Dahlias are subtropical plants native to Central America, and gardeners all over the world grow them for their colorful, diverse, and spectacular blooms.
The secret to successfully growing dahlias in cooler climates lies underground in their tubers. During the growing season, each dahlia plant produces a clump of tubers up to six inches deep underground. Dahlia tubers, which look somewhat like a small sweet potato, are swollen underground stems that store energy to enable plant growth the following spring.
Dahlia tubers are not true bulbs. Bulbs are round and consist of one swollen root while dahlia tubers are food storage vessels that come in various shapes and sizes and form a clump. Bulbs differ from tubers because bulbs contain the embryo of next year's blooms while dahlia foliage is produced from eyes located in the crown of last year's plant.
Because dahlias are hardy only to USDA hardiness zone 9, gardeners in colder climates must dig up dahlia tubers before the first autumn frost and overwinter the tubers indoors and replant them in the spring.
When to Dig Up Dahlia Tubers
Mother Nature will alert you when it's time to dig up your dahlia tubers. Dahlia foliage cannot tolerate freezing temperatures. The first heavy frost will blacken and turn to mush the flowers, stalks and foliage. However, deep underground the tubers are still alive and waiting for you to dig and store them for the winter.
Don't be too hasty and dig your tubers before the first frost. Frost stimulates dahlia tubers to set eyes for next year. Wait for 10 to 14 days after the first frost to dig up the tubers, but make sure to dig them up well before a hard freeze is expected.
Before the first frost hits is a good time to affix a label to the base of the dahlia stalks so that you'll be able to identify the variety next season when you re-plant the tubers.
Equipment / Tools
- Garden fork and spade
- Gardening gloves
- Hand pruners
- Long-sleeve shirt and pants
- Dust mask
- Spray bottle
- Sulfur dust
- Peat moss
- Cardboard box
Cut down the Dahlia
As soon as the first autumn frost blackens the dahlia foliage, cut down the dahlia stalks to a few inches and wait at least 10 days before digging up the tubers.
Dig Up the Tubers
After waiting at least ten days after you cut back the blackened foliage, loosen the soil around all four sides of the dahlia by gently angling and pushing the garden fork or shovel into the soil about about a foot away from the main stalk. Be careful not to let tools come in contact with the tubers, which are easily damaged. Gingerly pry up the soil as you go, freeing the tubers. With one hand on the stem and the other under the clump of tubers, carefully lift and remove the entire tuber clump.
Inspect, Clean, and Prune the Tubers
Inspect your dahlia tubers and trim away any tubers or tuber parts that are rotten or damaged. Don’t save every tuber, save only tubers that are in good condition. Clean off most of the soil and trim back the roots.
It's important to remove and discard the mother tuber— it's located in the center part of the structure. The mother tuber will look different; it has a more crinkly texture and is darker than the other tubers. The mother tuber produced the plant this year and won’t have any more energy left for next year. In the case of first-year plants, this is the original tuber that you bought and planted. The mother tuber is more likely to rot and could spread to the other tubers in storage.
Divide the Crown into Individual Tubers
You might want to divide the tubers before you dry them for storage rather than waiting until spring. Dividing a tuber clump is an optional step; it is perfectly fine to store the entire tuber clump as one unit.
Dividing means cutting apart the crown and retaining one or more tubers along with it. The crown is located at the base of the stem, and it contains the eyes where new growth emerges. To produce a plant next year, each division must have a piece of the crown that contains at least one or two eyes. You can usually assume that if you have a big piece of crown, it has an eye
Let the tuber divisions sit for a day or so to allow the cuts to callous over.
Sometimes the eyes, which is where new growth emerges, don't appear until after the winter storage period. In this case, it can be more time-efficient to wait until spring to divide the tubers.
Dividing a tuber clump is optional, and it's worth noting that storing and replanting an entire clump of tubers could offer a higher chance of success next year because you preserved the entire crown, which contains multiple eyes.
Hang the Tubers to Cure
Hang the tubers upside down with twine and allow them to cure out of direct sunlight for two weeks. Only cure them outdoors if temperatures are above freezing. If rain is forecasted, move the tubers indoors to a location that does not receive direct sunlight. Otherwise, cure them indoors in a cool and dry location, perhaps a garage, shed, or basement.
Once the dahlia tubers are dry, use pruners to remove most of the remaining stalk.
Apply Sulfur to the Tubers
To prevent rot and deter fungus during the overwintering process, many gardeners opt to sprinkle sulfur dust (a type of fungicide) onto tubers before storage. Come spring, you can plant the tubers, dust and all, to continue to thwart decay before they sprout.
Protect yourself from the sulfur dust by wearing long pants, a long-sleeve shirt, a dust mask, and gloves. Sprinkle the tubers liberally with sulfur dust. Take care to apply the powder to any areas that are nicked or cut.
Prepare the Storage Box
Line the bottom of a cardboard box with newspaper to cover any cracks. Pour in two inches of peat moss. Use a spray bottle to slightly moisten the peat moss. Make sure not to saturate the peat moss, which can cause tubers to rot. Instead, strike a balance between dry and wet, erring on the side of dry.
Tuber storage medium is a matter of debate. Instead of peat moss, some gardeners opt to use vermiculite or even dry sawdust as their tuber storage medium. Other gardeners individually wrap each tuber in newspaper. Still others wrap tubers in plastic wrap. The point is that there is no single answer for storage medium and methods. You need to experiment with what storage process is successful for you year to year.
Place Tubers in the Storage Box and Store in a Cool, Dry Place
Lay the tubers flat on the peat moss, and then cover them with an additional inch of peat moss. Make sure tubers or tuber clumps are not touching one another. You can place dry sheets of newspaper between layers of tubers.
Store the box in a dark area—such as a basement, garage, or root cellar—that will remain cool for the winter but will not freeze. The ideal storage temperature is above freezing but below 50 degrees Fahrenheit.
Check the Tubers Periodically
Periodically check your tubers during the winter. If they seem overly dry, spritz the peat moss with a spray bottle. And if a tuber feels mushy, discard the rotten culprit before it infects the rest of the tubers.
Tips for Overwintering Dahlia Tubers
Experiment with your storage methods until you find an overwintering system that works best. This might mean wrapping your dahlia tubers in newspaper to soak up excess moisture before storing them or opting to store them in paper bags or milk crates instead of a cardboard box.
You can add peat moss to a zippered bag, place the dried tubers into the bag, add a liberal sprinkling of sulfur dust, seal the bag, and then gently shake it to distribute the dust. Then, the tubers are ready to be stored.
Before foliage is blackened by frost, label the dahlias if you grow different varieties, so you know which cultivars you're working with when spring rolls around. You might even consider using separate storage boxes for each dahlia variety.
Digging, Dividing, and Storing Tubers. The American Dahlia Society Website