What is the best way to store tender bulbs such as dahlia over the winter?
After the first hard frost, cut back the blackened stems and gently lift the clump of roots with a spading fork. To prepare them for winter storage, label each clump with the variety name and stand the clumps upside down to drain away any moisture. After about a week of drying, place them in a deep box and cover the clumps with slightly damp peat moss or wrap them in newspaper before storing. They are best stored in a place where the temperature is between 35 to 50 degrees. Be sure to examine the tubers for any rotting during the winter and remove decaying portions immediately.
Courtesy of the NYBG Plant Information Service