In the colder climate zones, dahlias have to be dug up in the fall and stored for the winter. I have been digging and storing tubers for many years. I have tried many different methods. Here is what works best for me.
As soon as the first hard frost comes here in Central PA. I cut the stems off the plants and dig out the tubers! I don't divide my tubers until I want to plant them in the spring! I usually store about 15 boxes of Dahlia's in a ground cellar! I try to get the tubers in the ground as early as possible in the spring, say April! I also do the same with my Cana Bulbs!!