Digging and Storing your Dahlias
Overwintering your plants
We are getting close to the end of the dahlia season, and I’m sure your garden is full of some of your most lovely blooms. One of the great pleasures of growing dahlias is having blooms in your garden so late in the growing season. I have found that my plants often go until early November, which is such a joy! Dahlias are what we call tender perennials. This means they only return as a perennial in climates where the ground does not freeze. If you get cold, harsh winters where the ground freezes you may want to consider lifting your plants out of the ground so that you know for sure that you can enjoy them next summer and fall! If you decide to leave them in the ground you can treat them as an annual or hope for a mild winter. Heavily mulching can sometimes keep the ground warm enough in sustained freezing weather, but there is still a risk that your dahlias may not come up again if the temperatures get too low. If you would like to lift and store your dahlias, read on and I will explain some of the methods I have learned over the years.
Lifting Your Tubers
1) Wait until a hard frost kills all the greenery of the plant. If there will not be sustained freezing temperatures that could freeze the ground, then it’s best to leave the plant in for a few days or a week or so after the hard frost. Plants overwinter better if you can do this. You will also be able to see the eyes of the plant better if you can leave them in a little longer after this killing frost.
2) Cut the stalk down to the base, making sure not to leave any of the hollow part of the stalk left, but also not to cut too deep into the crown of the plant. I usually leave about an inch of stalk. (I’ve included a video at the end from White Flower Farms about digging storing; you’ll be able to see where they cut the stalk in that [1:32 minutes in])
3) With a pitchfork, carefully dig a circle around the stalk. You want your circle to have a radius of about a foot. There will be a clump of tubers in the ground, so you want to try to avoid piercing them if possible — a tuber that’s been pierced or broken won’t be able to overwinter, but don’t worry if you pierce one or two; there should be plenty of them left. You can use a shovel for this, but if you make a mistake and hit a large section of the clump of tubers, the shovel will cut right into them and may cause more damage, so it is better to use a pitchfork.
4) Lift the plant carefully out of the ground with the pitchfork. If you only have one or a few plants you can skip rinsing it off and place it directly in a pot filled with dirt, and store it somewhere that won’t freeze but will stay cool, such as a garage or basement. Make sure to cover the clump complete with dirt too. You don’t want it in a place that freezes because that will kill the plant, but you also don’t want it so warm that it triggers the dahlia’s growth cycle. An average temperature of about 40 degrees is ideal.
This is the most low-maintenance method, and it’s great for anyone with a few plants and limited time. But if you’ve got a lot of plants (like we do!) you’re going to need to make more elaborate arrangements, so read on:
5) Spray the clump with a hose to remove excess dirt. Get as much dirt out of all the crevices as you can. Let the clump dry in a cool spot somewhere out of direct sun for about a day. A garage or basement is fine, but make sure it is not placed on a cement floor. In a box on a shelf would be best.
6) Because dahlias are not true to seed, one of the only ways to be sure you’ll get the exact same plant is if you propagate them by dividing the cluster of tubers while it’s out of the ground. You can divide them in fall after you dig, wash and dry them, or in spring right before you put them back in the ground; the advantage to dividing in fall is that the tubers are easier to cut apart at this stage and they are also easier to store in bulk. The disadvantage is that the eyes of the plant are harder to see in fall and every plant needs eyes to grow a new stalk next season. Not every tuber has an eye, and only a fully intact tuber with a neck and part of the crown (with eye) will produce a new plant.
The advantage to dividing in the spring is that the eyes are usually more visible, but the disadvantage in spring is that the plant is tougher to cut apart and storing a clump of tubers over the winter takes up more space.
You don’t have to split them at all, if you don’t want to; you can just store them as a clump and replant them next summer as a clump. This is the easiest way to do it; the clump will get bigger and bigger every year. Eventually it will get so big that the plant stops producing flowers, and just looks like a big bush. Even if you decide not to split your tubers when you dig them out, it’s still best to divide them every 2-3 years if you want to keep getting blooms.
Storing Your Tubers
Tubers can be a little bit finicky about staying good over winter. You need to have cool but not cold temperatures, and also consistent humidity that’s not too low or too high — too low and the tubers will shrivel and dry out, too high and they’ll grow mold and die. We originally stored ours in the basement, but if you put them on a cement floor, the cement will pull the moisture out of the plant and it will shrivel up. If you have to store in a basement, make sure your container is on a shelf and off the floor. We store ours in my husband’s library in the unheated attic (he has mixed feelings about this).
1) Cardboard box filled with newspaper and peat moss.
This is a common method, but it’s falling out of favor recently. Peat, which is a form of sphagnum moss that has been compressed over hundreds or even thousands of years, is not an easily renewable resource, and is also a vital part of its ecosystem. You have to use fresh peat because it actually can’t reabsorb moisture once it’s been harvested, and digging up tons of peat moss for one-time uses has come under criticism by a lot of growers, especially in England, where many historic peat bogs have been nearly depleted.
That said, individual gardeners periodically using small amounts are probably not going to have a huge effect on the industry, and some of the other methods also have environmental problems. This is the method that I use, because it’s provided the best results so far, but you’ll have to decide if it’s right for you or if you want to explore other methods.
First, layer newspaper at the bottom of the box, then a layer of peat. Again, it’s important to use fresh peat because your tubers need consistent humidity, neither too dry nor too moist.
Then, put your tubers in the box; try to keep them apart from other tubers or clumps of tubers as much as possible. You can mist them lightly with water and cover with another layer of peat and newspaper over top.
If you have split your clumps already, you may want to pack them in layers: one layer of newspaper, peat, tubers and then repeat until you fill the box. Check once a month over the winter to make sure they are not rotting or shriveling. If you see signs of shriveling, you can mist them again to keep them hydrated. If they’re rotting, you need to throw any rotting tuber out right away so that it doesn’t spoil the rest — just like apples, if one tuber starts to rot, it can spread to the others.
2.) Vermiculite: You can use plastic containers filled with vermiculite, a clay-based mineral you can buy at the hardware store. The vermiculite absorbs humidity in the air, but does not pull from the tubers, so this can help regulate moisture. The disadvantage to vermiculite is that it can be expensive (though you can re-use it from year to year).
Another concern is the possibility of vermiculite being contaminated with asbestos; this was a problem pre-1990s, but since then all vermiculate is tested regularly for the presence of asbestos, it is otherwise non-toxic. It can be quite dusty though, so I’d at least use a mask, especially if you’re working indoors.
3.) Plastic Wrap Method: We’re going to try using this method this year — we’ll wrap the tubers in plastic wrap individually to store them. We were hesitant to use this, both because we didn’t want to contribute more to the amount of plastic in the world, and also because it can be time consuming. We plan to give it a try and once I have more experience with it, I can comment more on the method and how it works. — keep an eye out for future newsletters, though, with our results!
Below I have two helpful videos from reputable dahlia farms about digging, storing and dividing!
How to Dig and Store Tubers
from White Flower Farms.
Dividing your Tubers
Here is a great video from Swan Island Dahlias explaining everything you need to know about tubers and how to divide them!