I see these huge, gorgeous Canna outside every year being maintained in large outdoor containers at a local NJ gas station, I am sure they do not remove the tubers..., they don't even mulch them. You can even see a lot of the tubers are not exactly buried under the soil. So I was wondering how cold hardy are Canna. This is NJ/Zone 7b. Anyone else just leave them in the ground in the North with success?