How to Overwinter Dahlias
Unlike most annuals and perennials, dahlias are at their best from late summer through early fall. But as they say, all good things must come to an end. By mid to late October, dahlias usually begin to slow down. And once there’s a hard frost, the season is definitely over.
At this point, you have two options. If you treat your dahlias as annuals, you can simply pull out the plants and plant fresh tubers next spring. Or, you can save the tubers from the varieties you really like and grow them again next year. Overwintering dahlias is easier than you may think. Read on for some easy, step-by-step instructions.
When to Dig Your Dahlias
Though the dahlias in the photo above look dead, they’re not. A heavy frost has killed the flowers, stems and foliage. But the soil is still warm and is protecting the tubers from freezing.
If you live in hardiness zones 8-10, where winter temperatures rarely fall below 20°F, it’s safe to overwinter the tubers right in the ground. Simply cut the plants back to just above soil level and leave the tubers where they are. Your dahlias will start growing again in spring.
In hardiness zone 7, dahlia tubers can survive winter outdoors if the soil is well drained and you cover the area with a thick layer of mulch for insulation. But some winters are colder than others. The only way to guarantee that the tubers will not freeze is to overwinter them indoors.
If you grow dahlias in zones 3-6, you will need to dig up the tubers and store them indoors. For this task, you need pruning shears, a shovel or digging fork, survey tape and marker, some damp growing mix, and either big nursery pots, black plastic trash bags or some large paper bags and boxes. Here’s how to do it:
How to Dig Up Your Dahlia Tubers
LABEL. Start by labeling your plants with survey tape (plastic plant labels are too easily lost). Be sure to do this BEFORE you get a frost so you can still see the flowers and evaluate which plants you want to keep. Save only the plants that are strong and healthy, and that really impressed you. There are tons of great dahlias out there and no reason to grow underwhelming ones.
CUT BACK. After the first hard frost, try to leave the tubers in the ground for a week or two. While waiting isn’t essential, it does help to toughen the skin of the tubers. Don’t cut down the plants until right before you dig them. This way water won’t accumulate in the hollow stems.
When you’re ready to dig, cut back all of the stems to within 3-4” of the ground. Re-tie the labels as needed, so they are securely attached. If your dahlias were grown in containers, skip down to the section on packing and storage.
DIG. Dig up each root ball, starting at least a foot away from the stem. Depending on the size of the plant, the root ball may be 12” to 18” across and equally deep. Go slowly and be gentle, as the tubers are extremely brittle. During storage, damaged tubers are more susceptible to decay.
DRY. If possible, let the clumps air dry for a day or two. But make sure they are protected from frost. Tubers may be divided at this point, or you can wait and do it during the winter or early next spring. If you plan to divide your tubers later, there’s no need to wash soil off the clumps. Just store the entire root ball as it came out of the ground. The soil gives them some natural protection from damage and rot. If you want to divide the tubers now (which will save on space if that’s an issue) skip ahead to the dividing instructions below.
Packing and Storing Your Dahlias
PACK. If you are not dividing the root balls until spring, you can store them in several ways. You can put them in large nursery pots with barely damp potting soil. Or store them in ventilated cardboard boxes or large plastic storage tubs that are partially filled with growing mix, peat moss or vermiculite. Another option is to store several clumps together in a large black plastic trash bag.
Once your dahlias are in storage, the goal is to retain enough moisture to prevent the tubers from shriveling, but allow enough air circulation to avoid rot. This means keeping the tops of the storage tubs or plastic bags loose. The humidity level in your storage area makes a difference, so it may take a few seasons to arrive at the best solution. If the storage area is relatively humid, start with paper bags and boxes. If dry, go with tubs or plastic bags.
STORE. Store the pots, boxes or bags in a cool, dark, humid place where the temperature will stay between 40 and 50 degrees F. An unheated basement works well in the coldest climates. In zones 5 and 6 you may be able to keep them in an attached garage. Just make sure there’s no chance that the tubers will freeze. A frozen tuber is a dead tuber.
CHECK. Check on your dahlias periodically through the winter. If the storage conditions are too moist, you may get some mushy tubers. Remove them and increase the ventilation to reduce moisture levels. If the tubers are wrinkled and dry, mist them or add some damp growing mix to help them rehydrate.
What to Do When Spring Arrives
In late winter or very early spring, pull out your dahlias and discard any tubers that are soft or have completely dried out. Then it’s time to start dividing the root balls into manageable-sized clumps. If you want to skip dividing and simply replant an entire clump, go ahead. Otherwise, read on.
When dividing a clump of dahlia tubers, each division must wind up with one or more growth eyes. These eyes occur in a very specific location. In the photo below, some of the eyes have begun to sprout, so it’s easy to see how they are clustered on the knobby part where the tuber is attached to the stem. No matter how good a tuber looks, if it doesn’t have at least one eye, it will not grow a plant.
It takes practice to see the eyes, and dividing clumps of tubers can be intimidating. If you wait until early spring, some of the eyes will start to swell and sprout. This makes it easier to see where to make the cuts. To be on the safe side, you can simply divide large root balls into halves or quarters. Cut down through the middle of the clump, making sure to leave some of last year’s stem attached to each division.
To learn more, you can watch this video on our YouTube channel: How to Overwinter Dahlia Tubers, and read some of the following articles on our website: All About Dahlias, How to Pinch and Stake Dahlias, How to Plant Dahlias (video), and Dahlias: 8 Great Looks.