I pull the rhizome's after our last frost kill's them. I've had great luck doing that, besides they need to be split. I'm amazed the amount of rhizome's that grow over the season.
Christine

Everytime that I pull cannas out for the season I am amazed at there root system which takes up the entire pot. I dont think they would do well if you left them in there pots without repotting them and cutting the roots back so they have room to grow again. To be on the safe side its probably best that you either plant them in the ground, or take them in during the winter after the first hard frost has killed them down.

Good luck!
-Alex

I am in North Mississippi and the ones I left potted last year all rotted, so I would not advise leaving them in the pots.