Some of the canna info here lists zone 7a, but most list much higher. Also on some of the threads people refer to leaving them in the ground in 7a, but they are much further south.
Are some varieties substantially hardier?
Will they survive some years and not others?
confused about hardiness
Canna's here come back and My mother-in-laws come back in NJ. And my friends come back in Up Sate NY. I've heard depending on where they are planted matters..
If in a protected well drained location, along a foundation, and especially at a southern exposure .. and mulched heavily under approximately 16" of leaf, straw, or like held in place with dark landscaping cloth, tar paper, etc .. the bulky larger rhizome cannas have a good chance to survive.
The whites and thinner rhizomed cannas probably will not.
Although we dig most of our cannas, I have successfully wintered the purpurea types, musas, 'Robert Kemp', and 'Wyoming' in zone 6/5. Yeah! :)