Colourful dahlias, native to Mexico, make a superb cut flower. They thrive in hot, sunny weather and rich, fertile soil, but they won’t survive freezing temperatures. This means you’ll need to store dahlia tubers over winter if you wish to plant them again next year.
But before learning how to divide and store dahlias, first decide if it’s worth the effort. Unless your dahlias were a spectacular success and you simply must have these exact ones again, why not explore the incredible variety of dahlias out there and buy new tubers next spring? They’re not expensive, and nurseries, dahlia societies and garden clubs would be thrilled to lead you on your dahlia adventure.
If you decide to keep them, next decide if you want to divide them. If you have plenty and no remaining sunny spots to plant more dahlias, skip the instructions on dividing and go straight to storage instructions.
The little potato-like tuber you planted in the spring has likely grown into a large, multi-stemmed plant, and the tuber has probably grown huge and multiplied, too. In fact, a dahlia continues to grow until frost turns the stems and foliage into a collapsed sorry version of its former glory. Leaving plants until they’ve been killed to the ground is critical to storage success. The tubers are toughening up and getting thicker skins (a.k.a. ripening). Tubers lifted too early won’t store well.
Cut off the stems, leaving a few inches (about 8 cm) above the soil; the shortened stems make handy handles for carrying the tubers once they’re dug up. Start at least a foot (30 cm) away from the stem and gently lift the whole clump. It’s fragile and there’s probably some water in the stems. Carefully remove loose soil, placing the clump upside down for a few hours to dry out. A warmish, late fall morning is best. They’re less likely to fall apart if they’ve had a chance to dry out a bit.
The second trick is to remember the variety. Record the height and save any photos you’ve taken to help jog your memory next spring. If you’re not planning to divide the clump, skip to storage instructions.
A note about container-grown dahlias: Plants in containers freeze solid earlier than tubers in the ground. Lift these plants as soon as their foliage has been killed. Otherwise, the procedure is the same as plants grown in the ground.
It’s easier to divide clumps in the fall after you’ve lifted them than to do so next spring. A clump left until spring becomes as tough as wood over the winter months and hard to divide. The question now is how many more plants do you want? It’s always a good idea to have some spares, but do you need 10 more? If you just want a couple more, then divide the clump into a few roughly equal pieces.
Start by washing the soil off with your garden hose. Cut off the remaining stem as close to the base as possible. Now, use a sharp knife to divide your clump from the top downwards through the stem base, like a pie. Each piece should have a section of the base (also known as the heel) and a single or cluster of tubers attached. Cut out and dispose of any damaged tubers; these will likely spoil in storage.
You may notice clusters of dark bumpy spots on the tubers. These are the “eyes” that will form stems when the dahlia is planted again in the spring. Let your tuber(s) dry thoroughly.
It’s most important that they remain dry regardless of where you decide to keep them. Once dry, you can wrap them in a few sheets of newspaper, taking care to label each piece. You can also put them into individual paper bags, loosely closed. An ideal location is somewhere dark, between 5 and 10 degrees Celsius. They mustn’t freeze.