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<br /> <br />KDOW100ke to abr* <br />.. -5- . - - <br />4-H /Youth Agriculture Family/Consumer Horticulture Natural Resources Nutrition Resources Home <br />no. 3.102 <br />Musk Thistle <br />by K.G. Beck 1 <br />Quick Facts... <br />• Musk thistle is a biennial weed that reproduces only from seed. <br />• The key to successful musk thistle control is to prevent seed production. <br />• Apply herbicides such as Tordon, Vanquish/Clarity or 2,4-D to musk thistle rosettes <br />in spring or fall. Apply Ally or Telar up to the early flower growth stage. <br />• Combine control methods into a management system for best results. <br />Musk thistle is an aggressive weed of foreign origin that occurs in pastures, rangeland, <br />roadsides and non-crop areas. It is a biennial weed, although occasionally it is an annual. <br />Because musk thistle reproduces solely from seed, the key for successful management is <br />to prevent seed production. <br />Germination and seedling establishment are correlated with moisture and light. Thus, <br />more seeds germinate and establish plants in open pastures and other degraded areas. <br />Vigorously growing grass competes with musk thistle, and fewer thistles occur in <br />pastures where grazing is deferred. However, musk thistle also can become a problem in <br />pasture or rangeland that is in good condition. <br />Phenology <br />Seedlings normally emerge early in spring, develop <br />into rosettes and spend the first season in this <br />growth stage. Seedling emergence also can occur in <br />fall. All seedlings grow into rosettes and overwinter <br />in that stage. Rosettes are usually large and compact <br />with a large, corky taproot that is hollow near the <br />crown (Figure 1). <br />Early in spring of the second year, overwintered <br />rosettes resume growth. Shoots begin to elongate <br />(bolt) in late March through May, depending on <br />weather and altitude. Musk thistle flowers and starts <br />to produce seed 45 to 55 days after it bolts. <br />C-1 <br /> <br />Figure 1. Musk thistle (Carduus nutans) <br />Musk thistle dies after it sets seed. It spends <br />approximately. 90 percent of its life cycle in a <br />vegetative growth stage. Musk thistle's tolerance to most herbicides increases after it <br />bolts. <br />Reproduction and Spread <br />Musk thistle is a prolific seed producer. One plant can set up to 20,000 seeds. However, <br />http://www.ext.colostate.edu/pubs/natres/03102.htmi 6/20/2005