Effects of Plant Growth Stage on Chlorsulfuron Suppression of Canada Thistle (Cirsium arvense) Shoots and Roots

Field research was conducted to evaluate the performance of chlorsulfuron {2-chloro- N -[[4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} applied to established Canada thistle plants [ Cirsium arvense L. scop. # CIRAR] in the spring or fall to young vegetative shoots, and in June to plants in the bud growth stage. Canada thistle shoot densities measured in the spring following chlorsulfuron applications the previous year were lower when chlorsulfuron was applied in the bud and fall growth stage as compared to applications made in the spring to young vegetative shoots. Suppression of Canada thistle shoots from chlorsulfuron application during the bud and fall growth stage increased with rate, and was greatest at 269 g ai/ha. Chlorsulfuron applied at 269 g/ha at the fall growth stage only partially controlled the root system of Canada thistle.