Northern Prairie Wildlife Research Center
Musk thistle is an erect (0.5-3 m tall) biennial or winter annual, taprooted forb. It occurs solitarily or with several stems from one base and is highly branched above. Initial growth is in the form of a well developed basal rosette. Rosette leaves are elliptic to lanceolate (15-70 cm long) and pinnately lobed with deltoid-shaped segments each further divided into a spine-tipped lobe. Leaf surfaces are glabrous to densely pubescent, and margins are whitish to purplish. Cauline leaves are alternate and have a decurrent attachment forming spiny wings (0.5-2 cm wide) along the stem. Cauline leaf blades are lanceolate to broadly elliptic, pinnatified into lanceolate to ovate spiny lobes, and glabrous to densely pubescent. Heads are terminal, discoid (1-7 cm broad), globose, usually nodding, and occur singly or a few in a cluster. Peduncles are naked or with a few bractlets. Involuceral bracts are lanceolate (9-27 mm long and 2-10 mm wide), numerous, imbricate, reflexed, and tipped with a spine 1-15 mm long. Flowers are perfect, tubular (10-14 mm long and 1.4 mm wide) and purplish to white. Achenes are oblong (3-4 mm long), glabrous, brown, and have a pappus of numerous, whitish, barbed bristles. Two subspecies are of musk thistle are generally recognized. Subspecies. leiphyllus (Petrovic) Stoj & Stef. is the most common in North America. Subspecies macrocephalus (Desf.) Nyman has not been reported in Minnesota may be found only in Wyoming, Montana, and South Dakota.
Musk thistle is a highly competitive, noxious weed. A native of Eurasia, it was first reported in North America during the period of 1853 to 1866. Both early and later introductions may have been for ornamental uses. It has now been reported in all Canadian provinces (except Prince Edward Island) and in more than 40 states within the United States. It can occur in either sparse or dense stands on disturbed sites within pastures, prairies, wooded bottoms, fields, and roadsides. Musk thistle does not appear to have any specific climatic requirements other that a cool period of vernalization for flowering. Musk thistle grows in all soil textures, but the soils must be well drained. It is most abundant in fertile soils but may be found in poorer soils. Musk thistle occurs on soils with a pH range of 6.0 to 8.9.
Although some research has shown a dormancy period of up to 8 weeks, germination usually begins 14 to 21 days after the seed is dispersed in the fall. Additional seeds may germinate the following spring. Seeds germinate at high rates in soil cracks or rough microtopographies with reduced evaporation. Although lower rates of germination occur under nearly all natural conditions. Up to 95% germination has been reported. The transition from seedling to rosette occurs rapidly. Musk thistle usually requires a vernalization period of a minimum of 40 days below 10°C to produce flowers. These conditions normally occur over winter, but sometimes occur after musk thistle seed germination in the spring, allowing these plants to flower and set seed the same year, thus behaving as annuals. Research has shown if a rosette reaches a diameter of at least 36 cm it will flower the following season independent of vernalization. Bolting begins as early as April and continues into June. Flowering begins in late May or early June and continues through mid-July, usually lasting 7 weeks.
Seeds mature and are dispersed 1 to 3 weeks following flowering. Each head produces between 25 and 11,000 achenes, with an average of 1,200 to 1,500. Seeds are dispersed primarily by wind. The majority of the seeds will remain within 50 m of the parent plant, and less than 1% of the seeds are carried further than 100 m from the parent plant. Musk thistle seed appears to remain viable in the soil for at least 10 years. Musk thistle does not reproduce vegetatively. Although, if the taproot is severed at ground level, it will again produce a plant capable of flowering.
An intermediate number of musk thistle plants occur at Pipestone National Monument (PIPE). They occur in patches that cover less than a total of 5 hectares. These plants are found in mid-successional sites that were disturbed in the last 11 to 50 years, but they can invade undisturbed prairie. Musk thistle has the long-term potential to modify or retard the natural secondary successional processes at PIPE by competing for successional resources. The plants have a minor visual impact on PIPE.
Biological, chemical, cultural, and mechanical control methods have all been used on musk thistle with varying levels of success. An important consideration is that the seeds will remain viable in the seed bank for at least 10 years, thus removal of living plants may not totally eliminate the plant. A further consideration is that many sources of new propagules surround the PIPE. Most control methods will have a detrimental effect on other plant species and may constitute a disturbance that will favor reinvasion by musk thistle or by other exotic species. Many native thistles are in the area and should not be subjected to control, making proper identification important.
