Our objective was to maximize Canada thistle control and plant community diversity in a waterfowl production area administered by the U.S. Fish and Wildlife Service. We tested three rates (1.5, 3.0, and 4.5 ai/ha) of glyphosate applied during spring, summer, or fall using two application methods. The lowest rate of glyphosate decreased the Canada thistle density by about 30% relative to the control. Glyphosate applied in the fall decreased Canada thistle density below that of the control more consistently than when applied in spring or summer. Wick application generally resulted in less Canada thistle biomass than did broadcast application. Species richness was generally higher when glyphosate was wick applied, and all rates of this application method increased species richness when compared with the control. We recommend fall wick application of glyphosate at 1.5 kg ai/ha to control Canada thistle near the riparian areas. This application provided optimum Canada thistle control, while maintaining species richness important for waterfowl.
Nomenclature: Glyphosate; Canada thistle, Cirsium arvense L. #3 CIRAR.
