Carduus nutans is part of a variable complex that has been treated as one to several species or as a single species with several subspecies or varieties. The New World plants apparently represent multiple introductions, probably representing more than one of these taxa. Various intermediates are evident, and many specimens cannot be reliably assigned. Insufficient evidence exists to reliably apply the names of the various segregate entities to North American material. In a biosystematic study, two subspecies of C. nutans were differentiated in Canada (A. M. Desrochers et al. 1988). Subspecies nutans was characterized as having arachnoid phylla-ries with the terminal appendage only slightly wider than the appressed phyllary base, moderately to densely pubescent leaf bases, and a head diameter of 1.5–3.5 cm. Subspecies leiocephalus in contrast has glabrous phyllaries with the terminal appendage definitely wider than the base, glabrous or slightly pubescent bases, and heads 1.8–7 cm in diameter. Subspecies nutans was distributed in eastern Canada from Newfoundland to southern Ontario and subsp. leiocephalus from Ontario to British Columbia. Whether the results of the study (Desrochers et al.) are applicable to all the populations of musk thistles occurring in the United States has not been determined.
Hybrids between Carduus acanthoides and C. nutans (C. ×orthocephalus Wallroth) have been documented from Ontario and Wisconsin and probably occur at other sites where the parental taxa co-occur.
Nodding thistle is one of the most serious weeds in North America. It is unpalatable to wildlife and livestock and often forms dense, impenetrable stands in pastures and rangelands. It readily colonizes disturbed sites in many different habitats. A single large terminal head can produce as many as 1200 cypselae. Efforts to control musk thistle infestations with Rhinocyllus conicus, a European seed head weevil, have met with some success, but concerns have been raised because this parasite also attacks native Cirsium species.
Response of musk thistle to fire appears to be primarily related to the
abundance of competitors and their response to fire. Hulbert's
summary [54] of fire effects on tallgrass prairie indicates that musk thistle can be
eliminated with or without fire on ungrazed prairie, although spring burning
was recommended to promote the vigor of native warm-season grasses. Similarly, observations in tallgrass prairie sites in South Dakota indicate
that late spring prescribed burning (when native species are still dormant) on a
4- to 5-year rotation (as per the historic fire regime) encourages the growth of
native plants and discourages the growth of Canada, bull, and musk thistles [18].
Additionally, Hulbert [54] suggests that late spring burning in these ecosystems results in fewer forbs
but greater grass production than fall or early spring burning.
Following the Long Mesa fire in Mesa Verde National Park,
response of musk thistle differed between the 2 plant communities that burned.
In pinyon-juniper woodlands where the overstory was killed and mineral soils were exposed,
invasive species including musk thistle dominated the postfire community.
Where shrubs dominated the prefire community (in the northern section of the
fire), sprouts from native perennials
repopulated the area within the 1st postfire year [34,35]. A pinyon-juniper site in Utah was burned in 1976, after which cheatgrass and other weeds (including musk thistle) established [39]. The area was then prescribe burned and
aerially seeded with crested wheatgrass (Agropyron cristatum) and other introduced perennial
species
in 1990. Parts of the burn were missed in the seeding treatment, allowing for
comparison of seeded and non-seeded areas. Frequency of musk thistle occurrence was lower in areas that were seeded following fire (756
plants per acre (1,871/ha)) than in areas that were not
seeded (7,849 plants/acre (19,424/ha)) [39].
More research is needed on long-term secondary effects of fire on musk thistle. See "Postfire
colonization potential" below for more details.
The Research Project Summary
Vegetation response to restoration treatments in ponderosa pine-Douglas-fir forests of western Montana
provides information on prescribed fire and postfire response of plant community species including
musk thistle.
Musk thistle in an invasive biennial, and sometimes summer or winter annual forb. Musk thistle 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 [118]. Correct identification of musk thistle is important if control strategies are planned since it can be easily confused with native thistles: some of which may be threatened or endangered, and the vast majority of which fill specific ecological niches and have traits useful to humans [11,108]. The following description provides characteristics of musk thistle that may be relevant to fire ecology, and is not meant to be used for identification. Keys for identifying musk thistle are available (e.g., [21,42,118]). A fact sheet provided by the Oklahoma Cooperative Extension Service provides photos and detailed descriptions of both invasive and native thistles in Oklahoma. Check with the Native Plant Society in your state for more information.
As a biennial, musk thistle initially forms a prostrate rosette. Rosette leaves can grow up to 10 inches (25 cm) long and 4 inches (10 cm) wide, and rosettes can be 2 feet (0.6 m) or more in diameter. Musk thistle rosettes have numerous small roots in the fall, and develop a large, fleshy taproot in the spring [97] that is hollow near the soil surface. The root crown and upper root tissues contain buds, normally suppressed by apical dominance, which may sprout following damage to plants [92]. Harris and Clapperton [48] report infection of musk thistle roots by vesicular arbuscular mycorrhizae.
Musk thistle may have 1 to 7 branched stems that grow 2 to 6 feet (0.6-1.8 m) tall. Stem leaves are 3 to 6 inches (7.6-15.2 cm) long and spiny. Stems have spiny wings their full lengths except for a few inches below flowerheads. Flowerheads are large, 1.5 to 3 inches (3.8-7.6 cm) in diameter, solitary, and terminal on shoots. Flowers are subtended by numerous large, lance-shaped, spine-tipped bracts that resemble a pinecone. The fruit is an achene bearing 0.3-0.5 cm seeds with a hair-like pappus [11,22,92].
Musk thistle is capable of forming dense stands, especially on highly disturbed sites where competition is low, or in overgrazed or disturbed pastureland. Population size may fluctuate in response to climatic conditions [22]. Musk thistle patches are usually denser than patches of other biennial thistles, but less dense than those of perennial thistles [72].
