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Musk 1'h~stle <br />Page 2 of 3 <br />only one-third of the seeds are viable. Musk thistle produces many heads. The terminal, <br />or tallest, shoots flower first, then lateral shoots develop in leaf axils. A robust plant may <br />produce 100 or more flowering heads. <br />Musk thistle flowers over aseven- to nine-week period. It begins to disseminate seed <br />from a head about two weeks after it first blooms. It is common to observe musk thistle <br />with heads in several stages of floral development and senescence. Thus, musk thistle <br />sets seed over an extended time period. <br />Most seed is disseminated within the immediate vicinity of the parent plant. This leads to <br />a clumped pattern of seedling development and results in intraspecific competition and <br />mortality. Wind and water are good dissemination methods and seeds are also spread by <br />animals, farm machinery and other vehicles. Less than 5 percent of seed remains <br />attached to the pappus when it breaks off the Flowering head and floats away on wind <br />currents. <br />Management <br />Cultural control. Maintaining pastures and rangeland in good condition is a primary <br />factor for musk thistle management. To favor pasture and rangeland grass growth, do <br />not overgraze. Fertilize only when necessary and accoMing to soil testing <br />recommendations. To successfully manage musk thistle, prevent seed formation. <br />Mechanical control. Musk thistle will not tolerate tillage and can be removed easily by <br />severing its root below ground with a shovel or hoe. Mowing can effectively reduce seed <br />output if plants are cut when the terminal head is in the late-flowering stage. Gather and <br />burn mowed debris to destroy any seed that has developed. <br />Chemical control. Several herbicides are registered in pasture, rangeland and noncrop <br />areas to control musk thistle. Tordon 22K (picloram), Curtail (clopyralid +2,4-D), <br />Banvel/Vanquish/Clarity (dicamba), 2,4-D, or Banvel/Vanquish/Clarity plus 2,4-D are <br />commonly used. Apply these herbicides in spring or fall to musk thistle rosettes. <br />Apply Tordon at 0.5 to 1 pint per acre (A), Curtail at 2 quarts/A, Banvel/Vanquish/Clarity <br />at 0.5 to 2 quarts/A, 2,4-D at 1.5 to 2 quarts/A, or Banvel/Vanquish/Clarity plus 2,4-D at <br />0.5 plus 1 quart/A. Cool temperatures (below 50 degrees F), particularly in fall, may <br />adversely affect 2,4-D control of musk thistle; therefore, use 2,4-D in spring. Tordon is <br />largely unaffected by cool temperatures. Banvel/Vanquish/Clarity can be adversely <br />affected but less than 2,4-D. Banvel/Vanquish/Clarity plus 2,4-D works well in spring or <br />fall. Apply any of these herbicides before musk thistle bolts or seed production still will <br />occur. <br />Ally-Escort (metsulfuron) or Telar (chlorsulfuron) also can be used. Use Telar in noncrop <br />areas only and Ally-Escort in pastures, rangeland or noncrop areas. Research from <br />Colorado State University and the University of Nebraska shows that Telar or Aily-Escort <br />prevents or dramatically reduces viable seed formation when applied in spring, up to <br />early Flower growth stages. The latest time to apply these herbicides is when developed <br />terminal flowers have opened up to the size of a dime. <br />Apply Telar at 1 ounce/A or Ally-Esmrt at 0.5 ounce/A. Add a good agricultural surfactant <br />at 0.25 percent v/vz to Ally-Escort or Telar treatments or control is inadequate. <br />Biological control. The musk thistle seed head weevil, Rhinocyllus conicus, can be <br />found throughout Colorado. The female deposits her eggs on the back of developing <br />flowers and covers them with chewed leaf tissue. After eggs hatch, larvae bore into the <br />Flower and destroy developing seed. The seed head weevil reduces seed production by 50 <br />percent on the average. If used alone, however, it is not an effective management tool. <br />Certain herbicides or mowing can be combined with the seed head weevil if these are <br />used during late flowering stages. This allows the weevils to complete their life cycle and <br />ensures their presence in subsequent growing seasons. <br />The Colorado Department of Agriculture has established another weevil, Trichosiroca/us <br />http://www.ext.colostate.edu/pubs/natres/03102.html 6/20/2005 <br />