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An Assessment of Exotic Plant Species of Rocky Mountain National Park

Carduus natans L.
Musk thistle, plumeless thistle (Asteraceae)

Current level of impact
Known locations in RMNP: Moraine Park, Horseshoe Park, Hollowell Park, Headquarters Area. Common in open grassy meadows and ponderosa pine areas on the east side of park.
Assessment: Thought to be increasing in RMNP. If added together, all populations would cover an estimated area between 11-50 hectares. Populations are widespread with a patchy distribution.

Origin: Introduced from Europe, native to southern Europe and western Asia.
Geographic distribution: Widespread throughout U.S. and Canada. Northeastern and north central states south to Missouri.
Ecological distribution: Disturbed sites along roads, fields, and pasture land. Spreading into sagebrush, pinyon juniper, and mountain brush communities. Meadows and waste places, an abundant weed in overgrazed or neglected fields and roadsides. Does not appear to have any specific climatic requirements other than a cool period of vernalization for flowering.
Soils: Mostly on dry gravely soils. Abundant on fertile soils, but may also be found in poorer soils. Occurs on soils with a pH range of 6.0 to 8.9.

Biennial forte, reproduces by seeds. Does not reproduce vegetatively, but if severed at the taproot, will produce a plant capable of flowering. Musk thistle dies after it produces seed. Flowers June to October, usually requires a vernalization period of 40 days below 10 C to produce flowers.
Seed production: Average productivity is approximately 4000 seeds/plant; however, a single plant can produce up to 20,000 seeds.
Seed longevity: Musk thistle seeds appear to remain viable in the soil for at least 10 years.
Seed dispersal: Seeds are dispersed primarily by wind. The majority of seeds remain within 50 m of the plant and very few seeds are carried further than 100 m from plant. Seeds can also be dispersed by water, and seeds may attach to animals, farm machinery, and vehicles.
Germination: Seeds germinate in fall, forming a rosette of leaves. Germination usually begins 14-21 days after seed is dispersed in fall.

Spreads rapidly and forms extensive stands which forces out desirable forage. Invades pasture, range, and forest lands along with roadsides, waste areas, and stream banks. A highly competitive plant. However, vigorous growing grasses can compete with musk thistle.
Level of impact: Has a "high potential for becoming a troublesome weed in the Park" (Weber 1988). May retard natural secondary succession processes and invade undisturbed prairie.
Response to shade: Shading may reduce seedling establishment.

Because of the long seed-viability, removal of the living plants may not totally eliminate the plant. The key to controlling musk thistle is to prevent seed production. Most control methods will have a detrimental effect on other plants and may cause a disturbance that will favor reinvasion by other exotic species. Natural areas should be monitored for the presence of musk thistle. Dense musk thistle stands along roadsides and in degraded areas can be treated by spot use of herbicides and in high quality areas by a persistent program of hand chopping.
Cultural: Maintaining pastures in good condition (by not over-grazing or fertilizing) is one important component to successfully managing musk thistle (Beck, 1991).
Mechanical: Repeated hand grubbing and mowing can be used to help control musk thistle. Musk thistle will not tolerate tillage and can be removed easily by severing its root below the ground surface with a shovel or hoe. Mowing can also reduce seed output if plants are cut when the terminal head is in the late flowering stage (Beck 1991). The crown must be completely removed because removing only the head will result in a multi-stemmed plant. Repeated treatments are necessary, and heads should be deeply buried or burned because seeds can mature and become viable after cutting.
Chemical: 2,4-D is the most common herbicide and should be applied during the rosette stage (1014 days prior to bolting). Banvel (dicamba at 0.5 to 2.0 Ibs. ai/A) has provided good control. Tordon (at 0.125 to 0.25 Ibs. ai/A) can be used during cool dry periods with less effect on non-target species.
Biological: A number of insects have been introduced worldwide to help control musk thistle. The weevil Rhinocyllus conicus has in some cases reduced populations to less than 10% pre-release levels. However, Beck (1991) reports that the musk thistle seed head weevil, if used alone, is not a successful management tool. Herbicides or mowing can be used in conjunction with the seed head weevil during late flowering stages. This allows weevils to compete their life cycles and insures their presence in subsequent growing seasons (Beck 1991). Other weevils (such as Trichosirocalus horridus) and fungi have also been used with some success.
Other: Fire is not an effective method of control. Spring burns do not reach a high enough temperatures to kill crowns. However, burns may increase warm season grasses and their competitive effects on musk thistle.


Beck, K.G. 1991. Musk thistle: biology and management. Colorado State University 
    Cooperative Extension. no.3.102.2pp.

Desrochers, A.M., J.F. Dain and S.l. Warwick. The biology of Canadian weeds. 89. 
    Cardaus nutans L. and Carduus acanthoides L. Canadian Journal of Plant Science 

Heidel, J.L. and J.M. Lee. 1987. Element Stewardship Abstract for Carduus nutans, 
    C. thoermeri, C. macrocephalus. The Nature Conservancy, Minneapolis.

Kok, L.T. and W.W. Surles. 1975. Successful biocontrol of musk thistle by an 
    introduced weevil, Rhinocyllus conicus. Environmental Entomology 3:429-430.

Kok, L.T., T,J. McAvoy, and W.T. Mays. 1986. Impact of tall fescue grass and 
    Carduas thistle weevils on the growth and development of musk thistle (Carduus 
    nutans). Weed Science 34:966-977.

Lacefield, G.D. and E. Gray. 1970. The life cycle of nodding thistle (Cardaus nutans 
    L.) in Kentucky. Proceedings. North Central Weed Control Conference 25:105-107.

McCarty, M.K. and J.L. Hadding. 1975. Effects of herbicides and mowing on musk 
    thistle seed production. Weed Research 15:363-367.

Politis, D.J., A.K. Watson, and W.L. Brukart. 1984. Susceptibility of musk thistle 
    and related composites to Puccinia carduorum. Phytopathology 74:687-691.

Reece, P.R. and R.G. Wilson. 1983. Effect of Canada thistle (Cirsium arvense) 
    and musk thistle (Cardaus nutans) control on grass herbage. Weed Science 

Rees, N.E. 1980. Life history, habits, and success of Rhinocyllus conicus as a 
    biological control agent of musk thistle in Montana. Proceedings ofthe North 
    Central Weed Control Conference 35:21.

Roeth, F.W. 1979. Comparisons of dicamba, picloram, and 2,4-D for musk thistle 
    (Cardaus nutans) control. Weed Science 27:651-655.

Stubbendieck, J., C.H. Butterfield, and T.R. Flessner. 1992. Carduns nutans L. 
    pp. 132-138. In An Assessment of Exotic Plants of the Midwest Region. Final 
    Report. Department of Agronomy, University of Nebraska, Lincoln.

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