We are nearing the end of the time period to control Musk thistles effectively. Musk thistle is primarily a biennial or winter annual species. As a biennial, seed will germinate in the spring or fall and plants remain as rosettes during the entire growing season. Upon surviving a winter, plants will bolt, flower, and produce seeds, taking parts of two growing seasons to complete their life cycle.
Musk thistle reproduces only by seed. Thus, the goal of any control program is to reduce and/or eliminate seed production. Control options include mechanical, biological, cultural, and chemical methods.
Mowing at the bloom stage will prevent seed production, but it usually takes two or three mowing’s at 2-4 week intervals to ensure that musk thistles do not produce seed. Another method to keep musk thistles from producing seed is to cut individual plants 2-4 inches below the soil late enough in the growing season that they don't have time to produce viable seed. The musk thistle head and rosette weevils can also help reduce seed production.
Cultural control practices are any methods which improve grass vigor and grass cover and would include prescribed burning and good grazing management. Burning by itself will not kill musk thistle but can remove excessive amounts of litter that prevent good coverage when spraying. Areas with musk thistle should be sprayed about 10-14 days after burning. Proper burning stimulates warm-season grasses that compete more favorably against musk thistle. Proper grazing that maintains and/or improves the vigor of competing vegetation can also help keep musk thistle populations down.
Musk thistle plants are most easily controlled by herbicides applied during the seedling and rosette stages of growth. Common herbicides such as 2,4-D, dicamba, and picloram as well as those that contain metsulfuron, chlorsulfuron, and aminopyralid are very effective controlling musk thistles in the rosette stage.
Once plants begin to bolt, products with aminopyralid, picloram, dicamba and metsulfuron will need to be used. Most of the previously mentioned products are available in combination with 2,4-D and are also excellent in controlling annual and perennial weeds. Examples of these would include: Escort XP, Graslan L, Grazon Next, Weedmaster, Chaparral, Cimmaron plus and Cimmaron Max.
Always read the label with particular attention to precautionary statements, grazing/haying restrictions, and rates of application.