Cannas are tender perennials and are always active, unlike some bulbs that require a period of dormancy to complete their annual cycle. Cannas will continuously grow for 52 weeks of the year and are ever-blooming. However, they are also frost-tender and cannot survive outdoors in areas where they will become frozen. Therefore, if you live in a frost-prone area, there are two alternatives. The first is to cover with deep mulch and let the plant take its chances; this is only possible in areas where there is light frost, such as coastal areas where a maritime influence prevails. The second is to lift it and store indoors over the winter. This article deals with that option. There is of course a third option, sometimes used by public parks departments, and that is to treat Cannas as an annual and to lift and dispose, replacing with new rhizomes next spring. However, that can also be an expensive option.
You have two basic options, the first is to simply remove the pot indoors into a conservatory or heated greenhouse, position where it can get sunlight and keep it growing throughout the winter as a houseplant, taking it back outdoors again when the threat of frost has gone, and after dividing and transplanting. The second is to cut the stems back down to the ground and store the pot and its contents in a frost-free place, such as a garage or greenhouse. Keep it barely moist throughout the winter, but do not over-water. Once the weather has warmed up you are ready to divide and transplant again.
Once the first frost has blackened the foliage, it is time to cut the stems back to just above ground level and dig out of the ground. It is best to leave enough stem so that you can handle the plant without damaging it, it can always be removed later. The detached stems and their attached leaves make a rich compost, so remember to add them to the compost heap, rather than just dumping them. After digging out of the ground the Cannas should be cleaned and old rhizomes removed and discarded. The remaining rhizomes should be cleaned off and rinsed in a chlorox (bleach) solution, made up by mixing 1 part chlorox to 9 parts water. The rinsing will reduce the disease risk and its spread over the winter months.
After cleaning the rhizomes should be dried for a day or two, this will dry out cut ends. Remove the dried roots and store in a seed tray (flat in the US) or box. The rhizomes should be covered with dry general-purpose compost (peat moss in the US) or vermiculite or perlite and store in a cool place in the garage or similar. In some old farmhouses, there is a root cellar, which is perfect for Canna storage. Temperatures should not fall below freezing point, as Cannas have no tolerance of freezing conditions.
To avoid excessive drying, it is best to cover the tray or box with a layer of cardboard or a plastic bag. Make sure there is some ventilation by making several holes in the bag, otherwise the rhizomes, which are only semi-dormant will be depleted of oxygen and start to ferment.
Check periodically over the winter. Squirrels and other semi-dormant mammals can find your stored rhizomes a welcome food source over winter if they can get access. If any are looking too dry then spray with a little water, just enough to keep the tray on the moist side.
Best of luck this winter...