bull thistle

Bull thistle seed heads are easy to see, the seedlings and rosettes are easiest to kill.

Fall Weed Control: Bull thistle is here, and now is the time to wipe it out

THE ISSUE: Bull thistle is a non-native biennial thistle that is making headway throughout Idaho. It is shorter than musk thistle (usually flowering at 1 to 4 feet tall) and not as prevalent, which makes it more difficult to spot. It flowers later than musk thistle and Scotch thistle. The magenta flowers are shaped like a shaving brush. Seed heads are usually 1 to 2 inches wide, with spiny bracts all over them. The flower stem is thorny all the way to the flower.

Bull thistle invades yards and gardens, pastures, ditch banks, rights-of-way and even cultivated fields.

Integrated Pest Management Options:

· Mechanical: Tilling, digging, hoeing and field cultivation of bull thistle rosettes in the fall works 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 flies and a weevil that attack bull thistle, but control is never complete.

· Chemical: Most broadleaf weed killers will work well on bull thistle rosettes. Some herbicides are not intended for homeowner use and many will damage trees and shrubs that are some distance away from the spray site. 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:

Weed control should be in our minds throughout the year. Fall is the best time to control bull 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.