Thistles can be controlled

By Leslie Scharafanow
Chisago County Master Gardener
Thistles are often troublesome weeds in Minnesota gardens and lawns. The first step to proper thistle control is their identification. Biennial thistles can be controlled by digging and cultivation; this is more difficult with perennial thistles because they spread by creeping underground stems (rhizomes). The most effective way to remove perennial thistles is through the use of herbicides. Broadleaf herbicides containing 2,4-D and MCPP can control thistles in lawns. In gardens, it may be best to spot treat thistles with a non-selective herbicide containing glyphosate, such as Round-up.
Herbicides must be applied when weeds are actively growing and air temperatures are roughly 60° to 85° F. The best times to control weeds are in the fall (September through mid-October) or spring (late April through mid-June). Always read and follow all pesticide label directions carefully.
Biennial thistles
Biennial thistles form a low-growing rosette of leaves the first year, followed by a taller, flower and seed-bearing stem the second year. They live only two years. It’s best to eliminate them the first year so they never have a chance to bloom and produce seeds. Common biennial thistles in Minnesota are bull, musk, plumeless and tall.
Perennial thistles
Perennial thistles come back each year from roots that survive the winter. They bloom and set seed yearly. Common perennial thistles in Minnesota are Canada and flodman.

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