Researchers have found some evidence of allelopathy in musk thistle (e.g., [125]). Aqueous extracts, leachates, and ground plant material from musk thistle all showed some inhibition of germination and radicle growth rate of several pasture species. Additionally, growth of musk thistle seedlings appears to be stimulated by adding musk thistle tissue to the soil. Musk thistle may thus stimulate recruitment of its own kind as it invades [125].
Beck [11] provides a discussion of the origin, history, and distribution of musk thistle; the following is a discussion based on that review, except where otherwise cited. Musk thistle is native to western and central Europe, northwards to Scotland and extending to Sicilia, central Yugoslavia, and Ukraine [118], western Siberia, Asia Minor, and North Africa [22]. It has since been introduced to North and South America, Australia, and New Zealand [92]. Worldwide distribution of musk thistle is limited to the temperate zones of the northern and southern hemispheres, although occasional sightings in the tropics at high elevation have been reported. 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. In 1976, Dunn [29] reported the presence of musk thistle in at least 1 out of every 10 counties in the U.S. As of 1999, musk thistle was reported to occur in 45 states in the U.S. and all of the southern Canadian provinces [59].
Subspecies of musk thistle described in North America each have a specific area of distribution as described by Kartesz and Meachum [59]. See the Plants database for distribution maps of musk thistle and described subspecies in the United States.
The following biogeographic classification systems are presented as a guide to demonstrate where musk thistle might be found or is likely to be invasive, based on reported occurrence and biological tolerance to factors that are likely to limit its distribution. Precise distribution information is lacking. Therefore, these lists are speculative and not may not be exhaustive.
Fire adaptations: Musk thistle can produce abundant seed and establish well in high light environments (see Successional Status). Fire creates conditions that are favorable to the establishment of musk thistle (i.e. open canopy, reduced competition, areas of bare soil), so if musk thistle seeds are present and competition minimal, musk thistle may be favored in the postfire community.
FIRE REGIMES: Musk thistle commonly occurs in tallgrass prairie ecosystems where fire can stimulate flowering of warm-season grasses and increase stem density. In Kansas, frequent burning of tallgrass prairie is said to be effective in keeping out nonnative plants such as musk thistle, especially on sites where prairie grasses are vigorous [54]. Effects of musk thistle on native FIRE REGIMES in temperate grasslands are unknown and research is needed in this area [41]. Musk thistle also occurs in pinyon-juniper communities where fire return intervals may range from 10 to 70 years or more [34,90]. These ecosystems tend to be heavily influenced by introduced species. Following fire, cheatgrass (Bromus tectorum) and other invasives such as musk thistle can establish and dominate in place of native grasses and forbs, especially where prefire pinyon and juniper crown cover is high [34,38,39]. Musk thistle established in these communities after the practice of fire exclusion began, and it is unclear how the presence of musk thistle might affect FIRE REGIMES in these communities.
The following table provides fire regime intervals for some of the communities in which musk thistle may be found. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
Community or Ecosystem Dominant Species Fire Return Interval Range (years) bluestem prairie Andropogon gerardii var. gerardii-Schizachyrium scoparium 64,90] Nebraska sandhills prairie A. g. var. paucipilus-Schizachyrium scoparium bluestem-Sacahuista prairie A. littoralis-Spartina spartinae sagebrush steppe Artemisia tridentata/Pseudoroegneria spicata 20-70 [90] basin big sagebrush A. t. var. tridentata 12-43 [101] mountain big sagebrush A. t. var. vaseyana 15-40 [3,14,85] Wyoming big sagebrush A. t. var. wyomingensis 10-70 (40**) [122,132] saltbush-greasewood Atriplex confertifolia-Sarcobatus vermiculatus desert grasslands Bouteloua eriopoda and/or Pleuraphis mutica 5-100 plains grasslands Bouteloua spp. blue grama-needle-and-thread grass-western wheatgrass B. gracilis-Hesperostipa comata-Pascopyrum smithii blue grama-buffalo grass B. gracilis-Buchloe dactyloides cheatgrass Bromus tectorum California montane chaparral Ceanothus and/or Arctostaphylos spp. 50-100 [90] curlleaf mountain-mahogany* Cercocarpus ledifolius 13-1000 [4,102] mountain-mahogany-Gambel oak scrub C. ledifolius-Quercus gambelii blackbrush Coleogyne ramosissima Arizona cypress Cupressus arizonica 90] beech-sugar maple Fagus spp.-Acer saccharum > 1000 [123] California steppe Festuca-Danthonia spp. 90] black ash Fraxinus nigra 123] juniper-oak savanna Juniperus ashei-Quercus virginiana Ashe juniper J. ashei. western juniper J. occidentalis 20-70 Rocky Mountain juniper J. scopulorum cedar glades J. virginiana 3-7 tamarack Larix laricina 35-200 [90] western larch L. occidentalis 25-100 [2] creosotebush Larrea tridentata Ceniza shrub L. t.-Leucophyllum frutescens-Prosopis glandulosa 90] yellow-poplar Liriodendron tulipifera 123] wheatgrass plains grasslands Pascopyrum smithii 90] Engelmann spruce-subalpine fir Picea engelmannii-Abies lasiocarpa 35 to > 200 pine-cypress forest Pinus-Cupressus spp. 2] pinyon-juniper P.-Juniperus spp. 90] jack pine P. banksiana 28] Mexican pinyon P. cembroides 20-70 [88,114] shortleaf pine P. echinata 2-15 shortleaf pine-oak P. e.-Quercus spp. 123] Colorado pinyon P. edulis 10-49 [90] slash pine P. elliottii 3-8 slash pine-hardwood P. e.-variable sand pine P. e. var. elliottii 25-45 [123] Jeffrey pine P. jeffreyi 5-30 western white pine* P. monticola 50-200 [2] longleaf-slash pine P. palustris-P. elliottii 1-4 [89,123] longleaf pine-scrub oak P. p.