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Management Of Musk Thistle
Musk thistle is a winter annual or biennial which, unlike other noxious weeds of Larimer County, reproduces solely by seed. Therefore, preventing seed production is a crucial aspect of musk thistle control.
Mechanical Control
Unlike Canada thistle, repeated mowing may control this weed particularly if this is done in the late flowering stage. Musk thistle will not tolerate tillage and can be removed by severing its root below the ground surface with a shovel or hoe. It is important to remember that this plant can produce a new plant from a severed taproot. Therefore, the crown must be completely removed. Repeated treatments are necessary and heads should be deeply buried or burned because seeds can mature and become viable after cutting.
Chemical Control
Apply 2,4-D Amine at 1.5 qts/A at spring rosette before flowering stalk lengthens or late Fall treatment of rosettes; or Banvel (1 pt./A) plus 2,4-D Amine (1 qt./A) or Veteran 720 (2 qts./A) during spring rosette before flowering stalk lengthens or late fall treatment; or Tordon (.5-1.0 pt/A) plus 2,4-D Amine (1 qt./A) during spring rosette before flowering stalk lengthens or late fall treatment of rosettes; or Escort (.75 oz/A) or Ally (1.0 oz/A) during Spring from rosette to early flower stage or fall rosette. These herbicides can be purchased at an agricultural products supplier (please check the yellow pages for specific stores).
Biological Control
The seedhead weevil Rhinocyllus conicus limits seed production of musk thistle. Females deposit eggs on flower bracts, and eggs hatch in six to eight days. Young larvae burrow into the flower receptacle and consume developing seeds. However, the decreases in viable seed production are highly variable. Research has shown decreases of seed production varying from 10% to 78%. It is important to note that biological control agents will never completely eradicate a weed but attempt to bring the weed to a manageable level. Larimer County may be able to supply these insects to landowners for free or a nominal cost. Please call our office if you have an infestation of at least a .5 acre and are interested in these insects.
Cultural Control
Maintaining healthy pastures with vigorous perennial grass stands is the best cultural control method for musk thistle. Musk thistle germination and establishment is favored in open areas and favored by disturbance. Therefore, reseeding or replanting with desirable vegetation in areas of soil disturbance is necessary to prevent musk thistle invasions.