One of the big fall chores we prepare for each year on our farm is digging and storing dahlias. This is a big job around here—I'm personally digging up a couple thousand plants within a week or so. But the beauty of these flowers is absolutely worth it to me when I see them bloom again the following year.
If this is your first time digging and storing dahlias, it can be a little intimidating, but it's worth a try if you wish to keep them year to year. Likely, every year you'll refine your storage technique and learn a little more about what works best for you, as there are many ways to dig and store dahlias. And to be honest, some years are just better than others for storage, and some varieties just store better than others. So don't feel bad if you lose a few—it happens to us all.
Do I Have to Dig Them Up?
I've had people ask me if they have to dig their dahlias up. I tell them only if they want them to come back! If you live in zone 4 or colder, then your dahlia plants will not survive outside over the winter. Some gardeners have been successful in zone 5 overwintering dahlias outdoors with heavy mulching and tarping strategies, but I'm not experienced with those as it's too cold here for that.
If digging and storing dahlias is not your thing, then leave the dahlia plants in the ground and they will rot nicely, adding nutrients back to the soil for your new dahlias next year. Some customers choose to buy new tubers each year, and that's fine too.
When Should I Dig Them Up?
Here in central Alberta on our zone 3 farm, I generally start digging my dahlias around the second to third week of October. I water my dahlias well in September while they're producing their tubers, but by the end of September/beginning of October I stop watering and give the soil a chance to dry out a little before digging.
The best practice for digging up dahlias is to wait until they've frosted and gone dormant. But make sure you watch the temperature closely and dig up your dahlias before it gets too cold. If a dahlia tuber freezes, it cannot be saved. Your dahlias are insulated by the ground during some of the frosty fall nights, so they should be fine when temperatures hit -1 to -4°C. Any colder than that and I start to worry a little (though really, I'm sure they're insulated in the warm soil for colder temps than that even).
How to Dig Your Dahlias
Wait for a hard frost. Sometimes we get a light frost in mid-September, but I prefer to wait until a real hard frost has fully killed off the plant closer to the end of the month or even early October. You can then wait a week or two to let them cure a bit more in the ground, go fully dormant, and eye up a little. Depending on weather and time, I sometimes skip the waiting.
Cut the stalks. Once dahlia plants have gotten a hard frost and leaves are dead, cut the stalks down to close to ground level, leaving a bit of a handle to work with.
Dig carefully. Dig approximately 12" around the main stalk to loosen the clump, going down about 1' deep. Use your shovel or garden fork to dig out the clump. Be very careful not to bend or break the tubers when prying it out. The tubers are especially delicate if the clump hasn't cured or hardened yet. If a tuber neck breaks, that particular tuber will not grow. This is of less concern to a home gardener who doesn't split their tuber clumps fully—one or two broken tubers is fine, as you can replant a whole or half a clump of tubers come spring. If your shovel cuts a large tuber in half, that's fine too—just allow that tuber end to dry and cure for a few hours before storing the clump.
Handle the dirt. Once I dig my tuber clump, I remove a bit of the loose dirt, but I actually keep most of the dirt on the clump. I find the dirt helps keep the tubers plump over the winter by keeping moisture in. That said, my dirt is usually not very wet anymore as I've stopped watering the dahlias a few weeks before I dig. If the dirt was quite moist, I would likely remove more of it to prevent a tuber from rotting from too much moisture.
It also works perfectly well to remove most of the dirt by hand before storage, as this is more common. Some people remove all the dirt and wash their tubers. I personally find this too time consuming and labour intensive for the number of tubers that I store. I also like that the dirt acts as a natural protector for the tubers. But there are definitely plus sides to storing tubers clean as well.
Drain excess water. Once I dig my tubers, I bring them inside. Sometimes I spread them out on a table and flip them over, stalk side leaned down, so any extra water drains out of the hollow stalk and the stalk has a day or two to dry and callous over before storage. Flipping them upside down has been pretty important, as water can easily pool in the stalk and lead to rot.
Storage Methods
Choose your container. I store dahlia tubers in large plastic tree pots (because these were donated to me). These have large holes on the bottom for drainage (or air flow in this case) and are open to allow some air circulation as well. I would also recommend large Rubbermaid totes for storing in.
Consider a storage medium. Because my tubers have dirt on them, I don't use a storage medium (until I split them apart later in the winter). The dirt holds in moisture and I store hundreds of large pots with tubers in a cold room that I keep the humidity quite high in. I sometimes have a fan on to allow some air movement as well.
For home storage, I would recommend putting the tuber clumps in a Rubbermaid tub and adding some animal bedding or shavings to absorb excess moisture. Some people use peat or vermiculite to store in as well, but there are costs, convenience, and renewability of resources to consider (as peat comes from digging up bog and swamp ecosystems). Animal bedding can be purchased in large bales from various farm stores, pet stores, or Peavey Mart/Co-op, but it can cause tubers to dry out a little more.
Find the right spot. Keep the Rubbermaid tub in a cool dark place for the winter—a slightly heated garage or a cool part of your basement. Keep the lid cracked or punch some holes in the lid to allow some air circulation.
Maintain ideal temperature. Ideally tubers should be stored around 2-5°C. However, I've known friends whose tubers are stored probably closer to 14°C in a basement and survive fine. They don't stay fully dormant in this situation, but rather grow very slowly. In spring, they just break off the spindly colourless shoots and let it resprout healthy shoots under the right growing conditions. This obviously uses up some of the energy that your dahlia has available to it in the tuber, but in general, the plant should fare fine come spring if a warmer than ideal spot is your only option for storage.
Avoid freezing. Make sure your tubers are not too cold. Unheated garages are too cold in our zone. They may be fine for part of the winter, but you would need to add a space heater or have them in an insulated area with a light bulb or something to keep them from freezing. I've seen many creative things that people have built to achieve this ideal storage space.
Monitor through winter. Monitor your tubers over the winter, ensuring that they're not too moist. In general, if a tuber gets a little dried out or shriveled it will likely be fine. If a tuber has too much moisture on it, it can rot, in which case, it likely cannot be salvaged. In earlier storage years I've even carefully added a bit of moisture to Rubbermaid tubs when tubers looked like they were drying a bit much. If you come across a mushy rotted part of a tuber in storage, cut off the rot, let the cut cure and dry for a few hours, and then return it to storage, tweaking your methods as needed.
Should You Split Tubers Before Storing?
I generally store my tubers in clumps for as long as possible. But, because I sell them, I do split some of them in mid-winter and store the clean split tubers in vermiculite. The ones I don't sell remain in the clumps until spring. For the home gardener, I do recommend storing them in clumps for ease and better storage results. More about splitting dahlia clumps in a future post!
Think Like a Potato
One last thing I find helpful to consider is thinking of dahlia tubers like potatoes or other root vegetables when storing them. More people have stored potatoes so this may help to think of what works for potatoes. You want them cool so they don't sprout and you want to maintain their humidity so they don't dry out, but you also need to ensure they're not too moist that they rot.
Good luck if you're storing your tubers. Remember, you may lose a few, but it's a new skill to develop—so even if a few store, it's fun to try!
Happy Gardening,
Diana, Blossom Hill Gardens
