Dahlias are hardy to Zone 8, which makes them tender in our 7a-5b gardens. (Depending on where you live in Western North Carolina, your garden falls into one of these zones.) Most dahlia growers dig up the tubers in fall, after a few touches of frost (when the stems are black), and either divide then or wait for spring, replanting after the chance of frost is gone.
Will the dahlia tubers survive if I don't dig them up?
Maybe. If you've lived in WNC long enough, you know the winters can be mild, or the winters can be a bear. If you decide to the skip the digging (I never dig my dahlias up except for division), you chance losing them. Many people treat them as annuals and budget purchasing more each spring. They don't stress if winter kills them.
Still, like all perennials, division is necessary every three years or the dahlias will stop flowering. (For how to divide perennials, click here.)
Follow the steps below for storage (and dividing).
Digging up Dahlia Clumps.
Dig your dahlias a few weeks after frost to allow them to go dormant which produces growth eyes and thickens the skin of the tubers. Dig at a wide angle, trying not to cut through tubers (they're like digging potatoes in this respect). If you do cut through some of them though, don't worry. Tubers self heal and chances are good--like this clump--you have more than enough if some don't produce. Wash the clump off and remove the root hairs. Allow the dahlias to dry outside for a few days.
Cut the stems off.
Cut the stems to 1"-2" above the crown--this helps minimize rot. Decaying plant material from the stalk can ooze down and cause tubers to rot. You're leaving the inch or two because the eyes are located on the neck of the tuber just below the stem. It helps to identify the eyes and not cut them off if some stem is left. You may want to wait until spring before dividing the tubers because the eyes will be very noticeable by then.
What are eyes?
Dahlias have eyes like potatoes have eyes. You must have a growth eye with each tuber or it will not flower. In the fall, these aren't as easy to discern, and leaving them in the ground a few weeks after frost is helpful as it causes the tubers to go dormant (producing eyes) and the skins to thicken. Flag the stems of the dahlias before they die back, so you'll find them easily when digging time comes. If frosts haven't occurred, don't wait much past mid-November to dig. The eye is the green growth bump you see in this picture. In spring these are much easier to recognize (This picture was taken last spring), as right now, they're just tiny growth bumps. You can store the clump through the winter and divide in April, if you choose. Just skip the division and store as noted below.
Division (After Drying)
When you divide, first remove the mother plant. It will no longer produce flowers. Use a knife like the Hori-Hori knife (Japanese garden knife) to gently separate the new tubers from the original plant. This is easier to do after the clumps have dried outdoors for a few days.
Cut off tubers that are growing out of another tuber, (these won't produce flowers), and throw them away. Begin identifying the eyes at the neck of the tuber, and cut just above that. At this time of year, it's not as easy to spot the eyes on the dahlias, but don't toss any of them out. Go ahead and store them. You'll be surprised how many do sprout.
There are a couple of ways to store your tubers. This box is an old seed tray with wooden slats on the bottom and sides for air flow. Shredded paper works to reduce evaporation and drying out of tubers. Cardboard boxes or paper bags can be used, and peat moss, coarse sand, sawdust, wood shavings or vermiculite work instead of shredded paper. There's a balance on keeping the tubers dry without drying them out. It's best to err on the dry side.
Where to store.
Store in a cool, dry place where the temperatures will stay above freezing but below 50 degrees. A garden shed, cellar, or garage are all possibilities. Plant in spring once soil temperatures are above 60 degrees (or chance of frost is past). Remember dahlias like full sun and dry feet.
Written by Cinthia Milner, garden coach and blog writer.
BB Barns Garden Center serves all of Western North Carolina, upstate South Carolina and Tennessee.