Storing Dahlia Bulbs
When it comes to storing dahlia bulbs, the rule of thumb is to keep them cool and dry.
As soon as frost hits them (the leaves turn black) cut off the stems, dig up the tubers (they're really tubers not bulbs) and then trim off the remaining stem down to one inch or so of the tuber.
Let them sit in the air for a day or three to dry off the dirt. Once the dirt will brush off, they are ready for storage. (Do brush off the dirt when it's dry but don't bruise or get too clean - the dirt isn't the problem, it's the moisture in the dirt that gets the tuber started rotting).
So - cool and dry until it is time to plant again.
p.s. the pic is variety 'Moonfire'