Store dahlia tubers in sand or peat moss in a dry area, such as a root cellar, at temperatures of 40 to 45 degrees Fahrenheit. Wait until the first frost blackens the dahlia leaves to dig them up. Then, hang them upside down to dry for two weeks before storing.
Throughout the winter, check on the stored tubers. Temperatures higher than 45 degrees cause the tubers to shrivel. If they shrivel, leave them submerged in water overnight to rehydrate, and then hang them up to dry before putting them back in storage. If tubers develop diseased spots, cut those parts off.