Should You Divide Your Dahlia Tubers in the Fall or in the Spring?

The lower sections of our field were nipped by frost earlier last week, so we’re going to start digging dahlias this week. It’s hard to believe this season is almost over! 

We dig our tubers each fall so that we can preserve them from freezing, wet conditions outdoors and so that we can assess their health. Like many gardeners, we also divide our dahlias each year so that we can multiply the number of plants we’ll have next spring.

Every year we get lots of questions about dividing dahlia tubers, so today we’re going to answer one of our most commonly asked questions about this subject:

Should you divide your tubers in the fall or in the spring?

Before we tackle the question though, I feel compelled to point out that its answer is very closely related to dahlia storage. Storing dahlias well over winter might be the most challenging part of growing and keeping a beautiful collection of dahlias! If you dig your tubers and pack them away before they’re fully dry to the touch or let them sit out to dry for too long, they likely won’t store well. Dahlias need to be stored over winter in an enclosed, high-humidity environment, at about 45ºF. I have more to say about this below, but keep the basics in mind as we discuss fall vs. spring dividing.

There are a few things to consider when deciding between fall dividing and spring dividing, and I’m listing them here in order of importance.

Ease of Seeing Eyes 

We’ve found that the eyes on the tuber clumps will often be more pronounced and easier to find if we wait until spring, because they begin to swell as days lengthen and temperatures rise. The eyes will eventually become your dahlias’ first shoots, so every tuber must have at least one eye to become a dahlia plant. Sometimes, especially if your tubers are on the warm side, the eyes will swell in fall, but it’s preferable to keep your dahlias cool and dormant after digging and throughout the winter.

Space + Number of Dahlias to be Divided

Assess how much storage space you have. Do you have enough room to store unwieldy clumps of undivided tubers? If the answer is no, then you may need to divide in fall and condense your storage. It takes time to divide dahlias, and it’s super-important that tubers are stored promptly after digging. Start your digging early enough, and pace it so that your dividing can keep up with your digging so that undivided clumps don’t sit out too long and desiccate. Alternatively, you can roughly divide your tuber clumps in half in fall and nestle them snugly together in storage bins until you’re ready to divide.

Personal Preference

Some people have busier schedules in the fall, or simply enjoy dividing their tubers in the spring. Others like to divide them in the fall so they’ll know just how many they have when they’re dreaming of their garden over the winter. When you’re ready to divide your tubers, whether now or in spring, we invite you to check out our tutorial about how to divide!

Before you divide your tubers, you’ll want to lift them from the ground, and prepare them for storage.

Cleaning and Drying Tubers

Successfully storing dahlia tubers for the winter starts the moment you finish digging and cleaning them. Whether you brush off the dirt to remove trapped moisture and any slug/snail eggs or go through the process of washing/drying, we recommend that dahlia tubers be mostly soil-free and completely dry to the touch before storing.

Since we sell and ship most of our tubers, we wash them to remove all field dirt and let them sit on nursery shelves to drain until they’re dry to the touch. Then we immediately pack them away. On a clear, sunny day this might take 2 to 3 hours. On a sunny & windy day, tubers can easily dry faster. On drizzly, calm, gray days, we move the shelves undercover, and drying can take over 24 hours.

The key is to carefully monitor your tubers as they dry. Don’t set them out to dry and go away for the weekend. Check them every few daylight hours until the tubers and any feeder roots are dried to the touch, and then pack promptly. 

Storing Dahlia Tubers

Dahlias need a few key things to overwinter well. They need:

• a space that is dark, 
• cool temperatures (but not freezing -ideally between 40 and 50 degrees F)
• relative humidity that stays around 80-90% in the storage area
• something to pack them in that will allow them to breathe a little bit, but will keep them safe and cozy all winter  

Without these three criteria, it will be difficult to successfully overwinter your dahlia tubers. A root cellar-type environment is perfect for storing dahlia tubers, but of course most modern homes don’t include one. My first choice as a home gardener would be to place clean, dry tubers in coarse pine shavings in a plastic bin with a lid in a cool basement. My second choice would be to put the tubers in a bin in an unheated garage that does not freeze. (You can find coarse pine shavings at your local farm store, often sold as horse bedding.) 

Be sure to check on your tubers periodically throughout the winter to make sure they are staying cozy,  and not drying out due to low humidity or molding due to too high humidity/pest damage. You could set a reminder on your phone to check them every month until the spring.