If just frost then the frost will kill the top growth down. Just cut those down and leave the rhizomes in the ground. You might want to mulch with shredded leaves and come next spring, when the temps start to warm and the days get longer, pull the mulch back and let the soil dry out. For your potted cannas I would move them to a protected area, like under an awning or trees, or even in a garage.

If your ground freezes, I think it might be best if you dig them up, cut away the dead or dying foliage and stalks, dry them for a few days, and store them in a dry area. To get them to an early start plant them into containers in the late winter/early spring and give them as much warmth you can.

I do know of a few people who live in an area where the ground freezes and have successfully overwintered their rhizomes in the ground. However, they put tons of mulch over the cannas to keep the ground from freezing. They've lost some and had others come back. I really think it all depends on the hardiness of the canna/cannas that you have. Some can take alot of abuse and severe temps while others can't. As long as the roots/rhizomes survive the cold months then you'll have cannas again next year.

Hope this has helped some.

Jenna

Jenna's advice is pretty comprehensive, but below is my experience of growing them.

Last year I dug up all my Canna after a frost had wilted the foliage (I think it was late Oct/early Nov). I washed off the soil, let them dry, dusted the rhizomes with sulphur powder (to help prevent fungal disease), packed them all in dryish compost and left them somewhere dark, cold but frost-free. Around April I potted them all up and put them in the greenhouse to get them started into growth.

However, this year I plan only to dig up my really special Cannas, and I'll leave the rest in the ground (as with Dahlias), just with a good covering of mulch. But as Jenna says, the ones in the ground will be slower to get going than the ones started in warmth.

Good luck with your overwintering!