Winter storage of dahlia tubers can yield a 90 to 95 percent success rate according to the American Dahlia Society. Storing strong, mature tubers that produced an abundance of blossoms during the summer will yield the best winter time storage results. Leaving the tubers in the ground even after a light frost kills the top foliage will allow them to have the longest growing season possible which will produce stronger tubers with a more abundant supply of wintertime nutrients stored within their depths. Dig the tubers up to store prior to the ground suffering a complete freeze.
Cut the foliage of the dahlia down to a few inches above ground level. Use a pair of pruning shears to remove the foliage. Discard the foliage into a trash bag.
Dig carefully one foot out from the cut foliage crown. Circle the foliage crown working your hand trowel down and under the tubers. Lift the tubers from the ground. Wipe away any excessive dirt clumps using your hands. Lay the tubers in the sun for two or three hours to dry out a little.
Hose the tubers off using a garden hose. Remove as much of the dirt as possible using the hose and wiping the tubers with your hands.
Divide the clumps of tubers using a sharp knife. Cut the tubers so there is a little bit of crown and at least one eye on each tuber. Remove all roots from each tuber and also remove the crown completely once the tubers are divided so there is no danger of rot developing.
Wash all remaining dirt away from the newly cut tubers. Dip the cut ends of the tubers into Cleary's 3336 fungicide. Allow the ends to be treated laying in the solution for at least 15 minutes. Remove the tubers and place them on newspaper to dry for 24 to 36 hours.
Place tubers in a small plastic bag filled with vermiculite. Tie the bag shut and store at a temperature that is below 50 degrees F. but above freezing. Place the plastic bags in a box with a lid so no light reaches the tubers
Inspect tubers every month to make sure they are not rotting. Discard any tubers that show signs of rot so other tubers are not infected, too.