You should dig dahlia tubers after the first hard frost kills the foliage and turns it black. This frost signal tells you the plant has stopped growing and started pulling energy back into the tubers for winter storage. In most areas this happens between mid-October and late November based on your climate zone.
Digging dahlias in fall calls for patience and good timing. After that killing frost hits, cut the stems back to 6 inches (15 centimeters) above the ground. Then leave the tubers in the ground for 4 to 8 more days. PSU Extension says this waiting period helps the growth eyes on the crown swell up and become more visible. Those eyes are what sprout into new plants next spring so you want them as strong as possible before you pull the clump out.
UNH Extension warns against cutting stems too early before you plan to dig. Dahlia stems are hollow tubes. An open cut collects rainwater that drains straight down into the crown. This trapped moisture causes crown rot that can wreck your entire tuber clump in just a few days. If rain is in the forecast after cutting, cover the stumps with aluminum foil or flip a small pot over them to keep water out.
When the waiting period ends and you are ready for lifting dahlia tubers, grab a garden fork instead of a shovel. I learned this the hard way during my second year growing dahlias. A shovel blade sliced right through a clump and I lost half the tubers in one careless push. Now I insert my garden fork about 12 inches (30 centimeters) out from the stem on all four sides. I lever the whole clump up in one piece. The tuber necks are fragile and snap with even slight sideways pressure so you need to be gentle.
Once the clump is out of the ground, rinse off the soil with a garden hose on low pressure. Look at each tuber for soft spots, cuts, or mold. Toss any tubers that feel mushy or smell bad. Label every variety with a permanent marker on the tuber body or a plastic tag tied to the neck. I made the mistake of skipping labels my first year and had no idea which variety was which by the time spring came around. Every tuber looks the same once the dirt is gone.
Let your cleaned tubers cure for 24 to 48 hours in a dry spot at 60°F to 70°F (15.5°C to 21°C) before packing them away for winter. This curing time lets the skin toughen up and small cuts seal over on their own. A garage or covered porch works well as long as the temperature stays above freezing at night. After curing, pack them in peat moss or vermiculite. Store the packed tubers at 40°F to 50°F (4°C to 10°C) in a dark spot until spring comes back around.
Read the full article: Dahlia Tubers: A Complete Growing Guide
