Tackle thistles before they bolt

AMES — Time’s running out to treat biennial thistles with herbicide this spring.

  Buy Photo

Meaghan Anderson, extension field agronomist with Iowa State University, said if you want to effectively use herbicide, you need to do so before the plants bolt.

“We are currently reaching the end of the first window of the year, and while these plants might be easier to see once bolting begins later this spring, they become much more tolerant to herbicide application at that point,” she said.

While goldfinches and butterflies enjoy those purple flowers, invasive biennial species allowed to produce seed can quickly create monocultures of plants that are unpalatable to livestock.

Musk thistle in field

“Once plants have bolted, cutting or mowing stems will reduce or perhaps prevent seed production,” Anderson said. “Chopping individual plants with a spade below ground level is also highly effective but labor-intensive.”

Musk and bull thistles are common invasive weeds in Iowa pastures, and both are biennial, meaning they can be easier to control than the perennial Canada thistle.

Biennial thistles of Iowa, an online resource from Iowa State’s Integrated Crop Management program, has descriptions, comparisons, and control options for musk and bull thistles.

For more information, contact your ISU Extension and Outreach field agronomist.