Dahlias have underground root structures that are often classified as bulbs but are really tubers, which resemble sweet potatoes. Because tubers are planted just below the soil’s surface, they must be dug up each year and stored. In addition, dahlias are self-propagating plants that need to be divided before they're stored for the winter.
Allow the tubers to stay in the ground until after the first frost. You can wait to dig them up until right after the first killing freeze, if desired. During this time, dahlia tubers will continue to mature and grow.
Cut the stalks down to a couple inches above the ground. This will make the tubers easier to handle.
Decide when to dig up the tubers. This should be done right after cutting the stalks, but some gardeners wait a couple of days, because the eyes--or buds--protrude further out, which makes them easier to see. If you wait too long, you risk water getting in to the crown of the tubers, which will cause tubers to rot. You can cover the tubers with aluminum foil to help reduce the risk of water getting in there.
Loosen the tubers. Use a shovel or garden fork and dig 1 foot away from each side of the plant to cut away all the loose roots from the tubers.
Lift the tubers out of the ground carefully. Use the shovel or garden fork to gently push down under the tubers from the side and lift them out of the soil.
Turn the clumps upside-down to drain any water in the stem. Leave them upside-down for a couple of hours. This will make the tubers less fragile when handling.
Remove the dirt with your hands. The tubers are still a bit fragile, especially around the neck, so be gentle. Use a garden hose to remove the rest of the dirt (don’t get water in the stem). Divide the tubers, if necessary, before storing them for the winter.