-Quercus spp. 6-10 [123] Pacific ponderosa pine* P. ponderosa var. ponderosa 1-47 [2] interior ponderosa pine* P. p. var. scopulorum 2-30 [2,7,68] Arizona pine P. p. var. arizonica 2-10 [2] Table Mountain pine P. pungens 123] pitch pine P. rigida 6-25 [13,51] pocosin P. serotina 3-8 pond pine P. serotina 3-8 loblolly pine P. taeda 3-8 loblolly-shortleaf pine P. t.-P. echinata 10 to Virginia pine P. virginiana 10 to Virginia pine-oak P. v.-Quercus spp. 10 to sycamore-sweetgum-American elm Platanus occidentalis-Liquidambar styraciflua-Ulmus americana 123] galleta-threeawn shrubsteppe Pleuraphis jamesii-Aristida purpurea eastern cottonwood Populus deltoides 90] aspen-birch P. tremuloides-Betula papyrifera 35-200 [28,123] quaking aspen (west of the Great Plains) P. t. 7-120 [2,45,83] mesquite Prosopis glandulosa 80,90] mesquite-buffalo grass P. g.-Buchloe dactyloides Texas savanna P. g. var. glandulosa 90] black cherry-sugar maple Prunus serotina-Acer saccharum > 1000 [123] mountain grasslands Pseudoroegneria spicata 3-40 (10**) [1,2] California oakwoods Quercus spp. 2] oak-hickory* Q.-Carya spp. 123] oak-juniper woodland (Southwest) Q.-Juniperus spp. 90] northeastern oak-pine Q.-Pinus spp. 10 to southeastern oak-pine Q.-Pinus spp. 123] coast live oak Q. agrifolia canyon live oak Q. chrysolepis blue oak-foothills pine Q. douglasii-Pinus sabiniana 2] northern pin oak* Q. ellipsoidalis 123] Oregon white oak Q. garryana 2] bear oak* Q. ilicifolia 123] California black oak Q. kelloggii 5-30 [90] bur oak* Q. macrocarpa 123] oak savanna* Q. m./Andropogon gerardii-Schizachyrium scoparium 2-14 [90,123] interior live oak Q. wislizenii 2] blackland prairie Schizachyrium scoparium-Nassella leucotricha Fayette prairie S. s.-Buchloe dactyloides little bluestem-grama prairie S. s.-Bouteloua spp. 90] elm-ash-cottonwood Ulmus-Fraxinus-Populus spp. 28,123] *fire return interval varies widely; trends in variation are noted in the species summaryMusk thistle may germinate and grow under a wide range of environmental conditions, infesting arid areas in Nevada and relatively high moisture areas of Virginia and the east coast. In the Intermountain region of western North America, musk thistle occupies habitats ranging from saline soils in low altitude valleys to acidic soils at 8,000 feet (2,400 m) [11]. In Canada, musk thistle infestations can be found in soils with pH ranging from 6 to 9, which represents most of the soils in the southern part of that country (Holm and others 1997, as cited by [11]). Distribution of musk thistle is restricted mainly by extremes in soil water content, nutrient deficient or acid soils, and competition from other plant species (Doing and others 1969, as cited by [22]).
Musk thistle is frequently found on grasslands in many parts of North America [22] and is more common in the northern portion of the temperate grassland region than the southern portion [41]. Musk thistle may produce viable seeds with as little as 10 inches (250 mm) of annual rainfall, but seems to prefer moist, alluvial soils [55] and abundant spring moisture. In Canadian prairies, musk thistle is common in areas covered by snowdrifts in winter such as gullies, fence lines, brush patches, and the lee side of stone piles (Holm and others 1997, as cited by [11]). In Ohio, musk thistle is commonly found on high, dry ridges and hillsides where the limestone or dolomite bedrock is less than 6 feet (1.8 m) below the surface [113]. Similarly, serious infestations of musk thistle in the northeastern U.S. are associated with fertile soils formed over limestone (Batra 1978, as cited by [22]). Abundant soil nitrogen may favor musk thistle [81,96]. A review of the habitat requirements of musk thistle in Australia and New Zealand is provided by Popay and Medd [92].
The following table provides some elevational ranges for musk thistle as reported by state:
Location Elevation range Reference California 330 to 3,960 ft (100-1,200 m) [52] Colorado 9,000 to 10,000 ft (2,700 to 3,000 m) (Holm and others 1997, as cited by [11]) Montana up to 6,000 ft (1,800 m) [67] New Mexico 4,500-8,500 ft (1,400-2,600 m) [74] Utah 4,400 to 8,100 ft (1,340-2,440 m) [128]Musk thistle may or may not be killed by fire. A review by Heidel [50] summarizes observations of land managers that suggest high-severity fire may kill musk thistle plants by destroying the root crown, although there is no direct evidence of this. One manager observed musk thistle plants bolting and blooming after the rosettes were scarred by a late spring fire in Nebraska, which concurs with information presented in a review by Smith [108]. Popay and Medd [92] suggest that combustion would only readily take place on mature, dry musk thistle plants, from which most seed would have already dispersed.
It is unclear what effects fire has on musk thistle seeds in the soil, although incidents of rapid colonization after fire suggest that musk thistle seeds may have either dispersed from off-site sources, or could have been present in the soil at the time of the fire and survived to germinate after the overstory was removed [34,39,50].
Impacts: Musk thistle is a problem on range and pastures because it competes with desirable forage, and its sharp spines can limit recreation, hinder movement, deter livestock - and presumably wildlife - from grazing [11,22,55,97]. Even at low densities of musk thistle, losses in production of native and/or forage species can occur because rosettes of musk thistle can grow greater than 3 feet (1 m) in diameter [106]. A review by Rees and others [97] suggests that 1 musk thistle plant per 16 square feet (1.5 m²) can reduce forage production by 23%. Musk thistle may also threaten rare or sensitive species, such as the Mescalero thistle (Cirsium vinaceum) in New Mexico, by crowding into populations [110]. Musk thistle spreads most rapidly along roadsides, fence lines, and sparsely vegetated areas. Though plants are hardier where there is little competition, musk thistle can also grow in native and seeded ranges, irrigated pastures, and wet meadows with dense stands of graminoids [55].
