Weedy thistles are difficult to eradicate. Thistles are perennial or biennial. Effective control of each class is slightly different, based on their different methods of reproduction.
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Timing For Biennial Thistle Herbicide
Apply herbicide for biennial thistles during their rosette stage of growth to prevent seeding. Biennial thistles include musk thistle (Carduus nutans), plumeless thistle (Carduus acanthoides) and bull thistle (Cirsium vulgare).
Weed Killers for Biennial Thistle
The University of Kansas weed control program recommends different chemical herbicides for spring and fall application for biennial thistles; however, they identify Picloram and Aminopyralid for use in either season.
Weed Killers for PerennialThistle
In a study at Iowa State University, the best weed killer for the perennial Canada thistle (Cirsium arvense) was picloram, though Stinger worked comparably well. However, they did not test other perennial thistles that are less invasive, such as Flodman thistle (C. flodmanii) and wavyleaf thistle (C. undulatum).
Warning for Perennial Thistles
Perennial thistles have extensive root systems, which are not obvious from above ground. Therefore, you may need multiple herbicide treatments.
Timing for Perennial Thistles
The University of Minnesota recommends applying herbicides to perennial thistles just after budding, but herbicides applied soon after seed dispersal are almost as effective.
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- Kansas State Department of Agriculture: Plant Protection and Weed Control: Musk Thistle
- Hutchinson Leader: Hit Canada Thistle with Herbicide or Cutting Now
- Iowa State University Weed Science Online: Canada Thistle Management
- North Dakota State University Extension Service: Perennial and Biennial Thistle Control