How to Overwinter Dahlias
Dahlias are heat loving plants that grow best with lots of sun and warm soil. Unlike most annuals and perennials, they are at their best in late summer and keep on blooming right through the fall. But as they say, all good things must come to an end. By October, dahlias finally start slowing down. And once there’s a hard frost, the season is definitely over.
At this point, you have two options. Treat your dahlias as annuals and just pull out the plants and plant a fresh batch of 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 might think. Read on for some easy, step-by-step instructions.
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, you can simply cut the plants back to the ground and leave the tubers right where they are. Your dahlias will start growing again in spring.
In hardiness zone 7, dahlias will usually survive the winter outdoors if the soil is well drained and you cover the area with a thick layer of mulch to insulate the tubers. To avoid any risk of freezing, it’s best to bring the tubers indoors. (Follow the instructions below for colder zones.)
If you garden in zones 3-6, you will need to dig up your dahlia 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 large 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). If your dahlias aren’t already labeled, 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 varieties that look 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. This helps toughen their skins (though it isn’t essential). Cut back all the stems to within 4” of the ground. Re-tie the labels as needed, so they are securely attached. If your dahlias are 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 30” across and as much as 18” deep. Go slowly and be gentle, as the tubers are extremely brittle. Damaged tubers are more susceptible to decay.
DRY. If possible, let the clumps air dry for a day or two. Just 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 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) see below for dividing instructions.
Packing and Storing Your Dahlias
PACK. The root balls may be stored in several ways. You can plant them into large nursery pots with damp potting soil. Or store them in ventilated cardboard boxes or plastic tubs that are partially filled with damp growing mix, peat moss or vermiculite. Another option is to store several clumps together in a large black plastic trash bag. Don’t seal the bag, just loosely gather the top so moisture stays in, but there’s still some air circulation.
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 is ideal. In zones 5 and 6 you may be able to keep them in an attached garage. Just make sure the tubers don’t freeze. A frozen tuber is a dead tuber.
CHECK. Check on your dahlias periodically through the winter. If storage conditions are too moist, you may get some mushy tubers. Remove them and increase ventilation to reduce the moisture level. 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 March or early April, go through all of your dahlias and discard any tubers that are soft or completely dried out. Then it’s time to start dividing the root balls into manageable-sized clumps. If you want to handle and replant an entire clump, go ahead. Otherwise, read on.
When dividing a clump of dahlia tubers, every division needs one or more growth eyes. These occur in a very specific location. In the photo below, it’s easy to see how the eyes are clustered on the knobby part where the tuber meets the stem. If you plant tubers that don’t have an eye, they will not grow.
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, watch our bulb expert Hans Langeveld in this video: How to Lift and Store Dahlias. For other info on dahlias, read: All About Dahlias, How to Pinch and Stake Dahlias, How to Plant Dahlias (video) and Dahlias: 8 Great Looks.