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We're in a 6b zone here in Detroit and they managed pretty well, but I know they benefit from warmer conditions. I read somewhere to cover the soil with clear plastic to help keep the soil warm. We didn't do it, but might be worth a shot.

Cheers.

I'm going to try out growing some plants in a low tunnel this year. That would probably be a great place to put sweet potatoes. It should give them a little extra warmth early in the season.