Musk Thistle is usually a biennial or winter annual, but it will occasionally occur as a summer annual. As a biennial, it produces a rosette the first year and develops flowers and seeds the second year. As a summer annual, this thistle completes its life cycle from germination through seed production in the same growing season. Seedlings may emerge anytime during the growing season, but the plants usually remain in the rosette stage for about 90% of their life cycle.

Musk thistle usually begins flowering in May and may bloom from seven to nine weeks. Seed dispersal begins seven to ten days after a head begins to bloom and an average of 11,000 seeds can be produced per plant with around 5,000 of the seeds capable of germination.

Musk Thistle was declared a noxious week in Kansas in 1963. Kansas law requires all persons and organizations prevent its spread and eradicate it.

There are several different methods for controlling thistle. The first line of defense is proper grazing management. Invasion of thistle is slowed on grasslands that are properly stocked to maintain a good cover of competing vegetation. Burning does not control the thistle, but it makes control more effective. Burn according to your burn plan, and then spray 10-14 days after. For the most effective control, apply herbicides while the musk thistle is in the rosette stage and actively growing, usually early May. For fall, October 1st to soil freeze is an excellent time. During the fall, most musk thistles are in the rosette stage and are most susceptible to herbicides. Use herbicides when they are growing rapidly under good soil moisture and favorable air temperatures (70-90 degrees F). A 6-12 hours rain-free period after application at the recommended rate is adequate to allow herbicide absorption.

For control in pastures that have a scattered issue, you can remove them by hand. Dig the plant below the crow to prevent further development. Dig, remove the plant, and burn the flowers that are in late bud or bloom stage. Research shows that mowing is the most effective at full bloom, but must be repeated as necessary for effective control.

For questions over what herbicides are labeled for use, consult K-State Research and Extension’s Chemical Weed Control guide. Available online or you can pick up a copy at most Extension offices.

Column adapted from publication L-231.

Alicia Boor is the Agriculture and Natural Resources agent with K-State Research and Extension – Cottonwood District. Contact her by email ataboor@ksu.edu or call 620-793-1910.