Musk Thistle Control in Permanent Grass Pastures
WS-19

Merrill A. Ross, Professor, Weed Science
Daniel J. Childs, Extension Weed Specialist,
Botany and Plant Pathology Department,
Purdue University
Cooperative Extension Service
West Lafayette, IN 47907

 

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Musk thistle (Carduus nutans), or sometimes referred to as nodding thistle, is a weed that is increasing in pastures throughout the western and southern counties of Indiana. Its rapid invasion of pastures can quickly decrease the size of grazing areas for livestock. Understanding the life cycle and growth habits of this weed is useful for implementing effective control practices.

 

Identification and Life Cycle

Musk thistle is a biennial weed. Biennials require portions of two growing seasons to reproduce. The first season the plant germinates from seed and produces a rosette of basal leaves. These basal leaves are characterized by having a wide, white midvein and whitish leaf margins and tips. The rosette overwinters and the cold temperature causes the rosette to send up a stalk the next season, flower and produce seed. Rosettes may reestablish any time during the growing season. Some rosettes may extend three feet or more in diameter by late fall.

Flower shoots are initiated in early May and the plant reaches full flower in early June. Plant size at flowering may reach a height of six feet. Seed production is usually completed in mid-to-late June. Once seeds mature, the plant dies.

The red-purple flower of musk thistle is about two inches in diameter, which is larger than other biennial thistles (i.e., bull and tall) and perennial thistles, such as Canada thistle. The drooping or nodding flower head is surrounded by broad, spiny-tipped bracts. One plant may produce as many as 50 flowering heads, with each flower producing about 50 to 80 seeds. Since musk thistle only reproduces by seed, destruction of plants prior to flowering is an effective means of preventing seed formation and subsequent spread. Dispersal of seed is mainly by wind and transport in thistle-infested hay.

 

Control Practices

The best time to treat musk thistle, or other biennial thistles, with herbicides is in late fall or early spring when the rosettes are present, but before flowering stalks are initiated. Musk thistle plants with flower stalks are more difficult to kill than the rosettes. Rosettes need to be treated when they are actively growing and not under drought stress. The younger the rosette, the more susceptible it is to the herbicide.

 

Foliar Herbicide Treatments for Selective Control in Grass Pastures

One properly timed herbicide treatment per year should prevent seed formation. Fall treatments should be made late enough to kill all rosettes germinated before winter. Late germinating rosettes that establish after early fall herbicide applications could flower the next growing season.

Early spring treatments should kill all overwintering rosettes and those rosettes germinating later in the season should not produce seed until the following year.

Several herbicides have been screened for their effectiveness in controlling musk thistle in pastures. These herbicides have provided good to excellent control of musk thistle rosettes in Purdue University trials.

 

 

Cultural and Mechanical Methods of Control

Weed control in pastures starts with good management practices. Forage grass plants are most competitive with weeds when conditions are right for optimum growth. Therefore, proper liming and fertilization as well as preventing the overgrazing of livestock is important.

As mentioned previously, preventing seed production is essential for long-term control. Mow pastures after stem elongation, but before flowers open. Some regrowth will occur, so a second or third mowing may be necessary. Also, to avoid spread of the seed, keep areas such as fence rows, adjacent pastures and farm lots free of musk thistle.

 

Musk Thistle Control in Conjunction with Pasture Improvement

Several of the herbicides listed above will injure or kill forage legumes. MCPA (Rhonox) is safer than most herbicides on legumes, but may still cause injury. Thus, treatments to control musk thistle need to be made prior to legume establishment. Spot spraying individual rosettes rather than broadcast spraying the entire pasture also spares the legumes. Since musk thistle seed can survive in the soil for a number of years, it may take two or more years of excellent control before seeds are reduced to the point that allows for legume establishment.

 

Common name 	Trade name 	Rate per acre 	Comments*
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2,4-D 		numerous 	1.5-2 pint 	both amine and ester forms are effective

dicamba 	Banvel 		1 pint 		will severely injure forage legumes

clopyralid 	Stinger 	1/3 pint 	legumes cannot be seeded into treated 
						areas for 12 months following treatment

metsulfuron	Ally 		1/3-1/2 oz. 	mixing with 2,4-D may help reduce injury 
methyl						to forage grasses

MCPA 		Rhonox 		1-1.5 quart 	less injurious to forage legumes than 
						other herbicides

triclopyr+2,4-D Crossbow 	2-4 quart 	also effective on many woody species

picloram 	Tordon 		1/4-1/2 pint 	longer soil persistence than other
 						herbicides(A restricted-use pesticide)
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* Read and follow label directions; particularly note the grazing and haying restrictions.

Acknowledgment

The authors would like to thank Dr. J.D. Green, University of Kentucky, and Kelly Patterson, Warren County Extension Educator, for their assistance with this project and publication.

 

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New 10/93

 

Cooperative Extension work in Agriculture and Home Economics, state of Indiana, Purdue University, and U.S. Department of Agriculture cooperating; H. A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the acts of May 8 and June 30, 1914. The Cooperative Extension Service of Purdue University is an affirmative action/equal opportunity institution.