Musk thistle

Musk thistle

Fall Weed Control: Musk thistle: It’s not too late to do something.

The issue: Musk or nodding thistle is a non-native biennial thistle that is continuing to spread like wildfire throughout Idaho. Musk thistle is fairly tall (usually flowering at 2 to 6 feet tall). It flowers early- to mid-summer. The large, magenta flowers are shaped like a powderpuff brush. The dark-green leaves have a nearly white margin. The upper portion of the flower stem does not have thorns. The dead skeletons of this year’s plants are easily identified, but now is the time to look down and find the rosettes. The seedling and rosette growth stages are easier to kill than the flower stage.

Musk thistle invades yards and gardens, pastures, ditch banks, rights-of-way, and even cultivated fields. It only reproduces by seed so focus on keeping it from going to seed.

Integrated pest management options:

· Mechanical: Tilling, digging, hoeing, and field cultivation of musk thistle rosettes in the fall work very well. Be sure to cut the root off a couple of inches below the rosette. Remove and destroy any flowering seed heads throughout the growing season before digging up a mature plant.

· Cultural: Encourage competition from desirable plants.

· Biological: There are two weevils, a tortoise beetle and a leaf and shoot miner fly that attack musk thistle, but control is never complete.

· Chemical: Most broadleaf weed killers will work well on musk thistle rosettes. Spraying in the spring may not guarantee the plants don’t produce seed while the plant is dying. Be sure the herbicide, crop or landscape situations are listed on the product label. Always read and follow herbicide label directions.

Combine them: Fall is the best time to control musk thistle. Combine different IPM options to help improve the effectiveness of your efforts.

For more information, contact Ron Patterson, University of Idaho Extension horticulture/agriculture educator in Bonneville County.