I overwintered one once buried in some chopped leaves near my compost pile. But I tried it last winter without success.

A vendor at the Northville Farmer's Market usually has them for $2.50 in case that helps.

In zone 6 mine always come back, but this last winter was the test and they did come back. It took mine several years to get big so I think keeping them is better than buying new. Have you tried chopping them down and covering with a deep pile of straw?

I grow mine in pots and put the pots in the cold cellar or a cold entry-way that mitigates the cold. I've had them survive both ways.