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.
Herbicides are available that control the weed if consistently used.