Family: Asteraceae (Aster Family)
Duration and Habit: Biennial, Perennial Herb
Musk, or nodding thistle is an aggressive, biennial herb with showy red-purple flowers and painful spiny stems and leaves. Mature plants range in height from 1? to 6 feet tall, and have multi-branched stems. Leaves are dark green, coarsely lobed, with a smooth waxy surface and a yellowish to white spine at the tip. The large disk-shaped flower heads, containing hundreds of tiny individual flowers, are 1? to 3? inches in length and occur at the tips of stems. Flower heads will droop to a 90-degree angle from the stem when mature, hence its alternate name, nodding thistle. Each plant may produce thousands of straw-colored seeds adorned with plume-like bristles.
Ecological Threat: selective grazing leads to severe degradation of native meadows and grasslands as wildlife focus their foraging on native plants, giving musk thistle a competitive advantage. Although musk thistle is infrequently found in dense forests, it can colonize areas subjected to natural disturbances such as landslides or frequent flooding. Meadows, prairies, grassy balds, and other open areas are susceptible to invasion.
Biology & Spread: Musk thistle is usually a biennial, requiring 2 years to complete a reproductive cycle, but may germinate and flower in a single year in warmer climates. Seedlings emerge in mid to late July and develop into a rosette where plants can reach 4 feet in diameter. Plants overwinter in the rosette stage until they begin to bolt in mid-March. During the bolting stage plants form multi-branched stems to a height of 6 feet. The number of seedheads per plant is site-dependent and ranges from about 24 to 56 on favorable sites and 1 to 18 on less favorable sites. Flowers emerge in early May to August and seed dissemination occurs approximately one month after the flowers form. A single flower head may produce 1,200 seeds and a single plant up to 120,000 seeds, which may be wind blown for miles. Seed may remain viable in the soil for over ten years, making it a difficult plant to control.
History: A native of western Europe, musk thistle was introduced into the eastern United States in the early 1800s and has a long history as a rangeland pest in the U.S. It was first discovered in Davidson County, Tennessee in 1942.
U.S. Habitat: Roadsides, waste places, fields, pastures, and rangelands.
U.S. Nativity: Introduced to U.S.
Native Origin: temp. Asia & Europe (Germplasm Resources Information Network); NatureServe Explorer
U.S. Present: AL, AR, AZ, CA, CO, CT, DE, GA, IA, ID, IL, IN, KS, KY, LA, MA, MD, MI, MN, MO, MS, MT, NC, ND, NE, NH, NJ, NM, NV, NY, OH, OK, OR, PA, RI, SC, SD, TN, TX, UT, VA, WA, WI, WV, WY
Distribution: Throughout much of North America, except far north; common in Rocky Mountain region.
List All Observations of Carduus nutans reported by Citizen Scientists
Canada thistle [Cirsium arvense (L.) Scop.][CIRAR], bull thistle [Cirsium vulgare (Savi) Ten][CIRVU], and Scotch thistle [Onopordum acanthium L. ssp. acanthium] may be confused with Carduus thistles.
Canada thistle is perennial, with creeping roots and small unisexual flower heads unlike Carduus thistles. Plants are either male or female (dioecious). In addition, Canada thistle has smooth stems and plumose pappus bristles.
Bull thistle is a coarse biennial with plumose pappus bristles and upper leaf surfaces covered with stiff bristly hairs that are rough to touch.
Scotch thistle and related Onopordum species are distinguished by having receptacles deeply pitted, honeycomb-like, with membranous extensions around pits, and not densely covered with bristles.
Prevention: Good range and pasture management techniques, including grazing, cutting and forage production, can reduce weed establishment and impact. This includes using certified seed, clean hay, bedding, and equipment, avoiding overgrazing and poor fertilization, keeping vehicles and grazing animals out of infested areas.
Mechanical: Mowing can help reduce seed production but mowing alone will not eliminate an infestation. Early mowing is ineffective for control of musk thistle. The optimum mowing timing is 2 to 4 days after initial flowering. Mowing 3 ft tall musk thistle plants to a 6 in stubble will prevent seed production, but thistles quickly recover from remaining buds near the base. Tillage can also be used to control musk thistle. However, this technique is not always practical in non-crop areas.
