Storing and dividing dahlias may seem like a pain in the butt, but with just a few key pieces of knowledge, you can save and regrow your own dahlias each year, and expand the quantities that you grow, or have some extras to give away to friends and neighbors!
Dahlias are grown as an annual here in Connecticut (zone 6b/7a) although I have seen them winter over successfully in coastal towns such as Westport and Greenwich in gardens literally on the water. Jenny Love, of Love ‘n Fresh Flowers in PA has amazing info over on her website about overwintering dahlias if you really want to get into it!! But otherwise, for most of us in New England, tubers must be lifted and brought inside for the winter.
The first, perhaps most important step is to label your dahlias before the frost so that you know which variety you are saving after the blooms and foliage are dead. Make sure all plants are labelled by early October just in case an early frost comes. The first frost will not damage the tubers; the ground insulates them for a bit. But you want to get them out of the ground as fast as possible after that first killing frost when the blossoms and foliage turn brown and limp. 
My son LOVED lifting dahlias. Cutting with the loppers and digging with the shovel. It’s like buried treasure!
Do not label any plants that seem diseased or unhealthy! These plants should not be saved. You can throw them in the trash (do not compost since this can spread disease). If you touch any diseased plant with your hands or your tools, be sure to sanitize with isopropyl alcohol before touching any other healthy dahlia plants or tubers.
Lift dahlias only in dry, clear weather. Lifting them in wet conditions is just asking for storage issues and rotting. Use a hefty pair of loppers or sturdy clippers to cut the stem about 1” above the soil line. Then use a shovel or pitch fork to lift the tubers gently out of the ground. Don’t stick your shovel in too close to the stem - this could damage the tuber. Keep about 4-6” away to be safe.
Using your hand, gently brush as much dirt off of the tuber clump as possible. This will make cleaning them easier later. Do not wash your tuber clump until you are ready to divide them. Just place them into a plastic bag dry, and make sure to label the bag, or throw your plant labels inside the bag so you are never in doubt of the variety. Put your bagged tubers in a cool, dark spot until you are ready to divide. I use my unheated basement. Just be sure to protect them from mice if you have issues with those - a hard plastic container is handy in this case.
When you are ready to divide, get your materials together. I like to divide on a warmish fall day to keep the mess outside and make sure the tubers are nice and warm. Dividing early will also help mitigate any rotting issues early on. I usually aim to have them all divided by Christmas. Check on your tubers at least every other week to make sure they aren’t too dry or too wet, both before and after dividing them.
To divide, you will need the following: a cutting board (or plank of wood to cut on), large chef’s knife, smaller pairing knife or hooked dahlia knife, clippers, scissors, hose with spray attachment, and something to spray the tubers off on (a table, pallet, or netted chair all work). A bucket or wheelbarrow to collect all the trimmings is handy too.
First things first. Using your spray attachment on the hose, blast as much soil off of the tubers as possible. Flip the clump upside down and try to get as much of the center of the clump clean as possible. This is where the eyes are, and you need to see what you are doing! Most often, you will need to spray them off a second time, after dividing the clump in half or quarters.
Secondly, if you have a careful toddler or young child around like I do who is super excited to practice their scissor skills, you can have them (carefully) cut all the little tendrils off the tips of the tubers. This helps get things cleaned up, and will help you make sense of the clump. My son loves doing this! But only you know your child so obviously monitor and watch them like a hawk. This is the perfect Montessori activity for the right kid!
Next, cut into your clump. Make sure the stem end is facing up, so the clump is resting on the table like an octopus. This is the scariest part, because you are always going to have some casualties. Don’t fret! Just cut in a spot where you will slice the least amount of tubers in half and don’t worry about losing a few. Your plants are going to multiply exponentially regardless. And of course, failure is the best teacher, so go for it!
Overall, the goal is to keep most tubers in tact, with a connected eye. The eye is the most important part - this is what will grow you a new plant next year.
What does an eye look like, you ask? Easiest way to describe it is a small nipple. Eventually, the nipple will sprout and grow the main stem of your plant. You can have multiple eyes attached to a single tuber, and that is great! A tuber without any eyes will not grow any plant for you. The tuber is simply “food” for the eye to get through the winter. So make sure all of your tubers are connected to an eye.
Sometimes eyes can be hard to see. All varieties are different, so some may be more obvious than others. However, you will notice in many varieties, a swell just above the neck of the tuber, and a surface that looks kind of bumpy instead of smooth. That is the area where your eyes will be. Chances are, if your tuber has a good amount of bumpy swollen area above the neck, you will sprout an eye.
Make sure that any rotted or green plant matter is cut out of the tubers. Anything that is black or slimy should be cut off or discarded. It will only make the other tubers rot too.
Ideally, tubers should be individual after dividing. Make sure all necks are strong and not broken. If you have two tubers connected after dividing, it is just more opportunity for breakage in storage. These are delicate little ladies!
After dividing all of your clumps, lay the tubers out on a surface to dry off. I put them on a dish towel on my kitchen counter and let them dry over night. Keep in mind, you want them to dry off, not dry out. 8-12 hours of drying is plenty for most places, but if your weather is super humid or super dry, you may want to adjust up or down. If you harvested your tubers in wet weather, definitely dry them off for at least 24 hr or even a couple days. You don’t want them to liquidate in the plastic bag after you store them.
After they have been adequately dried off, place them (gently) into a clean plastic bag (I use a kitchen trash bag, but you can even use a ziplock if you only have a few). Label them and store in a dark, cool place that won’t freeze for the winter. An unheated basement is great! I repeat, do not let them freeze. An unheated garage may freeze and be too cold for them, so take care depending on your location and low temperatures in winter. I put the bags into hard plastic containers to keep mice out as well. You can also use cardboard boxes to keep the light out, if there are windows in your storage room.
Every couple weeks or so, you will want to check on your tubers to make sure they aren’t rotting. If there is any condensation on the bag, take them all out to dry off for a while before storing again. Cull anything that is black, slimy or smelly. Tubers that are super wrinkled may be starting to dry out, but that shouldn’t be a problem with storage in plastic bags.
Then in March or April you can take them out and pot them up if you want to get a start on the season. Remember not to plant them outside until after the last frost. For us that’s around mid-May depending on the weather. Dahlias grow well when planted directly in the ground too.
And now, from one plant, you have several! It’s like magic! While dahlias can seem like an investment in the beginning, you can have flowers forever if you learn how to save them! And after only a few years, you can have loads of plants. It‘s an awesome investment!
Happy gardening!