Rosettes reestablish during summer and early fall. The best time to treat biennial thistles with herbicides is in late fall or early spring when the rosettes are present but before flowering stalks are initiated. Musk thistle and bull thistle plants with seed stalks are more difficult to kill than the rosettes. Thistle rosettes need to be treated when they are actively growing and not under drought stress. The younger the rosette, the more susceptible it is to herbicide.
Several herbicides provide good to excellent control of thistle rosettes.
Fall treatments should be made late enough to kill all rosettes germinated before winter. Late germinating rosettes that establish after early fall herbicide applications could flower the next growing season. Spot treatment only or when Canada thistle is present. Forage legumes will also be killed. Mixing with 2,4-D frequently reduces injury to pasture grasses.