Musk thistle [Carduus nutans L.][CRUNU][CDFA: A] Photographs Map of Distribution Biocontrol

Giant plumeless thistle or Plumeless thistle [C. acanthoides L.][CRUAC][CDFA: A] Photographs Map of Distribution

Italian thistle [C. pycnocephalus L.][CRUPY][CDFA: C][CalEPPC: B] Photographs

Slenderflowered thistle [C. tenuiflorus Curtis][CRUTE][CDFA: C]. Photographs

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GENERAL DESCRIPTION:Erect thistles with prickly winged stems and leaves. Plants exist as basal rosettes until flowering shoots develop at maturity. Refer to the table Carduus thistles for a quick comparison of distinguishing characteristics. Also see Comparison of spiny-leaved thistles. The thistle head weevil (Rhinocyllus conicus), an introduced biocontrol agent, attacks Carduus species and several other thistles, including some native thistles (Cirsium spp.). Control of Carduus thistle infestations by the weevil varies by species and regionally from excellent to poor. The weevil is established in California and much of the northwestern and north central U.S. The fungus musk thistle rust (Puccinia carduorum) has recently been found in California and may soon be state approved as a biocontrol agent to help control musk thistle.

SEEDLINGS:musk thistle: Cotyledons nearly sessile, oblong, with tips often squared, 7.5-15 mm long, 2.5-6 mm wide. Cotyledon veins are white and broad. First 2 true leaves appear opposite. Subsequent leaves are alternate and form a basal rosette. Leaves are pale green, waxy, oval to elliptic, shallowly lobed, and irregularly prickly toothed. Hairs are sometimes scattered on the upper surface and the main veins of the lower surface. Seedling descriptions are unavailable for plumeless, Italian, and slenderflowered thistles, but given the similarity of mature plants, seedlings of these species probably closely resemble those of musk thistle.

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MATURE PLANT:Stems branched near the top. Basal leaves elliptic to lanceolate, pinnately lobed, and with prickly-toothed margins. Stem leaves alternate, reduced, with bases that extend down the stem forming spiny wings (decurrent).

ROOTS and UNDERGROUND STRUCTURES:Taproots long, thick, fleshy, occasionally branched, capable of penetrating the soil to depths of 40 cm or more.

FLOWERS:Heads consist of deeply lobed, purple to pink, (rarely white) disk flowers. Phyllaries spine-tipped,overlapping in several rows. Receptacles flat, densely covered with cream-colored bristles interspersed among the disk flowers. Insects pollinated.

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FRUITS and SEEDS:Achenes elliptic, curved, slightly compressed, sometimes slightly 4 to 5 sided in cross section, smooth, glossy, golden to brown. Pappus bristles numerous, cream-colored, fine, minutely barbed (with magnification), united at the base to form a ring and deciduous as a unit.

POSTSENESCENCE CHARACTERISTICS:Foliage is killed by hard frost, but plants remain intact for an extended period after death. The persistent spiny character of the foliage helps to distinguishes plants.

HABITAT:Thistles typically colonize disturbed open sites, roadsides, pastures, annual grasslands, and waste areas.

DISTRIBUTION:At publication time, populations of musk and giant plumeless thistle are limited to specific regions. Italian and slenderflowered thistle are widely distributed.

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PROPAGATION/PHENOLOGY:Reproduces by seed. Seeds fall near the parent plant or disperse by wind, water, birds, small mammals, and human activities.

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ADDITIONAL ECOLOGICAL ASPECTS:Musk thistle seeds appear to possess allelopathic qualities. They can inhibit germination and radicle growth in other pasture species, but stimulate or have no affect on other seeds of their own species. This suggests that the allelopathic potential of musk thistle seeds may be an evolved mechanism to encourage its own establishment. Emerged musk thistle plants can also weaken other pasture species by an allelopathic interaction at the early bolting stage, when the larger rosette leaves are decomposing and releasing soluble inhibitors, and at the stage when bolting plants are dying and releasing insoluble inhibitors. No specific chemicals have been identified. Although musk thistle is sometimes associated with fertile soils, it is more likely to increase in situations of declining fertility. Furthermore, it has the potential to induce long-term decline of soil nitrogen input. This appears to be related to its allelopathic activity. Decomposing rosette leaves have a strong potential to inhibit white clover (Trifolium repens) nitrogen fixation. Thus, dense musk thistle stands create conditions that favor their proliferation and are unsuitable for white clover establishment, persistence, and ultimately nitrogen fixation.

MANAGEMENT FAVORING/DISCOURAGING SURVIVAL:Thistles compete poorly with healthy established grasses and other vegetation. Disturbances such as fire, overgrazing, or trampling create prime sites for thistle colonization.

SIMILAR SPECIES: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.

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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. The table below lists the herbicides, rates, and timing that provide effective control of musk thistle. Picloram (0.25 lb ae/A) also gives excellent control of both musk thistle seedlings and mature plants following a spring or fall treatment. However, this compound is not registered for use in California.

Table 1. Herbicide recommendations for musk thistle control.

Herbicide Rate Timing Remarks
2,4-D 1 - 2 lb ae/A March to early April in rosette stage Newly planted legumes may be killed
dicamba 0.5 - 1 lb ae/A March to early April in rosette stage May kill all legumes
dicamba + 2,4-D 0.75 + 0.25 lb ae/A March to early April in rosette stage May kill all legumes
MCPA 1 - 1.5 lb ae/A Spring applications in rosette stage Safer on legumes that most growth regulator herbicides
clopyralid 1.5 - 4 oz ae/A Can apply up to the bud stage Will kill annual legumes and damage perennial legumes
glyphosate 1 - 2 lb ae/A Apply in spring or up to rosette stage Non-selective. Do not use with perennial grasses
chlorsulfuron 0.75 - 2.25 oz ai/A Late season applications for reduced seed production Will injure some grasses as well as broadleaf species

Combinations of 2,4-D with either clopyralid or glyphosate can also be effective. Rope wick applications of glyphosate and 2,4-D shows good activity on musk thistle up to 2 ft tall. However, only bolted plants are controlled with this treatment. Those plants that have not yet elongated above the forage canopy will not be contacted by the wick. It is important to recognize that there are grazing and cutting restrictions for many of these postemergence herbicides. This restriction period is provided on the herbicide label. In New Zealand, MCPA resistant musk thistle developed in dairy and sheep pastures following continuous use of the herbicide. This population was found to be cross resistant to 2,4-D, MCPA, and MCPB, but not resistant to dicamba, clopyralid or picloram. Even without the use of 2,4-D, MCPA or MCPB it is unlikely that the proportion of resistant individuals will decrease over time, as these plants are no less competitive than the susceptible individuals within the population. In addition to musk thistle, a population of Italian thistle is also suspected of having developed resistance to the phenoxy herbicides in New Zealand.

Integrated Weed Management: Using an integrated approach, the application of sublethal applications of phenoxy herbicides such as MCPA, 2,4-D amine and 2,4-DB amine in conjunction with heavy stocking rates of grazing livestock (sheep) has been a long accepted control method in Australia. Integrating late herbicide application (bolting or bud stages) and infestation with the musk thistle seed head weevils can provide excellent control and reduced herbicide use and costs.

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