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. 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. As such, all landowners are required to control the plant if it is growing on their property.
Control is considered to be prevention of seed production.
County prosecuting attorneys are required to notify offending landowners in a prescribed manner before leveeing penalties. Control Recommendations Recommended Practices in Natural Communities of High Quality Prescribed fire can be effective in controlling this species and is a preferred treatment. Late spring burns, between May and June, are most detrimental to this noxious weed and should be used when possible. Prescribed burns to control this plant should not be conducted early in the spring, as early spring burns can increase sprouting and reproduction of this species. During the first 3 years of control efforts, burns should be conducted annually.