MANHATTAN, KAN. -- The musk thistle control season has arrived, and the Kansas-wide noxious weed is alive and doing well, said Walt Fick, range and pasture management specialist with Kansas State University Research and Extension.

Knowing the musk thistle's life cycle is important to getting a good control strategy in place, Fick said. The weed is primarily a biennial or winter-annual plant.

"As a biennial, seed will germinate in the spring and the plants will remain as rosettes during the entire growing season. Then, after surviving a winter, the plants will bolt, flower, and produce seeds, thus taking parts of two growing seasons to complete their life cycle," he explained. "As a winter annual, musk thistle emerges in the late fall with moisture. The plants go through the winter, then produce seed the following year."

Musk thistle only reproduces by seed. Thus, the goal of any control program is to reduce and/or eliminate seed production, Fick said.

Control options include mechanical, biological, cultural, and chemical methods, the agronomist said. These options include:

  • Mowing at the bloom stage will prevent seed production, but killing musk thistle usually takes two or three mowings at 2- to 4-week intervals. Individual plants can be cut off 2 to 4 inches below soil level.
  • The musk thistle head and rosette weevils can both help reduce seed production.
  • Cultural control practices, including prescribed burning and good grazing management, can help keep musk thistle populations at reduced levels. Burning by itself will not kill musk thistle but it can remove the excessive amounts of litter that prevent good coverage when spraying. Plus, proper burning can stimulate warm-season grasses that compete favorably against musk thistle. Spraying areas with musk thistle should follow about 10 to 14 days after burning. 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. Such common herbicides as 2,4-D, dicamba, and picloram are very effective on rosettes. Products containing metsulfuron, chlorsulfuron, and aminopyralid are also effective on musk thistle.

  • Once plants begin to bolt, products such as picloram + 2,4-D (Tordon 22K + 2,4-D), metsulfuron + 2,4-D (Escort XP + 2,4-D), metsufuron + chlorsulfuron (Cimarron Plus), metsulfuron + dicamba + 2,4-D (Cimarron Max), or aminopyralid alone (Milestone) or in combination with 2,4-D (ForeFront R&P) are more effective. For bolted to bud stage thistles, products containing clopyralid (Curtail and Stinger) provide excellent control.

    "In other words, treat musk thistle before it starts to bloom," Fick said. "Although some herbicides, such as metsulfuron, have proven to reduce seed viability when applied at the bloom stage, they are unlikely to eliminate all seed production. And, it only takes one seed to keep the population going."

    Herbicide recommendations for musk thistle control are available in "2009 Chemical Weed Control for Field Crops, Pastures, Rangeland, and Noncropland." That publication is available at any county or district K-State Research and Extension office, as well as on the Web.

    "Always read the herbicide label, too," Fick advised, "paying particular attention to precautionary statements, grazing or haying restrictions, and rates of application."

    SOURCE: K-State.