Control: Musk thistle should be accurately identified before attempting any control measures, since several native species of thistles have a similar appearance. See General Botanical Characteristics for information on proper identification.
The key to successful management of musk thistle is to prevent seed production. Control data suggest that the threshold for viable seed production by biennial thistles is zero to achieve long-term population decreases, although zero seed production may not be a realistic goal. The transition from seedling to rosette may be the most precarious stage in the life cycle of musk thistle. Seedling and rosette growth stages are the most logical to target for control efforts in biennial thistles [11].
Combining control methods into an integrated management system will result in the best long-term population decreases. Desirable plant competition is important in any biennial thistle management strategy to deter establishment of thistle seedlings and the transition to the rosette growth stage. Recovery of infested areas should not be considered complete until a diverse population of desirable plants has replaced invasive biennial thistles and the thistles are a minor to nonexistent component to the plant community [11]. The requirement of light for musk thistle seed germination highlights the importance of maintaining dense pasture canopies in order to suppress thistle germination [106]. Beck [11] suggests that land managers control musk thistle diligently for 15 years or more to eradicate it, because of the long-lived seed bank. He also encourages monitoring and evaluating weed management programs to determine whether and when to repeat and/or modify control treatments.
Prevention: Prevention is the most effective method for managing invasive species, including musk thistle [11,55,72,104]. The best way to prevent or reduce musk thistle invasion is to deny it a suitable habitat. Maintaining a healthy stand of desirable vegetation will prevent or slow musk thistle invasion, since seedlings are intolerant of intense competition, especially for light [55,81,92]. Pastures that are growing rapidly, especially at the time of peak musk thistle recruitment in the fall, can suppress musk thistle germination [93], enhance competition upon seedings, prolong the rosette stage of thistle development [30], and ultimately reduce musk thistle population density and seed production [92]. Careful grazing management will help keep pastures and rangeland healthy by enhancing grass competition and deterring thistle survival from seedlings to rosettes [11]. Overgrazing by livestock and wildlife should be avoided, because survival of musk thistle rosettes is said to increase as grazing intensity increases [54,70,100], and bare spots caused by over-grazing are prime sites for musk thistle germination and establishment [11].
Preventing or dramatically reducing seed production and dispersal decreases the spread of musk thistle. It is important to clean mowers, vehicles, and other equipment after operating in an infested area. When seeding is necessary, use clean, certified weed-free seed and mulch to ensure that thistles and/or other weeds are not being sown [11]. Remove single plants and control small infestations so they do not expand, as they will furnish adjacent areas with abundant seed for infestation [55]. Control measures must be followed-up to prevent reinfestation, and monitoring programs developed to locate any new infestations [11,55].
Integrated management: The goal of any management plan should be to not only control invasive plants, but to also improve the affected community, maximizing forage quality and quantity and/or preserving ecosystem integrity, and preventing reinvasion or invasion by other invasive species, in a way that is complementary to the ecology and economics of the site [25,56]. Effective long-term control requires that invasive plants be removed and replaced by more desirable and weed-resistant plant communities [56]. Once the desired plant community has been determined, an integrated weed management strategy can be developed to direct succession toward that plant community by identifying key mechanisms and processes directing plant community dynamics (site availability, species availability, and species performance) and predicting plant community response to control measures [103]. This requires a long-term integrated management plan [11].
Most often, a single control method is not enough to sufficiently control an invasive plant, but there are many possible combinations of methods that can achieve the desired objectives. Methods selected for removal or control of musk thistle on a specific site will be determined by land use objectives, desired plant community, extent and nature of the infestation(s), environmental factors (nontarget vegetation, soil types, climatic conditions, important water resources), economics, and effectiveness and limitations of available control techniques [98].
Herbicide applications in spring followed by dormant seeding of competitive perennial grasses in the fall is an example of an effective management system for biennial thistles in the western United States. Similarly, integrating herbicides and biological control agents is likely to be more effective than using biological control insects alone [11] (see "Biological control" below, for more information). For information on integrated weed management without herbicides, see the BIRC website.
Some examples of combined approaches and considerations are presented within the following sections. Managers are encouraged to use combinations of control techniques in a manner that is appropriate to the site objectives, desired plant community, available resources, and timing of application.
Physical/mechanical: Any mechanical or physical method that severs the root below the soil surface will kill musk thistle. However, it is essential to revegetate the site with desirable plants, particularly competitive grasses, to compete with biennial thistles that may reinvade from seeds left in the soil. Mechanical methods are not always practical on rangeland and natural areas, but can be effective in improved pastures and roadsides. Tillage, hoeing, and hand pulling may provide effective control of musk thistle, providing these operations are done before the reproductive growth stage to prevent seed production [11].
Hand pulling is a common and effective practice for musk thistle infestations of low density or as a follow-up operation after broad-scale treatment of dense or large infestations. Plants must be severed 2 to 4 inches (5-10 cm) below the surface to prevent resprouting [33,92,108]. This may be done with a sharpened shovel or other implement. Plants may be less likely to regrow if allowed to bolt before severing the root crown [108]. Any musk thistle flowerheads must be removed and burned or otherwise destroyed to eliminate seeding [33,55], since florets are sometimes capable of maturing and producing viable seed on severed plants [92]. Hand chopping at ground level just before flowering eliminated musk thistle from a 350-acre pasture on the Konza Prairie [108]. Hand pulling of musk thistle reduced the density of infestations following fires in Mesa Verde National Park [34].
Mowing or slashing can improve the appearance of thistle-infested pastures [92] and affectively reduce the population size, but proper timing is critical. Plants cut before the appearance of the terminal flower bud are likely to regrow and produce viable seed. Viable seeds can be also be produced from heads severed later than 4 days after anthesis [76]. The greatest reduction in seed production is when musk thistle plants are mowed just before flowering [55,108]. Mowing within 2 days of the 1st terminal flowerheads in a plant population showing anthesis eliminated the production of germinable seed from all mowed stalks [76]. If cut after plants have flowerheads, they should be burned or otherwise destroyed so the seeds will not mature [55]. A single mowing will not control a musk thistle infestation, because infestations often consist of plants of variable age, and stands therefore have nonuniform development and flowering [11,76]. For this reason it is necessary to mow several times each year to effectively minimize seed production [11,72,92]. Combining mowing with chemical control may improve results [72].
