Cut back dahlia stems to 6 inches tall after the first autumn frost blackens the leaves. Use sharp shears and sterilize the blades with household disinfectant before and after you cut back stems and remove remaining leaves. Dig dahlias immediately after the first frost. Additional frosts can permanently damage the roots.
Loosen the soil around the dahlia using a hand-held garden fork. Leverage a spade under the dahlia and gently lift its tuberous roots from the soil.
Hang the bulbs with the stems pointing down in a dry place for two weeks. Place them in mesh bags to keep the stems in place and allow air flow around the bulbs.
Fill boxes with dry peat moss. Place the bulbs in the peat moss so they aren't touching one another. Store in a dry, dark place at a temperature between 40 and 45 degrees Fahrenheit. Substitute dry vermiculite for peat moss if it is available.
Check the bulbs every two to four weeks. Look for soft spots, shriveling or other signs of disease or rot. Cut off any damaged parts with a sharp, clean knife sterilized with household disinfectant before and after you cut damage away.
Place any cut bulbs that have begun to shrivel in a bucket of water for 12 hours to rehydrate. Hang them for an additional two weeks so that excess moisture dries. Then return the dahlias to storage.