Dahlias are beautiful flowering plants often used in floral arrangements. While they have root structures similar to bulbs, dahlias are not actually true bulbs. Dahlias have a tuber root system that in just one year can multiply into several tubers. Because of this, and because they are planted near the soil's surface, dahlia tubers need care in order to survive year to year.
Cover the dahlias with mulch during the growing season, which maintains soil temperature and retains water for the tubers to absorb and grow.
Cut the stems at about six inches above the soil after the first killing frost, or wait until late October or early November if no frost has occurred in your area. After cutting, wait a week to dig up the tubers so the eyes will emerge.
Dig straight down with a shovel or garden fork about a foot from the plant on all four sides. This will loosen the tubers from the fleshy roots. Then angle the shovel or fork toward the tubers to lift them up and out of the soil.
Turn the dahlias upside for a few hours to drain excess water from the stem, which will make them easier to handle. Then carefully remove clumps of dirt with your hands and wash the tubers with a garden hose to remove any remaining dirt. Do this carefully as the tubers are fragile, especially around the shoulder and neck.
Dry your tubers overnight and then divide them the next day. Cut each tuber so that you get a piece of the stem, an eye (bud), and a tuber with each slice. Be sure not break the shoulder, which is the narrow part between the stem and the main section of the tuber.
Soak the dahlias in a bleach solution that is one part bleach to three parts water. Soak for about 20 minutes. This will kill any bacteria or fungus. Allow the tubers to dry, which may take a couple of days.
Store about five to eight tubers in a large, sealable plastic storage bag. Add a couple cups of vermiculite to each bag. Seal and stick the bags in a container to be stored in an area that is dry and cool (40 to 45 degrees Fahrenheit) until spring when they can be replanted. Check on your dahlia tubers every few weeks. Discard any rotting tubers and add more vermiculite to help absorb excess moisture, if necessary. If the tubers are shriveling up because they are too dry, spray the vermiculite with a bit of water.