Musk thistle rarely occurs in croplands suggesting that it is intolerant of repeated cultivation procedures. Evidence suggests that musk thistle seed longevity is prolonged by burial; however, shallow tillage may help control musk thistle by promoting more rapid depletion of seed stocks and killing seedlings [92]. Tillage is not appropriate in wildlands and rangelands since it can damage important desirable species, increase erosion, alter soil structure, and expose the soil for rapid reinfestation by musk thistle and other invasive species [70].
Fire: See Fire as a control agent for information on this topic.
Biological: Biological control of invasive species has a long history, and there are many important considerations to be made before the implementation of a biological control program. The reader is referred to other sources (e.g. [98,129]) and the Weed Control Methods Handbook [117] for background information on biological control. Additionally, Cornell University, Texas A & M University, and NAPIS websites offer information on biological control.Biological control of musk thistle has had substantial success in at least part of its range. Several agents have been considered and tested for musk thistle control, and those in the following table have been introduced in North America:
Biological control agent Mode of action Areas established References thistlehead weevilRhinocyllus conicus was introduced from Europe to Montana and Virginia in 1969. It does not destroy 100% of musk thistle seeds, and decreases in seed production are highly variable, ranging from 10 to 78%. It can be extremely effective by itself in those areas where the plant and insect life cycles are synchronized. A review by DeLoach [20] indicates 90 to 99% reduction of in musk thistle stands in Montana and Virginia and 80 to 90% reduction in Missouri. Rhinocyllus conicus will use Carduus, Cirsium, Silybum, and Onopordum genera as hosts but prefer the Carduus nutans group [11,97]. Several strains of R. conicus have been identified and they vary in their utilization of various thistle species. At least 1 of these strains does attack some native Cirsium species [20,71,97], and reviews by Beck [11] and Wilson and McCaffrey [129] indicate that it is known to attack native and rare thistles. Therefore, before releasing insects in a new area containing native Cirsium species, investigate whether any of the local species may be attacked [97]. A detailed discussion of the biology of R. conicus is given by Harris and Shorthouse [49].
Trichosirocalus horridus was introduced to the U.S. in 1974. This weevil uses thistles of the subtribe Carduinae, including musk thistle, plumeless thistle (Carduus acanthoides), Italian thistle (C. pycnocephalus), Canada thistle (Cirsium arvense), bull thistle (C. vulgare), and Scotch thistle (O. acanthium). Reports of suppression vary from slight to great. Trichosirocalus horridus is more effective when used in conjunction with R. conicus [97]. In areas of Missouri where R. conicus and T. horridus have been present for over 15 years, an 80 to 90% reduction in thistle population has occurred [108].
Integrated management systems may be developed utilizing knowledge of thistle and weevil life cycles, to encourage survival of the weevils over the long-term, while reducing the size or limiting the spread of bull thistle infestations in the short term. There is evidence that both Rhinocyllus conicus [116] and Trichosirocalus horridus [62] are somewhat tolerant of certain phenoxy herbicides, although more research is suggested. It may be useful, in the meantime, to create spatial or temporal separation of herbicide use and biological control. For example, insects can be released in the center of a biennial thistle infestation, and the borders sprayed to prevent further weed spread. Or, herbicides can be applied at a time when direct exposure of insects to herbicides will be minimized, such as when insects are pupating [11].
Cattle and domestic sheep and goats sometimes consume musk thistle flower and seedheads, and it has been suggested that domestic goats can help to minimize the presence of both bull [131] and musk [92] thistles. There is concern regarding whether cattle and domestic goats may facilitate musk thistle seed dispersal by eating seedheads. However domestic goats may prefer early flowering stages [92].
Chemical: Herbicides are effective in gaining initial control of a new invasion or a severe infestation, but are rarely a complete or long-term solution to weed management [16]. Herbicides are more effective on large infestations when incorporated into long-term management plans that include replacement of weeds with desirable species, careful land use management, and prevention of new infestations. Control with herbicides is temporary, as it does not change conditions that allow infestations to occur [133]. See the Weed Control Methods Handbook for considerations on the use of herbicides in natural areas and detailed information on specific chemicals. Chemical control of musk thistle is reviewed by Beck [11].
Clopyralid, dicamba, MCPA, picloram, 2,4-D, metsulfuron, and chlorsulfuron and some combinations thereof will all kill bull and musk thistles. Herbicide choice and rates are influenced by growth stage, stand density, and environmental conditions (e.g., drought or cold temperatures) [11]. Auxin herbicides are effective only when applied during periods of active growth of seedlings or rosettes, usually during spring or fall [11,22,84,96]. Fall is a good time to control biennial thistles with herbicides because all live plants will be seedlings or rosettes, though cold weather may decrease the effectiveness of some chemicals [11]. Popay and Medd [92] and Medd and Lovett [82] suggest that spring application of herbicides may be more effective since musk thistle seedlings may still emerge in the spring after fall treatments. Herbicide tolerance of musk thistle increases after bolting [33,76]. Spraying thistles after they have bolted may reduce, but not eliminate, viable seed production. Because of the large numbers of seeds produced by musk thistle, reduction of flowering and seed production is unlikely to lead to an overall reduction in levels of infestation. Since seeds accumulate in soil, even effective herbicide treatment may have only a short term effect on musk thistle populations [92]. Clorsulfuron and metsulfuron are effective when applied in the bolting and early bud stages and may, therefore, be compatible with biocontrol agents [11].
