Canada Thistle Control

Abstract

Canada thistle (Cirsium arvense) control is easier once its biology and physiology are understood. This weed is a perennial which spreads by seed and underground roots. It is dioecious, so not all plants produce seed. Canada thistle occurs in the Northern Temperate Zone due to day length response and a high temperature limitation on growth. Flowering occurs when day length is 14 hr or more and temperatures are high enough. There is considerable variability in types of Canada thistle, and they can differ in their response to herbicides. Canada thistle seedlings can become perennial within three weeks after germination, producing buds on roots. Buds can develop at any location on the root and at any time if environmental conditions are favorable. Root bud growth was maximized when air temperatures were 15/5 C and root temperatures were 20 C under a 13-hr day length. As soil fertility increases, so does Canada thistle growth. Photosynthesis was as great in October as in June. Several herbicides are effective when applied to Canada thistle regrowth in the fall. However, on rangeland, soil moisture is often limiting in the fall and no regrowth occurs; hence, the best time for control is at or before flower bud stage, normally in June.