Answer: Mint has a tenacious root system and the smallest piece of root can sprout anew. The good news is that it pulls out pretty easily, especially if soil is moist, so you can keep after it just by pulling it as soon as it appears. If you don't want to hand weed, you might want to try glyphosate, which is a systemic herbicide. This means when it is sprayed on a plant, the plant absorbs and distributes it throughout its system. Eventually, it kills the entire plant, including the roots. The spray can drift to other plants, particularly if there is any breeze. Three feet is not very far away. Spray in the morning when air is usually calmest and cover any nearby plants. Another solution is to dab the hard-to-control weeds with glyphosate in a wick applicator (VERY low chemical use that way), which is preferable in a food producing garden.
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