Across the sunny growing season, especially in the cooler weeks of autumn before the first frost, dahlias will be flowering at their finest. If you grow dahlias in USDA winter hardiness zones 3 through 8, you need to lift the tuberous roots to overwinter in a cool, dry, frost-free location indoors. Once the first fall frost occurs and kills the above-ground stems and leaves, cut the plant stems to a length of 4-inches above the soil to act like a handle. Carefully dig the dahlia tuber mass, wash off the soil and air dry them. Dividing the tubers yields more plants for the next growing season when time for planting.
Place the spider-like clump of dahlia tubers on a table to examine the size, health and quality of the tubers you will be dividing.
Locate the mother tuber--the tuber that you planted last spring that created the plant and stem and yielded all the new tubers. The mother tuber should be in the middle of the clump mass and swollen or larger in size but can also be darker in colour and dry or shrivelled. Cut off the mother tuber and discard it by making a pruning cut at its place of attachment to the clump with a hand pruners or knife blade.
Use caution when handling the tuber mass, especially taking care not to tear or cut into the area where the plant stem attaches to the tubers. You want to preserve all the "eyes" or small bumpy warts on the tubers near the stem, as these are needed for the tubers to sprout new stems and leaves next year.
Examine the remainder of tubers in the cluster. Cut away any soft diseased tubers or those that are broken or have pierced skin that were obtained during the digging process. Discard these damaged and unsatisfactory tubers into the compost pile or garbage.
Select the largest, firm and plump tubers to divide and save for overwintering and planting next spring. Cut them carefully away from the point of attachment to the base of the plant stem with a pruners or knife, making sure there are some eyes intact on each tuber as you cut and divide them.
Air-dry the tubers for three to five days on the counter so that the pruning cuts naturally callus. Once dry, they can be stored in shredded newspaper or coarse sawdust in a crate and placed in the cellar to overwinter.
If two or more tubers fuse together into a massed cluster with a shared neck or collar of eyes, carefully slice into the neck with the blade to provide eyes to each tuber you wish to save. Tubers that are small and thin (less than 1-inch in diameter) likely won't survive the winter as the dormancy will cause them to shrivel and not have enough energy to sprout next spring. You can save them, but realise they may not be particularly healthy and plump next spring and sprout into weak plants.
Tubers that do not have any eyes retained on the tips where cut from the stem base will not develop into plants when sowed next spring. Storing freshly cut tubers that have not dried and callused can lead to unwanted mould or rotting of tubers over the winter storage months. Such tubers will have to be thrown out next spring.