Combinations of herbicides and nitrogen fertilizer were tried in Nebraska, with results suggesting that the added nitrogen favored the thistle over the perennial grasses [96]. Similarly, Austin and others [6] determined in a greenhouse experiment that yield of musk thistle increased in response to increasing nutrient concentration.
Musk thistle was found to be resistant to 2,4-D in New Zealand and may also be cross-resistant to other synthetic auxin herbicides [47]. When herbicides are used for musk thistle control, managers should consider an herbicide rotation to prevent development of resistance [73]. See weedscience.org for more information on herbicide resistance.
Cultural: No matter what method is used to kill weeds, reestablishment of competitive, desirable plant cover is imperative for long-term control. Reseeding with competitive, adapted species is often necessary in areas without a residual understory of desirable plants [98]. Vegetative suppression is applicable both before weed establishment and after weed control.
Bull and musk thistle germination and establishment are favored in open areas and by disturbance [11]. Vigorously growing grass competes with musk thistle, and fewer thistles occur in pastures where grazing is deferred. However, musk thistle can also become a problem in pasture or rangeland that is in good condition [9]. In New Zealand, pasture species and legume swards inhibited seedling shoot and root growth of both bull and musk thistles. Rate of musk thistle seedling emergence was also negatively correlated with pasture cover density (r=-0.666; p<0.05), presumably as a consequence of alteration of light quality by the pasture [124]. On Konza Prairie, musk thistle was present in heavily grazed spots, and burning favored establishment of musk thistle seedlings in areas where grazing had reduced vigor of prairie grasses. However, where the native prairie plants were vigorous, musk thistle was crowded out, whether burned or not [54]. A field plot study in Virginia indicated that musk thistle growth was seriously reduced by competition with tall fescue (Festuca arundinacea), more than infection by T. horridus, and R. conicus. Musk thistles planted simultaneously with tall fescue produced flowering plants but with reduced size and number of seeds per plant. When musk thistle was planted in 1-year-old tall fescue, germination was low and seeds that did germinate did not reach reproductive age. Because of its extensive root system and its ability to effectively compete for nutrients, seeding tall fescue where it is well adapted may be a practical method of musk thistle control in pastures, waste areas, and roadsides [63]. Tall fescue is not appropriate in drier areas [70]. Tall fescue is also an exotic, invasive species and is not, therefore, appropriate for natural areas.
Re-establishment of desirable vegetation after controlling existing populations of musk thistle will usually be necessary for successful control. Choice of species to sow will depend upon climate, location, and management objectives. The Natural Resource Conservation Service and land grant universities are sources of information about appropriate species for a particular purpose and location [11]. Reece and Wilson [96] express the importance of controlling weeds for more than 1 year to provide grasses with a competitive advantage that will help grasses recover fully. Following musk thistle population explosion after fire at Mesa Verde National Park, planting native perennial grasses slowed but did not stop the spread of musk thistle. Aerial seeding was the most effective treatment for preventing the spread of musk thistle when compared with hand pulling and biocontrol [34]. Pasture competition decreases germination and increases mortality of musk thistle seedlings, and slows growth of survivors. Encouraging early, rapid growth of pasture in fall can help reduce numbers of established musk thistle seedlings [30]. Management that allows grasses to grow taller in spring can also shade musk thistle seedlings, thus decreasing musk thistle establishment and growth [11].Livestock rarely eat musk thistle foliage; however, cattle and domestic sheep and goats have been observed consuming flowers and seedheads. It is unclear whether this results in musk thistle seed dispersal. A number of species of birds also graze on mature musk thistle seed in Australia [92]. Dense infestations of the plant discourage animals from occupying that portion of the field in which it grows [97].
Palatability/nutritional value: No information
Cover value: No information
Musk thistle is typically a biennial, but it may complete its life cycle as an annual [11,92]. Populations may have plants of mixed ages and of differing life cycles [92].
Breeding system: Musk thistle florets mature progressively from the periphery of the capitula inward. Each floret bears male and female organs and fertilization is primarily by outcrossing [57,78,92]. Reports of outcrossing rates vary [109,126]. Musk thistle can also produce a large number of seeds through self-pollination. Therefore, an isolated plant has the potential to expand into a large infestation [11].
Pollination: The sticky nature of musk thistle pollen prevents wind dispersal. Nectar secreted in the corolla tube is harvested by a wide range of short- and long-tongued insects that effect pollination, but these relationships have not been studied in great detail [92]. Due to asynchronous development of florets, autogamous, geitonogamous, and xenogamous pollination (see the FEIS Glossary) is effected mostly by insects [92] including bees, bumblebees, and various Lepidoptera [22,31,78,109].
Seed production: Seed production is quite variable. It is determined by habitat conditions, size of plant at flowering, and duration of flowering. Hull and Evans [92] observed musk thistle plants growing in a stand of native vegetation in southeastern Idaho. They varied from 8 to 40 inches (20-100 cm) tall with 1 to 40 or more seedheads. A single musk thistle growing nearby with little plant competition was 6 feet (1.8 m) tall with 643 seedheads [55]. The life cycle exhibited by a particular musk thistle plant also influences seed production, with biennials producing more than winter annuals, and winter annuals producing more than summer annuals [92,106]. It is best to assume that even individual and sparsely separated plants can potentially add thousands of seeds to the soil seed bank [106].
The first flowerheads to emerge (terminal and topmost branch) are usually solitary, and are the largest and produce the most seeds [78]. The number of seeds per inflorescence decreases over time along with inflorescence size [57]. Musk thistle can continue to produce flowers and seeds throughout the growing season if soil moisture is adequate. The amount of seed produced is therefore markedly affected by spring and summer rainfall patterns [106]. Lower branches often develop secondary and sometimes tertiary flowerheads. Terminal flowers average about 1,000 seeds per head, while the last ones to bloom produce about 125 seeds or fewer per head [78]. A review by Desrochers and others [22] reports an average of 165 to 256 seeds per flowerhead and 10,000 to 11,000 achenes per individual musk thistle plant. Hull and Evans [55] report that 10 large, terminal seedheads from musk thistle plants in southern Idaho, northern Utah, and western Wyoming had an average of 535 seeds, and 10 smaller flowerheads, midway up the stem, averaged 298 seeds. Seeds collected from these plants in September averaged 81% germination when placed in moist sand in the greenhouse [55]. Stuckey and Forsythe [113] report that many seedheads do not have fully developed achenes at the time of dispersal.
