Dahlias are spring flowering plants that are winter hardy in U.S. Department of Agriculture plant hardiness zones 8 and warmer. In those warm winter climates, you do not have to do anything to save your dahlia tubers except cut off the dead foliage in the fall. They will then grow and thrive as perennials in your garden bed. On the other hand, in zones 7 and cooler, you must dig up your dahlia tubers in the fall and store them in the winter in order to save them and have them regrow the next season.
Cut off the foliage in the fall after the first killing frost turns it brown. Use pruning shears and cut off all but 2 to 3 inches of foliage. You will now still know where your tubers are located.
Leave the tubers in the ground for an additional one to two weeks. According to Iowa State University, this helps the tubers cure before storage. Curing helps dry tubers, seals any wounds, and prevents the tubers from decaying in storage.
Remove the tubers from the ground. The tops of the tubers are usually about 3 inches beneath the soil. Since the tubers are delicate, use a sand shovel or potato fork to gently dig them up.
Shake off the soil and gently wash the tubers and crowns (where the foliage and tubers meet) in water. Lay them out to dry indoors for a day or two.
Place your dahlias with the crowns down (tubers up) in a box. Then, cover them entirely with peat moss, vermiculite or fine wood shavings. Place them in an area that is ideally between 40 and 55 degrees Fahrenheit until spring's last frost, when you can plant them again.
Check on your dahlia tubers every month in winter storage to be sure the material (e.g., peat moss) in which your tubers are buried is still moist and that the tubers are not rotting or drying out. Add a little bit of moisture to the material if necessary to keep your tubers from drying out. At this time, also throw out any rotten tubers.