Canada thistle should be accurately identified before attempting any control measures. It is scattered throughout the northern two-thirds of Missouri. Missouri distribution is poorly documented due to a lack of collecting of weedy thistles by botanists. It can invade sedge meadows and wet prairies from adjacent disturbed sites. This thistle does not do well in undisturbed prairies, good to excellent pastures, or in woodland.
Plants are tall and lax, with few flowers, on sites that are shaded most of the day.
This dioecious, weedy perennial occurs in patches, commonly in disturbed areas. Introduction to new areas occurs mostly by windborn seed or sometimes by run-off in ditches. It spreads rapidly by rhizomes or root segments. Lateral roots 3 or more feet deep spread from a fibrous taproot. Aerial shoots are sent up at 2 to 6 inch intervals.