Around here, in a similar zone, it appears that the cannas that overwinter tend to be tallish red flowered ones. I've been able to bring 'Red King Humbert' through one winter so far (but it was a mild one), but 'Orange Punch' and 'Tropicanna' never make it. A layer of mulch on top over the winter helps - I cut down the stalks after they are browned by the frost and lay them on top for mulch over the winter.

Cold, wet spring, especially in clay soil, are bad - they rot out the tubers. I'll be interested to see if my outdoor ones come back - this has been a nasty spring, cold and wet.

I'm in zone 5 in Ohio. I leave some of the "Robert Kemp" in the ground over the winter, and they occasionally make it. I would think the would do ok in zone 7.

I'm in zone 7a, and all of my 30-40 varieties have came back every year for the past 20 years. The only year I've ever lost cannas was the winter of 2000, which was extremely wet. I believe it was the wet winter more than the cold temps that rotted the rhizomes. I lost everything except for 17 Canna 'indica' and had to start my collection over again.

I believe 'Indica' is the most hardy canna I've ever grown. I would think 'Purpurea' would be just as hardy though. I have very sandy soil which is probably a factor in the cannas being so hardy here. You might consider amending your soil to make it lighter. A trick I have used on brugmansia is cutting them back to the ground before the first freeze, and covering the roots with a garbage can lid or with pots to prevent the roots from getting wet and rotting. Covering them with a tarp might work as well. You just need something to keep them from getting too wet and rotting over winter.