Since most thistles spead by both seeds and roots, you need to kill the entire plant and to also cut off the bloom before it can seed. Spray carefully just the thistles with a thistle killer - it take special products designed to kill thistles. Make a shield out of paper or cardboard so only the thistles at hit by the spray. Now the bad news. It may take several years to complete rid yourself of them.
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Weeds Taking Over Wild Flower Gardens