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

SYNONYMS:

• musk thistle: nodding plumeless thistle, nodding thistle, plumeless thistle, Carduus armenus Boiss., Carduus thoermeri Weinm., Carduus coloratus Tamamsch, Carduus schischkinii Tamamsch., and others.
• giant plumeless thistle:bristly or spiny thistle, Carduus fortior Klok.
• Italian thistle:slender thistle, Plymouth thistle.
• slenderflowered thistle: Italian thistle, seaside thistle.

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.

• musk and giant plumeless thistle: Biennials (or winter annuals) to 1.5 m tall. These species readily hybridize with one another, and plants with intermediate characteristics may be found where their ranges overlap. Introduced from Europe.
• Italian and slenderflowered thistle: Winter annuals (or biennials) to 2 m tall. Italian and slenderflowered thistle infrequently hybridize since slenderflowered thistle is typically self-pollinating and chromosome numbers are variable (Italian thistle: 2n = 62-64, slenderflowered thistle: 2n = +/- 54, 62-64). Some taxonomists believe that Italian and slenderflowered thistle may not be distinct species. Italian and slenderflowered thistle are introduced from the Mediterranean region and central Europe, respectively.

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).

• musk thistle:Stems narrowly winged, glabrous to woolly. Leaves 1- to 2-pinnate lobed, glabrous to sparsely hairy. Lower leaves mostly 10-40 cm long.
• giant plumeless thistle:Stems strongly winged, glabrous to lightly woolly. Leaves 1-pinnate lobed, typically sparsely hairy. Lower leaves mostly 10-20 cm long.
• Italian thistle:Stem wings to 0.5 cm wide, often interrupted near flower heads, glabrous to slightly woolly. Leaves mostly covered with woolly hairs. Lower leaves 10-15 cm long, mostly 4-10-lobed.
• slenderflowered thistle:Foliage resembles that of Italian thistle, except stem wings may be up to 1 cm wide, wings usually extend continuously to flower heads, and basal leaves are usually 12-20-lobed.

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.

• musk thistle: June-September. Heads (hemi-) spherical, 2-7 cm diameter, solitary, often nodding on stalks usually more than 2 cm long. Disk flowers purple, 20-25 mm long, with tubes 12-14 mm long. Phyllaries lanceolate to ovate, mostly 3-8 mm wide, spreading to reflexed at the middle, glabrous to sparsely woolly.
• giant plumeless thistle: May-August. Heads (hemi-) spherical, 1-3 cm diameter, solitary or clustered, on stalks less than 2 cm long. Disk flowers purple, 14-20 mm long, with tubes 7-10 mm long. Phyllaries narrowly lanceolate, 2 mm wide or less, glabrous to pubescent, tips erect to spreading.
• Italian thistle: May-July. Heads +/- cylindrical, 1-2 cm diameter, +/- sessile, usually 2-5 per cluster. Disk flowers pink to purple, 10-14 mm long, with tubes 5-8 mm long. Phyllaries linear-lanceolate with erect tips, 2 mm wide or less, with persistent patches of woolly hairs at the bases.
• slenderflowered thistle: May-July. Heads, flowers, and phyllaries resemble those of Italian thistle, except the following: heads often 5-20 per cluster, flower tubes mostly 4-6 mm long, phyllaries with narrowly membranous margins and +/- glabrous to sparsely woolly at the bases.

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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.

• musk thistle: Achenes 4-5 mm long, with longitudinal dotted stripes. Pappus bristles 13-25 mm long.
• plumeless thistle: Achenes 2-3 mm long, with faint lengthwise stripes. Pappus bristles 11-13 mm long.
• Italian thistle: Produces 2 types of achenes 4-6 mm long: central achenes cream-colored, initially sticky, with ~20 longitudinal stripes and pappus bristles 10-15 mm long. Outer ring of achenes golden to brown, smooth, lack pappus bristles.
• slenderflowered thistle: Achenes and pappus bristles resemble those of Italian thistle, except central achenes have ~ 10-13 longitudinal stripes.

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.

