Dahlias are tubers that offer large bright blossoms during the summer. Hardy in zones 7 to 11, most dahlia growers must dig up the bulbs in the fall and replant them in the spring. Gardeners who grow dahlias in warm climates do not need to remove them from the ground and store them over winter. In colder climates, you can wait until after the first frost and the dahlia leaves have turned black before you dig them up to prepare them for dormancy and storage.
Cut the stems back on your dahlia plants to six inches tall. Loosen the soil around the bulb with a fork and gently lift it out of the ground. Remove the excess soil from the dahlia bulb. If your soil is mostly clay, you can leave the bulbs in the sun for a day to dry the clay and make it easier to remove.
Label each of the dahlia bulbs if you have more than one kind. You can tie a tag to the root and write the names with a permanent marker.
Place the dahlia bulbs upside down in a dry area with good circulation. Leave the bulbs alone for two weeks so that the moisture can drain out of the stems. Your dahlia bulbs need to be completely dry before you store them in the winter.
Line the bottom of a tray or cardboard box with peat moss or dry sand. Place the dry dahlia bulbs in the tray in a single row; do not stack them. Cover the bulbs with a thin layer of peat moss.
Place the tray in a basement or any cool, dry space with temperatures that consistently range from 40 to 45 degrees F. Never store dahlia bulbs in higher temperatures as the bulbs will shrivel and will no longer be viable. Another way to store dahlia bulbs over the winter is to place the dried bulbs into a black plastic bag. Seal the bag to prevent dehydration and place it in a frost-free area with 40 to 45 degree temperatures. The dahlia bulbs may sweat and rot if you choose this method.
Inspect the dahlia bulbs every two to three weeks during the winter. Look for shriveling and disease. If the bulbs show signs of either, remove the affected tubers and cut off any diseased areas. If the bulbs have shriveled, place them in water overnight and they may become plump again. If the bulb plumps up, repeat the drying process by placing it upside down for about two weeks before returning it to the peat moss tray.