Canna bulbs are relatively winter hardy, and in growing zones seven and below they can be wintered in the ground. In zones where they must be stored indoors, they can be treated much like other summer bulbs. The only difference is that cannas should not be allowed to dry out, and should be checked and moistened as needed throughout the winter.
Cut the foliage back to 3-4 inches above the bulb after the leaves have turned yellow and died.
Use a spade to loosen the soil a few inches around each canna bulb. When it is loose enough, gently lift it out of the soil by the stem. Gently brush off the excess soil, taking care not to damage the roots or the skin of the bulb.
Inspect the bulbs. If any of them look diseased or rotten, toss them into the compost pile. Also consider getting rid of any bulbs that are much smaller than the others. They will not produce well next year.
Take the bulbs indoors and lay them in a dark, cool place and allow them to dry for 3 days. Once they have dried, brush of any remaining dirt and cut the foliage back to the bulb.
Punch several holes in the plastic bags. Line each bag with peat moss and then sprinkle the moss with a few drops of water. Place one bulb in each bag with the top up and the roots down. Tie the bags shut.
Hang the bags in a cool, well-ventilated, dry location like a basement or unheated garage where the temperature remains between 40 to 50 degrees Fahrenheit all winter long.
Check up on the bulbs periodically. Compost any bulbs that are rotten, diseased or infested. If any bulbs look like they have dried out, sprinkle a little more water on the peat moss.