Cultural: Prescribed burning will remove dense stands of mature thistles and create a good environment for subterranean clover to germinate and grow. However, burning may not completely control plants still in the basal rosette stage. Thistle establishment is less likely if desirable vegetation remains dense throughout the year. Many thistle problems occur when range or pastures are overgrazed in summer and early fall, or when conditions, such as drought stress or poor fertility leave bare soil. Targeted grazing of thistle with goats and other farm livestock provides a useful technique to control thistle. Cattle and sheep prefer the vegetative tissues of musk thistle. In contrast, goats virtually ignored the leaves of musk thistle, but relish the flowers. Even in the presence of palatable subclover and grass pasture, goats seemed to prefer musk thistle flower heads. Thus, goats will drastically reduced average seed production per plant. Seeds ingested by goats and other ruminants are nearly all digested and are not spread within the feces. The use of goats and other livestock can represent an important management technique and can be effective in a long-term integrated approach for the control of musk thistle. Musk thistle germination is greatest on bare ground. Control of musk thistle is maximized when range or pasture cover is dense during the weed seedling emergence period. It has been reported that allelochemicals released by some pasture and range species could be partially responsible for inhibition of seedling emergence and growth. Grasses inhibit seedling emergence and subsequent growth and survival of rosettes to a significantly greater level than legumes. Once seeds have germinated, dense grass and legume cover provides shading of developing rosettes and suppresses the growth of musk thistle. Perennial grasses are more effective than annual grasses or legumes.
Biological: Three insects have become established for the control of musk thistle; thistle head weevil (Rhinocyllus conicus), thistle crown weevil (Trichosirocalus horridus), and thistle crown fly (Cheilosia corydon). Rhinocyllus conicus was the first species released in the United States for control of musk thistle. It has one generation per year. It lays its eggs on bud bracts and the larvae infest the seed head or stem. The larvae feed on the seeds and are more destructive than other insect stages. The thistle head weevil infests a number of host genera in the thistle tribe, including species of Carduus, Cirsium, Onopordum, and Silybum. It has proven to be a very effective control agent on musk thistle. Trichosirocalus horridus also has one generation per year. Its larvae feed on the growing tip of the thistle rosette and the adults may also slightly defoliate plants. Like Rhinocyllus conicus, it can attack other thistle species, including plumeless thistle, Italian thistle, Canada thistle, bull thistle, and Scotch thistle. Suppression of musk thistle is only slight thus far, and requires other biocontrol agents to be present. Cheilosia corydon is a fly that also produces one generation per year. Its larvae damage the leaves, stems, and crown of musk thistle, Italian thistle, and plumeless thistle. This organism can lower total seed production and can kill the plant when it infests roots. It was only released in 1990, so little information is available on its effectiveness.
Chemical: Few herbicides provide effective preemergence control of musk thistle in rangelands and pastures. Chlorsulfuron has both pre- and postemergence activity. Preemergence application with chlorosulfuron (0.75 - 1.5 oz ai/A) in the fall are not very effective for control of seedlings or mature plants. However, treatment with chlorsulfuron (0.37 - 0.75 oz ai/A) in early bloom stage reduced seed production by over 99%. Several postemergence herbicides will control musk thistle. Typically, spring treatments give better control than fall herbicide applications, as many new seedlings which emerge after a fall treatment will escape injury. Dicamba, 2,4-D, clopyralid, MCPA, glyphosate and combinations of these compounds provide excellent control with a spring application, and somewhat less control with a fall treatment.
Beck, K.G., R G. Wilson, and M. A. Henson. 1990. The effects of selected herbicides on musk thistle (Carduus nutans) viable achene production. Weed Technology, 4:482-486.
Heidel, Bonnie. 1985. Carduus nutans: element stewardship abstract. The Nature Conservancy, Arlington, VA.
Hull, A.C., Jr., J.O. Evans. 1973. Musk thistle (Carduus nutans) an undesirable range plant. Journal of Range Management 26(5):383-385.
Kok, K.T., W.W. Surles. 1975. Successful biocontrol of musk thistle by an introduced weevil. Environ. Entomol. 4(6):1025-1027.
Lacefield, G.D., E. Gray. 1970. The life cycle of nodding thistle in Kentucky. Bowling Green, KY: Department of Agriculture, Western Kentucky University.
Lambdin, P.L., J.F. Grant. 1992. Establishment of Rhinocyllus conicus (Coleoptera: Curculionidea) on musk thistle in Tennessee. Ent. News 103(5):193-198.
Monks, D.W., M.A. Halcomb and E.L. Ashburn. 1991. Survey and control of musk thistle (Carduus nutans) in Tennessee field nurseries. Weed Technology. 5:218-220.
The Nature Conservancy. Musk Thistle: Element Stewardship Abstract. In: Wildland Weeds Management & Research Program, Weeds on the Web.
APWG WeedUS Database; Encycloweedia, California Department of Food and Agriculture
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