Answer: As you've discovered, mint is terribly aggressive and nearly impossible to eradicate because even a tiny piece of root left in the ground can develop into a plant. Disturbing it just seems to make it want to grow that much more. Aside from hand digging the bed and screening out the last remnants of roots, you might try using Round Up on the mint plants. Round Up is a vegetation killer so if it comes in contact with your other plants, it will kill them, too. But, if you carefully paint the chemical on the plants you recognize as mint, it should be translocated to throughout the root system and kill out the mint. It will take time and patience, but it can be done. Wish there were an easier solution!
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