Stewardship Overview: Integrated control options for natural areas managers are constrained by the need to restrict damage to native species. Although it appears that burning in June (Jaeger pers. comm.) or later (Smith 1985) is more effective for thistle control that early burning, this practice is also more harmful to native species.
It appears that the best available option for control of thistle in native prairies is to strengthen the component of native species by spring burning and follow-up by cutting or spot application of glyphosate on Canada thistle in late bud or early bloom.
Species Impact: Cirsium arvense competes with crops for moisture, nutrients, and light. It is responsible for millions of dollars of direct crop loss annually, with additional costs for control (Hodgson 1968, Messersmith 1978, Wilson 1980). Its spiny nature and patchy growth render infested pastures unusable to livestock. It interferes with the harvest of horticultural crops (Boldt 1981). It harbors destructive insects and pathogens (Linck and Kommedal 1958) and has been reported to have an allelopathic effect on sugar beet, wheat, alfalfa, corn, edible beans, and flax (Wilson 1981, Helgeson and Konzak 1950). Studies in Colorado indicate that species diversity in an "undisturbed" study area was inversely proportional to the relative frequency of Canada thistle (Stachion and Zimdahl 1980).
Within the context of natural areas, Canada thistle is more often a legal and public relations issue than a biological threat. However, in the west it can invade areas that are subject to heavy deer and elk grazing and areas newly exposed as a result of control of another weed, the tansy ragwort (MacDonald pers. comm.). In the Midwest it can invade established prairie where a mixture of blown-in snow and dirt leaves annual accumulations of new soil surface (Heitlinger pers. comm.), where surface runoff results in accumulation of litter, where erosion creates newly exposed soil surfaces (Winter pers. comm.), and where mist application of nonselective herbicides has set back succession (Jaeger pers. comm.).