Rosettes reestablish nearly any time during the growing season. Some rosettes may be three or four feet in diameter by late fall. Musk thistle is extremely aggressive. Bull thistle normally initiates flower shoots in July and reaches full flower in August. Seed production is usually completed in late summer.
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.