Cannas are tropical plants that love the heat and will struggle when grown outdoors if the winter gets cold. Good drainage like in a raised bed is ideal but you should also try covering the tubers with an upside down nursery pot to keep out the rain. Often it is not the cold but the wet soil that rots the bulbs. Another way to grow them is in pots so that in the fall you can store the containers inside a cold but not freezing garage or shed.  The mulch you use on top of the bulbs should be one that repels water such as brown fern fronds or wood chips. Do not use compost that will hold moisture on top of the bulbs. This is why an upside down pot (yes, I know it looks dumb) is the best way to prevent cannas from winter rot. Keep growing, Marianne Binetti