Dahlias are grown from tubers, or bulbs, which produce blossoms in a variety of shapes, colors and sizes during the mid- to late-summer and early fall. Dahlias are hardy in USDA zones 7 through 11, but they do not appreciate frosty weather. Most dahlia growers must remove the bulbs from the ground in the fall and replant them again each spring. The best time to remove dahlia bulbs is after the first frost kills the foliage.
Cut the stems of your dahlia plants back to between 4 and 6 inches from the ground. This helps the dahlia bulbs produce new sprouts on the tubers. Wait 1 week before you remove the dahlia bulbs from the ground, unless the frost has already killed the foliage. The first frost will also help the dahlia tuber to produce new sprouts.
Loosen the soil around the dahlia bulb with a garden fork. Dig a circle with the fork about 12 inches around the plant stem. Lift the clump gently out of the ground, being careful not to harm the new sprouts.
Remove the soil from the dahlia bulb by spraying it gently with your garden hose. You can leave the clump in the sun for a day to dry out first, if your soil is mostly clay. Break away the dried clay and then spray the remaining soil around the dahlia tuber with a garden hose.
Place the dahlia bulbs in a cool dry place for several days, until the bulbs are dried out. You can turn them upside down to help the moisture drain from the stem. After the tubers are dry, you can separate them before you store them for the winter. You can also separate dahlia bulbs in the spring, before planting.
Inspect the separated tubers, or the clump, to look for damaged bulbs that may have been cut as you removed them from the ground. Discard any bulbs that are cut or damaged. Label the remaining bulbs with plant tags and a marker, if you have a variety of dahlias.
Store your dahlia bulbs in a cool, dry place over the winter. You can place them in a single layer in a box lined with peat moss or sand, then cover them with more sand or peat moss. Dahlia bulbs prefer temperatures that range from 40 to 45 degrees Fahrenheit.