One year of poor thistle control will result in having to start the control program over. Most of the herbicides used for control of bull and musk thistle also kill pasture legumes. Spot spraying individual plants or patches rather than broadcast spraying the entire pasture also spares the legumes. Musk thistle normally initiates flower stalks in early May and reaches full flower in early June. Seed production is prolific and usually completed in mid to late June.
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.