I've kept mine in pots just fine. I'm fairly near the water north of Edmonds and dropped down to about 10 degrees last winter. The only ones I lost were due to a douglas squirrel digging into the pots to bury her hemlock cones.

The biggest problem is rot and I keep my potting soil almost completely dried out all winter.
Right now mine have not been watered for at least a month and are on a shelf, in the open air but under an overhang cover, so they get no rain at all. They do just fine with the normal cold damp air all winter.

In the spring I give them a little drink from below to get them started rooting again.

Ever since I lost a prized 6 inch diameter tuber to rot, I'm really careful about the water.