In USDA winter hardiness zone 6 or colder, you must dig up your canna plants each fall and store them indoors if you want to keep them from year to year. Here are directions on how to dig (lift) and store canna rhizomes (cannas are technically rhizomes — not roots or bulbs) for the winter so you can replant them the following spring.
After frost kills the foliage, trim the plant off short to about four inches. Dig it up in the next week or two, any time before the ground freezes.
Use a garden fork or shovel to dig up the rhizome (root) being careful not to injure it. The rhizome will be much larger than what you planted, so start digging a foot or more away from the stem. If pieces break off, save them, too.
Gently brush off excess soil. Rinse clean with a garden hose if you wish. Air dry at room temperature (minimum 45 degrees) for a few days before storing.
There are many different packing and storage methods so there is no one single right way to do this. I have unceremoniously dumped them in a cardboard box and shoved it in the back corner of my dark, unheated basement with good results. But, that is probably not the best way to keep your cannas!
Ideally, pack the roots in a single layer in a cardboard box or paper bag, separated and covered by clean dry shavings, peat moss, perlite, or sand. The rhizomes should not touch each other. When stored, make sure the storage area is not too dry and the air is still circulating.
Your goal in storage is to keep the roots cool (not cold), dry and dark. Aim for a temperature between 45 and 55 degrees. Kept too cold, the rhizomes rot. When stored too warm, they may try to grow prematurely. Experiment and see what packing method and storage place work best for you.
Check your cannas periodically. Remove any that soften or begin to rot. Rotting can be caused by cold temperatures or by storing damaged or overly wet rhizomes.
If they start to shrivel, mist with water and consider using a plastic bag (see above) to help keep them from drying out.
Most canna storage problems are related to the storage temperature. Check the temperature inside the storage container using a min-max thermometer and adjust as needed.
Good luck with your cannas this winter!