F gave us some canna rhizomes the other day. Last year we tried to overwinter our canna rhizomes and they all rotted, as did F's. So I did some YouTube research and this is what I learned: The cause of rot is usually that there is wet soil clinging to the rhizome or an adjacent one has started to rot and is touching its neighbor. To prevent rot you must remove the rhizomes from the ground and first prune away the stems or leaves as well as any black-looking part that is starting to rot.
What you have left will look something like the above--next year's buds, which can look pinkish or magenta as ours do, and some roots. The rhizome part is the underground horizontal stem along which the buds grow. Drawing 5895 has three good pink buds but also has a suspicious dark bud that may be rotten over on the left of the rhizome. I'm waiting to be sure, and if it is still black after all the dirt dries and falls off, I will cut away the rotten part. Drawing 5896 has three good buds and no rot; 5897 has three good buds but a slightly suspicious brown thing on the left end.
Step two is to let the rhizomes and their buds dry for a couple of days in a cool place that won't freeze. I'm keeping ours in a cardboard box, spread out so the things don't touch each other; I keep it on the shady back porch in the daytime but bring it inside on freezing nights. After things look dry, you brush off the dirt. The rhizomes have enough moisture inside of them for the buds to feed on during the dormant period.
Then when everything looks ready, you place them so they're not touching and lightly cover them with dry potting soil, not hay, which easily rots, and put them in a basement or crawlspace or unheated garage that will keep them cold but not frozen. You should check them every couple of months, and if they look too dry, mist the air around them but don't get them or the soil soggy.