Cultural control methods have been used for musk thistle with mixed success. Retaining a high canopy cover to reduce light reaching seedlings and maintaining high litter levels to prevent seeds from coming in contact with the soil helps reduce seedling establishment. However, one study indicated that light litter levels conserved soil moisture which encouraged musk thistle seedling establishment. Fire has not been effective in controlling musk thistle. Spring prescribed burns do not develop high enough temperatures to kill the crowns. However, prescribed burns may improve growth of warm-season grasses that will increase competition pressure on musk thistle. A mid-spring prescribed burn, used to promote warm-season grasses, can make musk thistle plants more visible for hand control efforts and will eliminate a large number of seeds. Grazing as a control method is ineffective, because livestock only eat a few flowers. Heavy grazing and associated disturbances near water, salt, and loafing areas will increase establishment of seedlings.
Repeated hand grubbing and mowing have been used to control musk thistle. Hand grubbing is highly labor intensive and may not result in the elimination of a plant. It is important to completely remove the crown. If only the top of the plant is removed, the lateral buds may be stimulated and may result in a multi-stemmed plant. Retreatments throughout the growing season are necessary to fully control this species. Viable seeds may develop in heads of severed plants if they are cut after anthesis. Therefore, heads must be removed from grubbed plants buried deeply or burned. Mowing within 2 days of anthesis prevents seed development and dispersal. Retreatment of the remaining plants after they have bolted is necessary.
Herbicides are not specific to musk thistle and may not be specifically licensed for the uses discussed here. It is important to read and follow all label directions. The most effective chemical control occurs when musk thistle is still in the rosette stage, and it quickly decreases after the plant has bolted. The most commonly used herbicide is 2,4-D applied either with a hand or tractor-mounted sprayer. This herbicide is most effective when applied 10 to 14 days prior to bolting during periods that are not overly cool or dry. Banvel (dicamba) has provided good control of musk thistle. In a Minnesota study, a combination of Banvel and 2,4-D ester provided 97% control of musk thistle. This combination can be made or obtained as the premix Weedmaster (dicamba and 2,4-D). Tordon (picloram) usually gives good spring control of musk thistle. However, another study found that Tordon was ineffective in control when applied at rates below 0.56 kg/ha. Tordon has an advantage over 2,4-D and Banvel in that it will provide good control during a cool, dry period when the other two are ineffective. Fall application of Tordon to rosettes when other plants are dormant is often effective and has less influence on nontarget species. Tordon will also provide good residual control, but it will also provide the greatest damage to non-target species. Another control method that will work well after bolting is a mixture of 2,4-D and Tordon. Other herbicides that provide control of musk thistle are Crossbow (triclopyr plus 2,4-D), Oust (sulfomethuron methyl), and Stinger (clopyralid).
Nearly 100 species of insects have been evaluated worldwide as potential control agents for musk thistle. Also, fungi and viruses have been investigated as control agents. One problem has been that the flowering period of musk thistle often exceeds the life span of the controlling insects. The most widely released insect is the weevil Rhinocyllus conicus Froelich (Coleoptera:Curculionidae). It has been released into musk thistle populations in at least 23 states and four Canadian provinces. In the spring, adults will feed on the leaves, mate, and oviposit on the bracts. The eggs hatch, and the larvae begin to feed by boring into the receptacle and peduncle reducing the ability of the plants to produce viable seed. This weevil has been most successful when released in the spring in cool climates. One study indicated that a minimum population of at least 20 larvae per head was necessary to eradicate seed production. However, other studies indicated that insect populations generally level off to three to six weevil larvae per head. In some cases, this insect has given good control by reducing thistle densities to less than 10% of the prerelease levels.
Another widely used European insect to control musk thistle is the weevil Trichosirocalus horridus Panzer [syn. Ceuthorynchidius horridus Panzer (Coleoptera:Curculionidae)]. Larvae infest the rosettes causing necrosis of the crown and destroying apical and lateral meristematic tissue which weaken the plants. Research in Canada showed the best control of thistles came from the infestation of smaller (spring) rosettes as compared to larger (overwintering) rosettes. Overall, the effect of this insect in controlling thistles is relatively small. A study using this weevil in combination with 2,4-D resulted in a 85% thistle mortality rate as compared to 7% mortality with weevils only and a 55% mortality for the herbicide alone. Another insect, through an accidental introduction, with a minimal effect on musk thistle, is the European insect Cassiba rubiginoa Muller (Coleoptera:Chrysomelidae).
Four fungi have been reported to infect musk thistle are Puccinia carduorum Jacky., P. galatica Sydow (syn. P. carduipycnocephali Syd.) (Uredinales), Ustilago carduri F. Waldheim, and U. violacea (Pers.) Roussel (Ustilaginales). Only Puccinia carduorum has been released in the United States. This fungus is introduced to the rosettes and reduces plant vigor. It should be noted that this species has also been found to attack five other members of the Carduus genus, eight members of the Cirsium thistle genus, and two members of the Cynara genus. The fungus is being tested in conjunction with insects to enhance control.
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