Dahlias are grown from bulbs which produce flowers in a variety of colors, sizes and shapes during the late summer and early fall. Hardy in Zones 7 through 11, dahlias do not appreciate cold, frosty weather. Most dahlia growers winterize the bulbs by removing them from the ground in the fall and storing them until the weather permits replanting again in the spring. The best time to remove and prepare to winterize dahlia bulbs is after the first frost in your area.
Remove the dahlia bulbs from the ground so you can winterize the tubers. Cut the dahlia plant stems back to 6 inches from the ground. If the frost has already killed the foliage, remove the plants from the ground immediately. If the foliage is still living when you cut the stems back, wait one week and then remove the dahlia bulbs from the ground.
Dig a circle 12 inches around the dahlia plant's stem with a garden fork to loosen the soil. Gently lift the clump out of the ground.
Spray the clump gently with a garden hose to remove the excess soil from the dahlia bulb. If your dahlias are growing in soil that's mostly clay, leave the clump in the sun for a day to dry the clay. This makes it easier to remove.
Allow the dahlia bulbs to dry out completely before storing them for the winter. Place them upside down in a cool dry place for up to two weeks; this lets the moisture drain from the stem. Separate the tubers after they are dry or wait to separate dahlias before you plant them in the spring.
Inspect the dahlias for damaged bulbs that may have been nicked as you were removing them from the ground. Discard any damaged bulbs. If you have a variety of dahlias, label the stems with plant tags and a marker before you winterize them.
Add a 3-inch thick layer of peat moss or dry sand to the bottom of a cardboard box. Place the dahlia bulbs in a single row on top of the storing medium. Do not stack the bulbs. Cover the bulbs with a thin layer of sand or peat moss.
Place the box in a cool, dry location where temperatures consistently range from 35 to 45 degrees Fahrenheit. If you store dahlia bulbs in temperatures higher than 45 degrees, the bulbs will shrivel and will not be viable for replanting.
Inspect the bulbs every three weeks during the winter. Look for disease and shriveling. If the tubers show signs of disease, remove the affected bulbs and cut off the diseased areas. If the tubers have shriveled, place them in water overnight. This may revive them and they may become plump again. If the dahlia bulbs plump up, repeat the drying process described in Step 4 before returning them to the box.