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SerV1Ce 1T1 ~°` ACTION Musk thistle: biologq and management fs.ceorge sack Buick Facts Musk UtisUe Is a biennial weed that reproduces only from seed. The key to successful musk thistle control is to prevent seed production. Herbicides such as Tordon, Banvel, or 2,4-D should be applied to musk thistle rosettes 1n spring or fall. Ally or Telar maybe applied up to the early !lower growth stage. Combine control methods into a management system for best results. Musk UusUe is an aggresstve weed of pastures, rangeland, roadsides, and non-crop areas. It is a biennial weed, although occasionally is an annual. Since musk thistle reproduces solely from seed, the key Cor successful management 1s to prevent seed production. Germination and seedling establishment are correlated with moisture and light. Thus, more seeds germinate and establish plants in open pas- tures and other areas. Vigorously growing grass competes with musk Un1sUe and fewer thistles occur m pastures where grazing 1s deferred. However, musk thistle also can become a problem !n pasture or rangeland that >s in good cortditlon. Phenology Seedlings normally emerge early In spring, de- velop into rosettes and spend the llrst season >n this growth stage. Seedling emergence also can occur in fall. All seedlings grow Into rosettes and overwlnter in that stage. Rosettes are usual}y large and rnmpact with a large, corky taproot that >s hoIlow near the crown. Early >n spring of the second year, werwlrn- tered rosettes resume growth. Shoots begin to elongate (bolt) 1n tale March through May depending on weather and altttude. Musk thistle IIowers and starts to produce seed 45 to 55 days after it bolts. Musk thistle dies after it sets seed. It spends ap- prwdmately 90 percent of its life eyrie in a vegetative t;ooperauve Extension so. s.>to2 tl I% growth stage. It is important to recognize that musk thlstie's tolerance to most herbicides increases after t bolts- Reproduction and Spread !dusk thistle is a prollflc seed ptoducer. A singae plant can set up to 20,000 seeds. However, only one- third of the seeds are viable. Musk thistie produces many heads; the terminal, or tallest, shoot flowers tlrst then lateral shoots develop to leaf axils. A robust plant may produce 100 or more Dowering heads. Musk UdsUe IIowers war aseven- to nine-week perto and begins to d~~~f~~te seed fpm a head about two weeks after 1t lust blooms. It 1s quite common to observe musk thistle with heads !n several stages of IIoral development and senescence. Thus, musk thistle sets seed over an extended time period. Most seed Ls dissemknated within the immediate vktinlty of the parent plant This leads to a clumped pattern of seedling development and results In iritraspe- dDc oompetfti~r and mortality. Wind and water are goo disseminatlon methods and with these methods seeds attach to antnnalss, farm machinery and other vehicles. However. few seeds pass than 50 peroenU stay attached to their pappus when II break off the Dowering head an. boats away on wind ctrrrrnts. 'K George Bock Caiaadv SFnfr [hrtuerslty Coopeatiue F.xterisfat uieed sc~erne specialist cold assistmlt projessn+ plontpathaiagy rnid u~ead sciane f7/91) ®Qoiorado Sf~e [hth~erslty Goopan~eE.deriskn. J99J. Issued In furtherance of Cooperative Eaterulon wart. I~CLS of May d and June b. 1911. In eoopcntion with [hc US. pcpanmcnt o! AQricultun:. Kenneth N. Bolen. director of Coopen[ivc Extension. (:olondo Su[e University, FOn Collins, Colorado Coopcn[Ive F.a[ertslon Drornms arc available w all wlntaw discrimim[ion. To simDlifs' rcchnical terminology. trade names of Droducu and WuiDmen[ otxvslonally will be used No <ndorsemcn[ of produce turned Is Intended nor Is criti- cism ImDlled of Draducu nw mentioned.