Answer: Thistles are a real problem because even if you dig them out, a small portion of root left behind can sprout into a new plant. Your best defense is to knock them down as soon as you see them sprouting (and certainly before the flower and set seeds). As you hoe them down, the roots will expend even more energy trying to resprout. If you keep at it, you'll eventually win the battle. One practice that seems to be successful is to dig the thistles out, roots and all, then spray Weed-B-Gon in the hole where you removed the weed. The chemicals will kill any root parts left behind.
Best wishes with your weed eradication project!
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