Seed dispersal: Wind, water, wildlife, livestock, and human activities disperse musk thistle seed. Musk thistle seeds are attached to a pappus, but less than 5% of seed remains attached to the pappus when it breaks off the flowerhead and floats away on wind currents [9]. Under controlled, windy conditions (up to 18.5 feet per second (5.6 m/s)), fewer than 1% of musk thistle seeds moved more than 330 feet (100 m), and most seeds were deposited within 160 feet (50 m) of the point of release [107]. Many musk thistle seeds fail to separate from the receptacle, so fruiting heads with seeds often fall to the ground. Thus, the majority of seeds are deposited in a dense pattern near the parent plant [107,113]. This may help to explain musk thistle's slow rate of spread into favorable habitats close to existing populations [92]. Similarly, in seedheads that are attacked by the thistlehead weevil, Rhinocyllus conicus, many seeds become tightly fixed in the seedhead. These may still germinate, although competition among germinating seeds will cause high rates of intraspecific mortality [107].
Musk thistle seed dispersal over long distances is most common along travel and water corridors, and as a contaminant in crop seed and hay. Musk thistle seeds may also be distributed by birds and mammals [11,97]. Upon contact with moisture, the seedcoat of mature musk thistle seeds thickens and releases sticky mucilage, thus allowing adhesion to moving objects [57]. Reviews by Desrochers and others [22] and by Popay and Medd [92] suggest that cattle and domestic sheep and goats may facilitate seed dispersal by eating seedheads, though more information is needed regarding the proportion of viable seed voided in livestock excreta. Musk thistle seeds also have elaiosomes (fleshy appendages) that promote seed dispersal by ants [91].
Seed banking: Scientific information on seed banking in musk thistle under field conditions is lacking, although it is suggested that musk thistle seeds may remain viable in the soil for 10 to 15 years or more [11,15,22,92]. Popay and Medd [92] indicate that depth of burial influences musk thistle seed longevity, with seeds buried in the top 2 cm of soil surviving 3 years, and seed buried at greater depths maintaining viability for longer periods. Examples of musk thistle invasion following disturbances such as fire [34,38,39], insect kill [75], or other vegetation removal [19] suggest that musk thistle seeds may be present and viable in soil if there is a history of musk thistle plants on site, or if a musk thistle population is nearby.
Popay and Medd [92] report densities of up to 477 musk thistle seeds per square foot (5,300/m2) in the top 2 inches (4-5 cm) of soil. Observations suggest that substantial losses may occur prior to recruitment. However, factors influencing the fate of seeds and the conditions of these high losses are incompletely understood [92]. Greenfield and others [43] suggested that losses of seeds (including musk thistle seeds) due to soil microbial action may be comparable to those caused by predation and environmental extremes.
Germination: McCarty [78] measured the weight and germinability of musk thistle seeds at 4 stages of seedhead maturity. At full maturity (just prior to seed dispersal), seeds were divided by weight into "quality" classes. About 32% of musk thistle seeds rated "good" quality (95% viability), 11% were rated "fair" quality (37% viability), and the remaining 57% were considered "poor" quality seeds (2% viability). Terminal heads produced the most viable seeds, with successive flowering producing progressively fewer good quality seeds. McCarty [78] estimated that a musk thistle plant producing about 11,000 seeds could potentially produce about 3,870 seedlings. Jessep [57] made a similar observation in a New Zealand musk thistle population.
Germination of musk thistle seeds in the field occurs over several months in the fall and spring [69]. A review by Desrochers and others [22] indicates that some researchers report no dormancy period for musk thistle seeds, and that germination of musk thistle seeds in the field usually occurs within 14 to 21 days of shedding. Several other studies indicate that musk thistle seeds may have a period of innate dormancy [66,81,92]. A dormancy period could prevent seeds from germinating all at once in response to transient summer rainfall, and allow time for some seed to become buried [92]. In a greenhouse study optimum levels of germination, survival, and growth occurred in treatments that provided microhabitats with reduced evaporation (a light covering of litter and/or cracked and irregular surface topography) [46]. Inadequate soil moisture can hinder musk thistle germination and establishment [46,81], although some musk thistle seeds may germinate and establish under dry soil conditions [11].
In the laboratory, germination of musk thistle seed is initiated under moist conditions, between temperatures of 59 to 86 degrees Fahrenheit (15-30 °C), and is strongly enhanced by white or red light [46,81,92]. This suggests that more seeds germinate and establish on bare soils in open pastures and other poorly vegetated sites [9], where there is abundant red light. Exposure to darkness or far-red light inhibited germination of musk thistle seeds, suggesting an induced dormancy. This dormancy may be broken when the light regime changes or possibly by other factors such as increased soil nitrate levels or seedcoat damage [11,81,92].
Seedling establishment/growth: Most musk thistle seedlings emerge in the fall, although seedlings can also emerge in large numbers in the spring [69,92]. Development from seedling to rosette occurs rapidly [22]. Most plants die before flowering, and the percentage of seedlings that survive to flowering can range from 0 to 46% [22,69]. Because most seed is disseminated within the immediate vicinity of the parent plant, seedling density tends to be high, resulting in intraspecific competition and mortality [9]. The greatest mortality occurs in late spring and summer [69], and whole cohorts of biennial thistles could conceivably die during very hot, dry summers [106]. Survivorship, flowering, and seed production in musk thistle are associated with site conditions, competition, time of emergence, and rosette size.
