. Canada thistle is present is every county in Idaho. It is one of Idaho's most widespread and damaging noxious weeds.