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If you live in an area where the ground does not freeze during the winter, you may be able to leave your dahlias "in place" and have them reappear the next growing season right where you planted the original tuber.
Most of us, though, need to take extra care if we wish to re-use our dahlias - or benefit from the fact that a single planted tuber will generally produce a tuber "clump" by the end of the growing season. These clumps often contain between 3 and 10 new viable tubers, each which can be "divided" and planted separately during the next growing season.
Anyone growing dahlias in an area prone to freezing temperatures will notice that the day before your first frost the dahlia plants look fine...but hours after a frost they look terrible. They are very tender and don't survive an exposure to freezing weather.
What we do: Wait about two weeks after the first killing freeze (when the above-ground portion of the plants have died). Then we cut and carefully remove the dead portion of the plants leaving about 6" of stalk protruding from the ground.
The first freeze, and the cold temperatures in the weeks that follow, will allow the underground tuber clump to "cure" and harden before you remove them from the ground for winter storage.
To "lift" the tuber clump from the ground we carefully dig under the clump around the central stalk. Some growers use a pitchfork to minimize the possibility of cutting through tubers, but we use a shovel (digging about 8-14" from the center) and lift the clump gently out of the ground.
PREPPING tuber clumps for storage
Many growers soon wash their tuber clumps free of soil, allow them to dry (usually overnight) and then proceed to "divide" their clumps prior to winter storage.
We do not divide our tubers prior to winter. Instead, we store the clumps over the winter in a relatively cold, somewhat moist environment safe from freezing temperatures. In our experience the easiest way to do this is by leaving dirt and soil in/on/around the clump in order to help "insulate" it from the hazards of winter storage.
The obvious downside to this approach: clumps with dirt can be somewhat heavy and take up a lot of storage space. But we don't have the time the divide our dahlias in the fall, they often do not have visible "eyes" which allow for easier dividing, and we have sufficient room to store clumps during the winter.
"Dividing" tubers is both an art and a science. We encourage you to do some internet sleuthing on the topic - you will find a large number of useful videos and articles. And you will notice there seem to be as many different techniques as there are growers!
We divide our clumps in the early spring just after the "eyes" have begun to form with the warmer prevailing temperatures. At some point we will supplement this page with more information on how we divide tubers, but everything we've learned has been through a combination of careful research and a bit of trial-and-error.
In general, we end up with 2 to 10 new "plantable" tubers from each clump. So, with proper care, you may find that your initial batch of a few purchased dahlia tubers can blossom into an enormous number of future plants.
Growers who divide tubers in the fall must protect the individual, divided tubers from two primary hazards: moisture and temperature. Tubers (whether divided or in clumps) need enough moisture not to dry out and die...but not so much they rot. Individual tubers which are insulated from airflow are less prone to desiccating, but more prone to rotting. Tubers which are subject to drafts are likely to dry out during winter's cold, low-humidity environment.
Growers who divide tubers in the spring must protect tuber clumps from those same perils. In our case, we can control temperature quite well, but humidity is more difficult. If our clumps become too dry, the dirt insulating them falls away and the tubers themselves eventually dry out and become non-viable. As necessary we re-hydrated clumps manually by spraying them with water - enough to keep them from drying out completely but not enough to encourage rot.