Musk Thistle (Carduus nutans L.)
DESCRIPTION
Musk thistle is a large biennial, growing to 6 feet (1.8 m.)
tall. Its flowerheads are rose-purple,
up to two and one-half inches (7 cm.) wide and are mostly solitary
and nodding at the tips of the branches. At the base of each
flowerhead are numerous, spine-tipped involucral bracts, one-eighth
to three-eighths of an inch (2 - 8 mm) broad, that are curved
away from the heads. The leaves are of two types. One
type forms basal rosettes during the first year, which overwinter,
staying green, and killing any plants underneath. These are deeply-lobed
leaves, growing up to 10 inches (25 cm) long
and 4 inches (10 cm.) wide, and possessing a prominent, nearly
white midrib. The upright
flowering stalk grows during the second year, producing smaller,
very spiny leaves. Stems are
commonly winged with spiny leaf tissue.
SIMILAR SPECIES
Musk thistle resembles several other thistles that are found in Missouri. Curly thistle or welted thistle (Carduus crispus), the most closely related Missouri thistle, can be distinguished by its smaller flowerheads 0.5 - 1.0 in. (1.5 - 2.5 cm broad), narrower involucral bracts one-eighth of an inch (2mm, or less broad), and by its clustered flowerheads that are ascending rather than nodding.
Other Missouri thistles, including bull thistle (Cirsium vulgare), field thistle (Cirsium discolor), tall thistle (Cirsium altissimum), and Canada thistle (Cirsium arvense), can be distinguished from musk thistle by the fine bristles that are attached to the seeds. Musk thistle has unbranched bristles whereas the thistles in the genus Cirsium have feathery or plume-like bristles. The thistles in the genus Cirsium also tend to have smaller flower heads than does musk thistle. Canada thistle (see page 27), also a noxious weed in Missouri, occurs in thick colonies because it is a perennial with creeping rootstocks. Biennial species like musk thistle die after flowering and therefore do not usually occur in dense colonies.
Musk thistle should be accurately identified before attempting any control measures. If identification of the species is in doubt, the plant's identity should be confirmed by a knowledgeable individual and/or by consulting appropriate books.
DISTRIBUTION
Musk thistle is a native of Europe that was introduced into the U.S. as early as the 1850's. It has become widely naturalized in the U.S. and Canada. In this country, it occurs in at least 40 of the mainland states from the East to the West Coast. Kansas and Nebraska are reported to have the most serious infestations. In Missouri, musk thistle is widespread and probably occurs in most counties today.
HABITAT
Musk thistle is found in waste ground, old fields, pasture, and along roads and railroads. It has become a major weed in range and pasture land, a nuisance pest along rights-of-way, and a potential weed in land placed in conservation reserve programs. It can invade native grasslands, even where existing dense prairie vegetation exists. Glade communities are also likely areas for establishment of musk thistle, especially those with grazing histories and with inadequate buffers of natural land.
LIFE HISTORY
Musk thistle is variable in its flowering strategy, acting as a biennial, a winter annual, or an annual. Plants typically overwinter as rosettes and send up flowering stalks the following spring. Flowering can occur from early June through October. Seeds mature and can begin dispersing within 7 to 10 days of flowering. As many as 11,000 seeds per plant may be produced. Most seeds fall near the parent plant and can remain viable for as long as ten years. Musk thistle infestations are economically important in agricultural systems because they compete with crops for light, space, nutrients, and water. The plants spiny tissue renders it unsuitable for livestock.
CURRENT STATUS
Musk thistle has been formally designated a noxious weed by Missouri law. As such, all landowners are required to control the plant if it is growing on their property. Control is considered to be prevention of seed production. County prosecuting attorneys are required to notify offending landowners in a prescribed manner before leveeing penalties.
Control Recommendation
Recommended Practices in Natural Communities of High Quality
-Cultural, mechanical, biological and chemical control methods have all been used on musk thistles with varying degrees of success in different parts of the country.
Cultural and Mechanical Methods: Musk thistle favors abandoned fields and overgrazed pastures. Rotationally grazed or deferred pasture is less susceptible to infestation than heavily grazed pastures. Although some investigators report thistle populations declined rapidly as natural succession proceeded, others report populations that persisted for twelve years in an unmanaged area.
Hand-cutting or mowing can provide control if repeated over a period of years. Hand-cutting with a sharpened shovel should insure that lateral as well as the terminal bud on the root crown are destroyed. A labor-intensive hand chopping at ground level just before flowering eliminated musk thistle at Konza Prairie, Kansas from a 350-acre pasture. The plants are apparently less likely to regrow if allowed to bolt before hand chopping the root crown. Mowing should also be timed to occur just prior to flowering.
