Thistle, although a national heraldic symbol of Scotland, is branded as invasive or noxious by most of the U.S. Getting rid of these distinctive pink to purple-flowered plants can involve a campaign that takes years for perennial varieties. Although annual and biennial thistles can be controlled more easily, they must be stopped before they let loose the hundreds of seeds they produce. Whatever the variety, you’ll encounter more success if you get started early.
Grow a thick lawn to discourage thistle growth; water deeply and fertilize to help grass keep roots healthy. Pull annual and biennial (plumeless, musk and bull) thistles as they form rosettes in summer before annual plants grow tall branches, or “bolt.” Once plants have bolted, they will quickly form flowers and set seed; if this happens, decapitate plants before they have a chance to bloom.
Mow lawns frequently to prevent perennial (Canada, Flodman or wavyleaf) thistles from “bolting” (the phase of growth that raises flower heads and sets seeds). Keeping perennial thistles limited to growth as ground-hugging rosettes weakens them over time. Perennial thistle roots are often deep and difficult to completely pull. A badly infested lawn may need to be cultivated with a rotary tiller, cleared of plants and re-seeded.
Treat thistles with herbicides containing 2-4D, dicamba or glyphosate in the early fall when perennial and biennial roots and rosettes are gathering strength to get though the winter. Use herbicides labeled for use on Canada and other perennial types or musk and biennial types.
Apply glyphosate to the ends of roots when pulling annual thistles or immediately after mowing other thistles. A systemic herbicide will enter wounds and kill the remaining roots. Late fall application of glyphosate also acts as a “pre-emergent”herbicide, preventing germination of biennial seed.
Contact your local state university extension to ask about biological controls if your interest is organic thistle control beyond digging and mowing. The Midwest Organic and Sustainable Education Service reports research has investigated organic controls such as an up to 20 percent solution of vinegar, the Pseudomonas syringae pv. Tagetis (Pst) thistle parasite and the use of bugs such as the Canada Thistle Stem Weevil (Ceutorhynchus litura) and Thistle Defoliating Beetle (Cassida rubiginosa).