Are Dahlias Annuals or Perennials?
With their enormous variety in form, size and color, dahlias are divas of the summer and fall garden, often blooming right up until frost, depending on the dahlia varieties.
Are dahlias annuals? No. Dahlias (Dahlia) are tender perennials that grow in USDA hardiness zones 7 to 10, but they are grown as annuals in many gardens.
Growing dahlias is not for the casual gardener, because you need to dig up the tubers in the fall, store them carefully over winter, and replant them in the spring. Even in climates in the warmer end of their range, dahlias benefit from the rejuvenation that occurs when you dig them up and divide them.
Why Lift and Divide Dahlia Tubers?
Even if you live in a climate warm enough for a dahlia to avoid the wet, cold soil that it dislikes so much, there are still good reasons for digging up and dividing your dahlia tubers:
- Dahlias will eventually develop a large mass of tubers underground. The larger the clump of tubers, the weaker the stalks and the smaller the flowers will be, come growing season.
- A huge clump of dahlia tubers is difficult to divide, so waiting several years between divisions means that you may lose many or most tubers due to damage.
- Wet soil can rot dahlia tubers, so they might not make it through the winter, even in warm climates.
Furthermore, digging up and dividing dahlias gives you the opportunity to share or swap with fellow gardeners.
If your garden is in the warmer part of the dahlia’s climate range, you can try lifting and dividing tubers in alternating years while allowing them to overwinter in the garden soil every other year.
Provide some winter protection via a thick layer of mulch, but remember to remove the mulch after all danger of frost is past and before they begin to regrow.
How to Lift and Store Dahlia Tubers
When the top growth of the dahlia dies back in the fall, it’s time to dig up the tubers. It’s ok if this is just after the first frost, because you want the tubers to stay in the ground as long as possible—the more mature they are, the better they hold up in storage.
Don’t cut down the dahlia stems until you are ready to dig up the tubers, because their hollow insides collect water that can result in rotted tubers.
Digging Up the Tubers
Loosen the soil around the plant using a spade or shovel, being careful to avoid severing any tubers or damaging the neck, which is delicate.
Gently lift out the dahlia tubers, taking care not to nick the skin if you can avoid it. Any cuts or abrasions in the tubers can invite disease and rot.
Cleaning the Tubers
Remove all the soil from the dahlia tubers using a hose or submerging them in water. The soil might contain microorganisms that contribute to decay. You can divide them at this point or wait until you get them out of storage. To divide them, use a sharp, sanitized knife.
Dry them for a few days in a location with good airflow and out of direct sunlight, in temperatures of about 60 to 70°F.
Storing the Tubers
The main goal when overwintering tubers is to keep them chilled below 50°F but above freezing. They should not be allowed to dry out, but too much moisture invites rot, so this is a delicate balance. Pack them in moist vermiculite, sand, sawdust or wood shavings inside plastic or paper bags.
Check them regularly. If any have developed soft spots or appear to be rotting, remove them so they don’t spread decay to the other tubers.
Next year in late spring after the last frost, replant them in full sun and well-draining soil. Then, sit back and enjoy your flower garden.
I garden in the Pacific North west, previously Hawaii where I had an avocado orchard. I have a Master Gardeners certificate here in Eugene, Oregon.