Answer: Thistle is probably one of the worst weeds to try to control! Thistle, a perennial, multiplies both by seed and through spreading horizontal roots called rhizomes. Most other thistles, such as bull and musk thistles, are biennials. Their first year, they produce 4- to 18-inch-wide rosettes, but must first go through a cold winter period before they flower, set seed and die the following season. Unlike Canada thistle, these biennials can be knocked out by keeping them from setting seed. As long as you keep cutting them down when they appear, the roots will eventually run out of stored energy and the entire root system will die out. 2,4-D is widely used for thistle control, but one application is seldom enough to kill the plants. I think the best time to spray thistles is in the early fall, when temperatures are still in the 70s F. Nearly all of the plants will be in the rosette stage and actively growing. At this point in the season, the thistles are transporting sugars and starches to their roots for winter, and so more effectively carry the herbicide down along with these nutrients than in spring. Keep hacking and hoeing this summer, and then spot treat with 2,4-D this fall. Best wishes with your landscape!
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