Musk thistle (Carduus nutans)

Author: George Beck, Colorado State University Cooperative Extension

Adapted from the Colorado State University Extension Natural Resources Fact Sheet "Musk Thistle"

Identification and Life Cycle

Musk thistle (Carduus nutans) is a biennial, although occasionally an annual, in the sunflower family (Asteraceae). Flower heads are deep rose, violet, or purple, or occasionally white. Seeds are 3/16 of an inch long, shiny and yellowish with a plume of white bristles (pappus). Leaves are dark green and deeply lobed with spiny margins. Leaves extend onto the stem giving a winged appearance. Stems can grow up to 6 feet tall. Seedlings form rosettes with wavy leaf margins and yellow spines. Rosettes are usually large and compact with a large, corky taproot that is hollow near the crown.

Habitats

Musk thistle can invade pastures, range, forests, roadsides, waste places, ditches, and stream banks.

Impacts

The weed can form dense stands and reduce desirable wildlife and livestock forage. It is unpalatable to livestock.

Biology and Ecology

Seedlings normally emerge early in spring, develop into rosettes and spend the first season in this growth stage. Seedling emergence also can occur in fall. All seedlings grow into rosettes and overwinter in that stage. Early in spring of the second year, overwintered rosettes resume growth. Shoots begin to elongate (bolt) in late March through May, depending on weather and altitude. Musk thistle flowers and starts to produce seed 45 to 55 days after it bolts. Musk thistle is a prolific seed producer. One plant can set up to 20,000 seeds; however, only one-third of the seeds are viable. Musk thistle produces many flower heads. The terminal, or tallest, shoots flower first, then lateral shoots develop in leaf axils. A robust plant may produce 100 or more flowering heads. Musk thistle flowers over a 7 to 9 week period. It begins to disseminate seed from a head about two weeks after it first blooms. It is common to observe musk thistle with heads in several stages of floral development and senescence. Thus, musk thistle sets seed over an extended time period. Most seed is disseminated within the immediate vicinity of the parent plant. This leads to a clumped pattern of seedling development and results in intraspecific competition and mortality. Less than 5% of seed remains attached to the pappus when it breaks off the flowering head and floats away on wind currents.

Management Approaches

Cultural methods that favor desirable plant growth can be combined with chemical or biological control by superimposing proper grazing management and seeding. To combine chemical and biological control methods, apply herbicides when they will not interfere with insect development. That is, allow the control insects to complete their life cycle. Or use herbicides in areas that are not sensitive to their use and biological control in areas where herbicides are impractical or environmentally unsafe.

Biological Control

The musk thistle seed head weevil, Rhinocyllus conicus, and the rosette weevil, Trichosirocalus horridus, have been released in the U.S. as a biological control for musk thistle. The seed head weevil is known to attack native thistles and is no longer being distributed by the USDA. Trichosirocalus horridus attacks the crown area of musk thistle rosettes and kills or weakens the plant before it bolts. Thistle seed head weevils can also negatively impact native thistles, including some rare species, so there is some concern about its continued use.

Mechanical and Cultural Control

Musk thistle will not tolerate tillage and can be removed easily by severing its root below ground with a shovel or hoe. Mowing can effectively reduce seed output if plants are cut when the terminal head is in the late-flowering stage. Gather and burn mowed debris to destroy any seed that has developed. Maintaining pastures and rangeland in good condition is a primary factor for musk thistle management. To favor pasture and rangeland grass growth, do not overgraze. Fertilize only when necessary and according to soil testing recommendations. To successfully manage musk thistle, prevent seed formation.

Chemical Control

Musk thistle should be treated during the rosette stage or prior to flowering. There have been reports of musk thistle resistance to synthetic auxins in New Zealand, although there have been no resistant populations reported in the United States. For more information on resistance please see http://www.weedscience.org/Summary/USpeciesCountry.asp?lstWeedID=42&FmCommonName=Go.

Examples of herbicides that can be used to manage musk thistle

Consult herbicide labels for additional rate, application, and safety information. Additional herbicide information can be found at http://www.greenbook.net.

Herbicide Active Ingredient trade name	Mode of Action	Product per Acre	Application Time or Growth Stage
Established Grasses in CRP acres, Pasture, Rangeland
Chlorsulfuron; metsulfuron-methyl	Inhibition of acetolactate synthase (ALS)
*Cimarron Plus		0.25 ounce	Apply in late fall or in spring before musk thistle bolts.
Rangeland and Permanent Grass Pasture
*Dicamba	Action like indole acetic acid (synthetic auxins)
Clarity		8 - 16 ounces	Apply when musk thistle is in the rosette stage, but before the diameter of the rosette exceeds 3 inches.
Aminopyralid	Action like indole acetic acid (synthetic auxins)
*Milestone		3 - 5 ounces	Apply in the spring and early summer to rosette or bolting plants or in the fall to seedlings and rosettes. Apply at 4 to 5 fl oz when plants are at the late bolt through early flowering growth stages. 2,4-D at 1 lb ae/acre should be tank-mixed with Milestone starting at the late bud stages.
Clopyralid	Action like indole acetic acid (synthetic auxins)
*Transline		0.3 - 1 pint	Apply when musk thistle is in the rosette to early bolt stage.
2,4-D; clopyralid	Action like indole acetic acid (synthetic auxins)
*Curtail		2 - 4 quarts	Apply in late fall or in spring before musk thistle bolts.
2,4-D; picloram	Action like indole acetic acid (synthetic auxins)
*Grazon P+D		2 - 4 pints	Picloram is a restricted use product. Apply 2 pt/acre at rosette stage. Apply 3 to 4 pt/acre in mid to late season from bolting to bud stage.
Picloram	Action like indole acetic acid (synthetic auxins)
*Tordon 22K		0.5 - 0.75 pint in fall or 0.5 - 0.75 + 1 pound 2,4-D in spring	Picloram is a restricted use product. Apply at the rosette stage before bolting in the spring or in the fall prior to soil freeze up. In spring apply before flowering at the rate of 3/4-1 pt of Tordon 22K +1 lb ae of 2,4-D/acre.

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.

References

For more information and images please visit IPM Bugwood. http://www.ipmimages.org/search/action.cfm?q=musk%20thistle

HPIPM:Musk thistle

Contents