@ardesia ,
we lived for a short time in Charlotte quite close to SC border and my cannas didnot survive; but they were not mulched well. Closer to the mountains at elevation it will be cooler still. So i think you'll need to dig them up for winter.
As far as St.Johns you can keep them going into 40F nites and cut the stalks and dig them up into pots for xfer. I would put them on the side to dry up somewhat to kick the dirt off and just pack the rhizomes.
