A week or two after the first frost, remove plants from the soil, dry them and clean away any dirt still clinging to the tubers. If you washed the plants, allow them to dry, and then trim off any fine roots and place tubers in storage.
After the first frost, cut dahlia plants to the ground. After seven to 14 days, use a fork to carefully remove plants from the soil. If you dig dahlia tubers too early, they are not likely to survive winter storage. Allow them to air dry and clean any remaining soil on the tubers if necessary. Trim the stems to 6 to 8 inches long. If tubers have been washed, hang them upside down in a cool place to dry for several weeks.
Discard any underdeveloped or unhealthy tubers. Trim any fine roots on remaining tubers, and then place them in shallow wooden boxes or trays and pack them with dry sand or a peat-free compost. Cover the tubers, leaving the crown exposed. Gardeners can also store dahlias in paper bags or in cardboard boxes lined with newspapers. Dahlia tubers survive best in relatively humid storage environments where the temperature stays between 40 and 50 degrees Fahrenheit at all times. Inspect tubers regularly throughout winter and discard any rotting or unhealthy tubers.