Lateral roots 3 or more feet deep spread from a fibrous taproot. Aerial shoots are sent up at 2 to 6 inch intervals. Basal leaves are produced the first year, flowering stems the next. Pollination is mostly by honeybees, and wind pollination is limited. Most seeds germinate within one year.
Some seeds immediately produce rosettes before winter and emerge to flower the next spring.
Seeds remain viable in soil up to 20 years in some cases. As frequency of Canada thistle increases at a site, species diversity decreases, possibly due to allelopathic substances. Canada thistle is an alien species capable of crowding out and replacing native grasses and forbs. Prairies, barrens, savannas, and glades are susceptible, particularly those sites that have been disturbed and are reverting naturally to native species, as well as those undergoing manipulative restoration management. Current Status Canada thistle is a noxious weed under Illinois law.