Synonym(s): Musk thistle, nodding thistle
Family: Asteraceae (Aster Family)
Duration and Habit: Biennial, Perennial Herb
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Musk, or nodding thistle is an aggressive, biennial herb with showy red-purple flowers and painful spiny stems and leaves. Populations in North America exhibit almost continuous variation in characteristics such as hairiness, leaf size, spine length, flower stalk diameter, width and shape of bracts, and corolla length. In general, mature plants range in height from 1.5 to 6 feet tall, and have multi-branched spiny stems. Leaves are dark green, coarsely bipinnately 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 1.5 to 3.5 inches in length and occur at the tips of stems. Phyllaries are spine-tipped and overlap with several rows. 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.
Native Lookalikes: NOTE: These are non-native invasive species. Canada thistle [Cirsium arvense (L.) Scop.], bull thistle [Cirsium vulgare (Savi) Ten], and Scotch thistle [Onopordum acanthium L. ssp. acanthium] may be confused with Carduus thistles.
Canada thistle (Cirsium arvense)
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. NOTE: Canada thistle is a non-native invasive species, although as of 2020 it has not been reported in Texas. See more information on this species.
bull thistle (Cirsium vulgare)
Bull thistle is a coarse biennial with plumose pappus bristles and upper leaf surfaces covered with stiff bristly hairs that are rough to touch. NOTE: Bull thistle is a non-native invasive. See its profile.
Scotch thistle (Onopordum acanthium)
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. NOTE: Scotch thistle is a non-native invasive. See its profile.
Ecological Threat: Musk thistle is a highly competitive plant that can replace much of the native vegetation in pastures and disturbed areas. Because it is unpalatable to wildlife and livestock, 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. What makes Carduus nustans a difficult plant to control is that 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. Furthermore, seeds may remain viable in the soil for over ten years.
History: The earliest records of musk thistle in North America are from central Pennsylvania in 1852, followed by several records of its occurrence along the east coast in the late 1800s, apparently associated with ship ballast. Musk thistle began to appear in the Midwest around the turn of the 20th century. By the early 1940s, musk thistle was regarded as a potential problem in North America.
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, ME, MI, MN, MO, MS, MT, NC, ND, NE, NH, NJ, NM, NV, NY, OH, OK, OR, PA, RI, SC, SD, TN, TX, UT, VA, VT, WA, WI, WV, WY
Distribution in Texas: Throughout much of North America, except far north; common in Rocky Mountain region.
List All Observations of Carduus nutans reported by Citizen Scientists
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.
For smaller infestations, fortunately, cutting or removing thistles can be effective in reducing populations. If cut or mowed within two days of flowering of the terminal blooms, plants will not produce seed or regenerate significantly. Timing in mowing is important; if mowing occurs four days after terminal bloom anthesis (full flowering), significant amounts of seed are produced. Please be sure to dispose of seeds, shoot and roots in a sealed garbage bag in the trash. Also, dispose of the seedhead carefully to avoid wind dispersal.
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.
Even though these weevils have proven very successful certain entities like the National Park Service are not able release it on their lands, due to potential impacts on native plants. Also, Cheilosia corydon has been observed in several native thistle species, so caution should be used when considering it as a biological control agent.
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. George. 1999. Biennial thistles. In: Sheley, Roger L.; Petroff, Janet K., eds. Biology and management of noxious rangeland weeds. Corvallis, OR: Oregon State University Press: 145-161.
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.
Fire Effects Information System - USDA Forest Service
APWG WeedUS Database; Encycloweedia, California Department of Food and Agriculture
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