Overwintering Dahlia Tubers
Dahlias are sub-tropical flowers indigenous to Mexico and parts of Central America. Even still, many North American gardeners grow this flower, coveting it for its ornate blooms. The secret to successfully growing this plant in cooler climates lies underground in the flower's tuber. Dahlia tubers (sometimes referred to as "bulbs") are not true bulbs at all, as they differ in both appearance and growth function. Bulbs are round and consist of one swollen root. Tubers, conversely, come in a variety of shapes and form in a cluster. Also, bulbs grow from one fixed point, where tubers (like potatoes) grow out of the "eyes."
Since the dahlia plant only thrives as far north as USDA plant hardiness Zone 9, cold-climate gardeners must learn to work with these finicky tubers by digging them up and storing them each year.
Related: 15 Gorgeous Must-Plant Dahlias
When to Dig Up Dahlia Bulbs
Mother Nature will alert you when it's time to dig up your tubers. When the first light frost of autumn arrives, the leaves on the flowers will turn brown. At this point, only the aboveground vegetation has perished. Deep underground, the dahlia's tubers are still alive waiting for you to uproot them and store until warmer weather returns.
Don't be hasty about digging before the first frost. Frost stimulates dahlia tubers to set eyes for next year. So hold off about a week or two after the frost has passed, but make sure to pluck them well before any hard freeze to prevent damage.
- Working time: 1 to 2 hours
- Total time: Up to 6 months
- Material cost: Under 30 dollars
What You'll Need
- Garden fork or spade shovel
- Gardening gloves
- Five-gallon bucket
- Hand pruners
- Long-sleeved shirt and pants
- Dust mask
- Cardboard box
- Spray bottle
- Soil sulfur dust
- Sphagnum peat moss
- Mesh bags
Digging, Trimming, and Washing the Tubers
You can dig up dahlia tubers with a garden fork (a chunky version of a pitchfork) or a standard shovel. Be careful not to let the tool come into contact with the tuberous roots, however, which can be easily damaged.
- Loosen the soil around the dahlia tubers by thrusting the fork or shovel into the ground about one foot away from the plant. 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 remove the entire mound from the ground.
- Poke the edges of the clump gently with a pencil to knock off most of the dirt, feeling for tubers with your free hand.
- Using pruners, trim off almost all of the aboveground vegetation that was killed by the frost, leaving only a little bit.
- Gently wash the dirt off your dahlia tubers by swishing them around in a bucket of water or hosing them down with a garden hose. Be very gentle—even the slightest puncture can introduce pathogens, causing your dahlia bulbs to rot in storage.
Applying Sulfur Dust to the Tubers
To prevent rot and deter fungus, gardeners opt to sprinkle sulfur dust onto tubers before storage. Come spring, you can plant them, dust and all, to continue to thwart decay before they sprout.
- Inspect your dahlia tubers, and trim away any parts that are rotten. Remove the "mother" tuber. In the case of first-year plants, this will be the original tuber that you bought and planted. This old tuber is more likely to rot than the new ones and such rotting could spread to the others in storage.
- Let the tubers sit for a day or so to allow the wounds to callous over.
- Protect yourself from the sulfur dust by wearing long pants and a long-sleeved shirt, a dust mask, and gloves.
- Sprinkle the tubers liberally with sulfur dust. Take care to apply the powder to any areas of the tubers that are nicked or cut.
Dividing and Drying the Tubers
Before you dry the tubers for storage, you may want to divide them now, instead of waiting until spring. However, sometimes the "eyes," crucial for reproduction don't appear until after the winter storage period. In this case, it can be more time-efficient to wait until spring to divide them.
- Divide the tubers (similarly to removing garlic cloves from a bulb), taking care to preserve at least one eye per tuber.
- Hang the tubers upside down with twine or place them in a mesh bag and allow them to cure outdoors for two weeks. (Only use this method if temperatures are above freezing and there is no rain in the forecast.) If rain is forecasted, move the tubers indoors to a location that does not receive direct sunlight.
- Once the dahlia tubers are dry, cut off the remaining stems with pruners.
Preparing the Medium and Storing Tubers
Some gardeners prefer storage mediums like vermiculite, coarse sand, sawdust, or wood shavings. But for this particular method, sphagnum peat moss works great.
- Line the bottom of a cardboard box with newspaper to cover any cracks. Pour in 2 inches of peat moss.
- Moisten the peat moss slightly using a spray bottle. Make sure not to saturate it, which will encourage rot. Instead, strike a balance between dry and wet, erring on the side of dry.
- Lay the tubers onto the medium and then cover them with an additional inch of peat moss. If you divided the tubers, make sure they are not touching one another.
- Store the box in a dark area—like a basement 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.
Tips for Overwintering Dahlia Tubers
Experiment a little with your preparation and storage method until you find a system that works best for you. This might mean wrapping your dahlia tubers in newspaper (to soak up excess moisture) before storing them in the box filled with peat moss or opting to store them in paper bags instead of a box.
Alternatively, 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 around to distribute the dust. Once you remove the tubers from the bag, they are now ready to be stored in your box filled with peat moss.
Label the dahlia tubers so that you will know what specific cultivars you are working with when next spring rolls around. If you have lots of varieties to store, consider using separate boxes or label each clump properly.
Periodically check your tubers during the winter. If they seem overly dry, spritz the peat moss with a spray bottle. And if a dahlia tuber feels mushy when you check it, discard the rotted culprit before it infects the lot.
Planting Dahlia Tubers
No matter which plant hardiness zone you reside in, there is a simple way to remember when to plant your dahlia bulbs. Plant them in the spring, just after the danger of the last frost has passed. If you're not sure when that is, ask an experienced gardener at your local garden center or check with your county's extension office. In some North American zones, you can use Memorial Day as a cue to plant most tubers.
Read Next: How to Grow and Care for Dahlias