Mike if we have a winter like we had last year you might have a chance.

If you do not have any other option but to leave them in the ground I would try covering them first with evergreen needles or limbs. Have some one save you some of their christmas tree limbs when they throw them out if you do not have any. Next get styrofoam boxes and put them over the plants. Put a rock over them so they do not blow away. cover the boxes with extra limbs.

I left mine outside for two weeks last winter the pots were frozen solid but they still survived. It is wet and freezing that kill cannas. If you can keep as much moisture out as possible and as I said we have a mild winter you have a good chance. I would lay the boxes and EG limbs around the planting area also.

This process will build a insulated house around your plants.