-Biological Controls: Two exotic weevils, the flower head weevil, (Rhinocyllus conicus), and the rosette weevil, (Trichosirocalus horridus) were purposely introduced from Europe and have become established in Missouri and at least ten other states. These biological control agents aid in reducing populations of the thistle. In areas of Missouri where the weevils have been present for the longest period of time (1976-1992), an 80-90% reduction in thistle population has occurred. These weevils are increasing and dispersing naturally. They have also been collected for recolonization throughout the state. As of 1992, flower head weevils were established and widely distributed in 65 counties, whereas the rosette weevil is presently known from 25 counties. Recent recolonizations of the rosette weevil (1990-1992) have been made in 13 additional counties. An integrated pest management system has been developed utilizing knowledge of thistle and weevil life cycles. This program encourages survival of the weevils through a harmonious use of herbicides and mechanical control methods. Biological control methods are intended to establish a balance between predator and target species. As a population of thistle is diminished by the weevils, weevil numbers decline. This allows the thistle to rebound and the weevil numbers to again increase. This scenario of fluctuating thistle and weevil populations does not totally eliminate musk thistle. If the thistle is eradicated from a site by herbicide application or other means, then the predator weevils will also be lost. Then future colonization of the site by musk thistle will require reintroduction of weevils or other means of treatment.
Two studies have addressed the effect of herbicides on biological control agents. In Montana, the effect of 2,4-D on Rhinocyllus conicus varied with weevil density. As larval populations increase, survival decreased. A Virginia study found that treatment with 2,4-D did not adversely affect either Rhinocyllus conicus or Trichosirocalus horridus.
Recommended Practices on Lands Other Than High-Quality Natural Areas
-Chemical Controls: The effectiveness of chemical control is influenced by the weather and the stage of growth of the thistle at the time of application. Chemical control of all types is most effective in the rosette stage and least effective when musk thistle is in flower. By law, herbicides may only be applied according to label directions.
Due to its low cost, 2,4-D ester at 2-4 lb per acre (4.48 kg per half-acre) is the most commonly used herbicide. Application can be made aerially, with a tractor-mounted sprayer, with a backpack sprayer, or in granular form. Effectiveness depends on application when temperatures are not too cool and when it is not too dry. This may limit its use in early spring applications or in fall applications in some areas. 2,4-D is most effective when applied 10-14 days before bolting in the spring. 2,4-D is less effective than picloram or dicamba after plants have bolted.
Dicamba can be used to allow treatment earlier in the spring than 2,4-D. Dicamba at the rate of 1 lb per a provided the same control as 2 lb per acre of 2,4-D in one study. Dicamba at 0.5 lb per acre (0.6 kg per ha) provided between 90 and 100 percent control in the year of treatment for three successive years, but residual control in the second and third years fell to 36% or less. Spring applications of dicamba (.25 lb per acre) in combination with 2,4-D ester (0.5 lb per acre) gave 97% control by the fall of the same year.
Picloram alone or in combination with either 2,4-D or dicamba gives the best late-season control but is more expensive and carries more restrictions. These include restrictions on use near groundwater and on the season of use. Reports of effective use of picloram vary in recommended rates from .27 lb per acre (0.3 kg per half-acre) to 0.5 lb per acre (.56 kg per half-acre). The greatest attraction of picloram is that it provides excellent control during the cool, dry autumn season when neither 2,4-D nor dicamba is as effective, and when non-targeted vegetation is less susceptible. Although picloram overcomes the effects of weather, it does not fully overcome the effect of growth stage. Control of dense, even height, bolted plants using picloram in a ropewick applicator was effective, but control was poor when plants were of varying heights. Concomitant with its greater effectiveness, picloram presents a greater risk of damaging non-target species. Damage to cool-season grasses was reported in a study using higher concentrations of picloram (1 lb per acre).
Failed or Ineffective Practices
Mowing, brushhogging, and spraying when thistles are in full bloom is ineffective. Cutting the stems does not kill the plant at this stage. Stems will regrow and still flower and seed. Similarly, if herbicide application does not kill the plant, which is not too uncommon, regrowth and seeding will occur.
Fire has not been effective as a method for directly controlling musk thistle. It is difficult to generate enough heat to kill the thistle's root crown and fire-scarred plants can bolt, flower, and fruit.