• Musk thistle grows best on moist alluvial soils but tolerates a wide range of conditions, from acidic to saline soils. Plants establish poorly on highly acidic or nutrient deficient soils, or soils with extremes in moisture content.
• Giant plumeless thistle often occupies similar but drier, better-drained sites. Serious infestations of musk and giant plumeless thistle are often associated with sandy, fertile soils.
• Italian and slenderflowered thistle infest many sandy and to clay soil types.

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

• musk thistle: Klamath Ranges, Cascade Range (c & e Siskiyou, n Shasta cos.) northern Sierra Nevada (s & e Plumas, e Sierra, c & e Nevada cos.), Modoc Plateau (Modoc, n & s Lassen cos.); to 4000 ft (1200 m); widespread in North America. Previous infestations now eradicated occurred in the South Coast (se Los Angeles, ne Riverside cos.) and Mojave Desert (se San Bernardino cos.).
• giant plumeless thistle:Eastern North Coast Ranges (se Humboldt, cw Trinity, w Glenn, ne Lake, e Colusa cos.), northern Sierra Nevada (w Nevada Co.), Modoc Plateau (ce Modoc co.), San Francisco Bay region (nw Marin Co.); to 4300 ft (1300 m); widespread in North America. A previous infestation now eradicated occurred on the Central Coast (cw Monterey Co.).
• Italian thistle: Southern North Coast, southern North Coast Ranges, Sierra Nevada foothills, Central Coast, San Francisco Bay region, South Coast Ranges; to 3300 ft (1000 m).
• slenderflowered thistle: North Coast region, North Coast Ranges, Sierra Nevada foothills, Central Coast, San Francisco Bay region, South Coast Ranges, South Western region; to 3300 ft (1000 m).

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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.

• musk and giant plumeless thistle: Plants appear to require a period of chilling to induce flowering. First flower heads can produce large numbers of seeds, sometimes 1500 or more seeds per head. Late flower heads produce fewer seeds, to less than 25 seeds per head. Most seeds (~99%) are dispersed within 50 m of the parent plant. Few seeds (<1%) are deposited further than 100 m from their source. Typically, seeds are dispersed 1-3 weeks after flowering. Reports of seed dormancy or an afterripening period in musk thistle seeds are variable. Most musk thistle seeds germinate 2-4 weeks after dispersal.
• giant plumeless thistle seeds appear to lack an afterripening period. The optimal temperature for germination of musk thistle seeds is near 20ºC, and light is not required. Seeds of both species typically germinate late summer through spring, depending on environmental conditions. Most plants are biennials, germinating in the winter/spring months and existing as a rosette until flowering in the spring/summer of the following year. Seeds that germinate in late summer/early fall can behave as winter annuals, flowering in the following summer months. Seeds can germinate in constant heavy shade, but seedlings seldom reach maturity under this condition. In one study, buried musk thistle seeds remained viable for 10 years. However, seeds of both species rarely persist in the soil seedbank. A seedbank study of soil in a permanent pasture indicated that seeds on the soil surface and in the top 2 cm of soil did not persist beyond 3 years because of germination. Similarly, a field seedbank study of giant plumeless thistle suggested that viable seeds of this species rarely persist in the soil seedbank due to decomposition and seed predation by insects, mammals, and birds.
• Italian and slenderflowered thistle: Plants do not require a period of chilling to induce flowering. Flowering is continuous until soil moisture is depleted. Seeds are highly viable and germinate under a wide range of temperature regimes. High temperatures such as those that typically occur in late summer inhibit germination. Seedlings can emerge from soil depths of 8 cm, but depths of 0.5-2 cm appear optimal. Seeds beneath about 1 cm of litter or soil germinate better that those on top of litter or soil. Seed germination on the surface of clay soils is greater than on other soil or litter surfaces. Central seeds are mostly wind or gravity dispersed and apparently lack an afterripening period. Seed coats contain germination inhibitors that can leach out within 2 days in the presence of adequate water. Evidence suggests that the mucilaginous coating can act as an adhesive for seed transport and that it increases water retention of seeds, improving germination on the surface of clay soils. On average, flower heads produce about 11 central seeds and 2-3 outer seeds. Outer seeds remain with the flower head and typically germinate in the head after it falls to the ground. Outer seeds are variably dormant and persist in the soil for up to ~ 7 years.

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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.

• 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.

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CONTROL METHODS:

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.

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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