Successful establishment of musk thistle may be rare in dense pastures due to thick litter and low light levels at the surface [46]. Established stands of musk thistle are thought to be self-perpetuating for several reasons. At high stand densities, the seedbed is devoid of competing vegetation. Additionally, dead flowering stalks of musk thistle can trap winter snow and thus provide additional moisture for spring seed germination [22]. One study also suggests that musk thistle litter may encourage germination of musk thistle seeds [125]. Early autumn germination favors high survivorship and early flowering of musk thistle, with larger, more productive individuals than those germinating late in the fall or spring [69,92]. McCarty and Scifres [77] observed that musk thistle plants growing with competition suffered more mortality than those with no competition and that seedlings emerging in spring had higher survival rates than those emerging in late summer and fall. Musk thistle plants with larger rosettes are also more likely to survive and flower and produce more seed [69].
Asexual regeneration: Vegetative reproduction has not been reported for musk thistle [22].
Musk thistle is an early successional species that establishes well on open, disturbed sites. Musk thistle was part of the earliest postfire successional stage in pinyon-juniper communities in Colorado [38,39]. On a pinyon-juniper site in central Utah that was chained to remove trees and seeded with a grass/forb/shrub mix, musk thistle density increased dramatically over the 3 years following treatment [19]. Similarly, musk thistle production increased on a site in north-central Colorado following death of the ponderosa pine overstory due to mountain pine beetle attack [75]. Another example can be seen at Mesa Verde National Park, where fungal and insect pathogens have killed thousands of Colorado pinyons (Pinus edulis), leaving patches in the woodland that have been invaded by nonnative plants. The largest invasion of musk thistle coincides with patches where black stain root disease has killed the Colorado pinyon overstory [35].
Established stands of musk thistle may be self-perpetuating [22]. However, increases in interspecific competition can cause musk thistle populations to decline. In Nebraska pastures, competition with Kentucky bluegrass (Poa pratensis) reduced musk thistle plant size and flower number and increased mortality [77]. In Australia, musk thistle is more productive in communities where levels of inter- and intraspecific competition are low [6]. Musk thistle seedlings may be sensitive to competition with neighboring plants for light. When musk thistle seedlings received 2, 8, or 14% of full sunlight, growth was reduced by 97, 68, and 35%, respectively [81].
In North American literature, the scientific name Carduus nutans L., commonly known as musk thistle,
has been applied to an aggregate of large-flowered plumeless thistles in the
aster (Asteraceae) family [92]. This report uses Carduus nutans L. as the scientific name
for the musk thistle complex as it occurs in North America
[8,17,37,42,53,60,67,74,87,94,112,128,130].
The C. nutans group has almost continuous variation in several
morphological characteristics, and intermediates between
taxa can be found [118]. Kartesz and Meachum [59] identify 4
subspecies in North America (see Synonyms), while Flora Europaea identifies
only Carduus nutans L. ssp. nutans
as a subspecies that occurs in North America [115]. Detailed treatments of the
taxonomy of Carduus species are available [21,115,118].
Hybridization and introgression between musk thistle and spiny plumeless
thistle (Carduus
acanthoides) have been reported. The hybrid is referred to as C.
ÃÂ orthocephalus Wallr. and has been studied extensively in Ontario ([22] and sources therein).
Carduus nutans, with the common names musk thistle,[1] nodding thistle, and nodding plumeless thistle, is a biennial plant in the daisy and sunflower family Asteraceae. It is native to regions of Eurasia.[2]
Carduus nutans is usually a biennial, requiring 2 years to complete a reproductive cycle. However, it may germinate and flower in a single year in warmer climates. Seedlings may emerge at any time from spring to late summer and develop a rosette. Plants overwinter in the rosette stage, sending up a multi-branched flowering stem in mid-spring of their second year.
Mature plants reach 2.7 metres (9 feet) in height with multi-branched stems. It has sharply spiny stems and leaves. The stem is cottony/hairy. The plants develop a rosette, with large leaves up to about 40 centimetres (16 inches) long.[3]
The leaves are dark green, coarsely bipinnately lobed, with a smooth, waxy surface and sharp yellow-brown to whitish spines at the tips of the lobes. They are more or less hairy on top, and wooly on the veins below.[4]
The plant bears showy red-purple flowers. The large globose flower heads, containing hundreds of tiny individual flowers, are 3–5 cm (1+1⁄4–2 in) (rarely to 7 cm) in diameter and occur at the tips of stems. The flower heads commonly droop to a 90° to 120° angle from the stem when mature, hence its alternate name of "nodding thistle". Each plant may produce thousands of straw-colored seeds adorned with plume-like bristles. They are 4 to 6 cm across, with purple-red bracts.
The number of flowerheads per plant is site-dependent and ranges from about 20–50 on good sites and 1–20 on poor sites. Flowering occurs from June to October,[3] 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 are wind dispersed. The seeds may remain viable in the soil for over ten years, making it a difficult plant to control.
C. nutans is a native plant of Eurasia.[2] It is an introduced species, and often a noxious weed, in other regions and on other continents.[2] It is abundant in region of the North American Rocky Mountains.[3]
The plant grows from sea level to an elevation of about 2,500 m (8,200 ft). It is found in neutral to acidic soils. It typically grows in meadows and grasslands, in heavily grazed land in areas such as pastures, and on open disturbed soil such as roadsides and building sites.[3] It spreads rapidly in areas subjected to frequent natural disturbances such as landslides and flooding, but does not grow well in excessively wet, dry, or shady conditions.
C. nutans is an invasive species in various regions around the world, including in disturbed and agricultural settings, and in natural habitats.[5]
Musk thistle was introduced into eastern North America in the early 19th century, and has been an invasive species there since. It is declared a noxious weed in many U.S. states, Canadian provinces, South Africa, New Zealand, and Australia.[2][6] Previous populations in Southern California were eradicated, but it remains in northern California.[7]
Carduus nutans, with the common names musk thistle, nodding thistle, and nodding plumeless thistle, is a biennial plant in the daisy and sunflower family Asteraceae. It is native to regions of Eurasia.
