In my limited experience, canna will not survive winter in zone 8 or less. I am in zone 7a. I have to dig up canna in the fall before the ground freezes. I wrap the rhizomes in newspaper and keep them in unheated area in the house.

in the spring, you can plant the rhizomes in the pot or in the ground.

Not all Z7's are alike...being in the South, cannas are weeds and majorly hard to eradicate (I know this from doing it for two years). I can't get them to die.

But being as you're NE kinda like shp (who says he needs to dig), that's probably true unless you have a warm microclimate.