Dahlias really shine in late summer and fall. They fill our gardens and vases with beautiful blooms long after most other flowers have faded. When cold weather arrives, it's always hard to say goodbye to such generous plants.
At the end of the growing season, you can either treat your dahlias as annuals and plant a fresh batch of tubers next spring, or 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.
Dahlias are heat loving plants that grow best with lots of sun, 80 to 90 degree temperatures and nice warm soil. They won't tolerate freezing temperatures, and in the fall, the first heavy frost will blacken their foliage.
Though the dahlias in the photo at left look dead, they’re not. Frost has killed the leaves, but the soil is still warm and has protected the bulbs from freezing. If you live in growing zones 8-10, where winter temperatures rarely fall below 20 degrees, you can simply leave your dahlias right in the ground. Cut back the foliage, and the plants will start growing again in spring.
In zone 7, dahlias will sometimes survive the winter if the soil is well drained and the area is covered with a thick layer of mulch. To be perfectly safe, follow the instructions for zones 3-6.
If you garden in zones 3-6, you'll need to dig up your dahlia bulbs and store them indoors. For this task you need pruning shears, a shovel or digging fork, survey tape and marker, damp growing mix, and some big nursery pots, black plastic trash bags or large boxes. Here's how to do it:
1. LABEL. Start by labeling your plants with survey tape (plant labels are too easily lost). If your plants aren't labeled, be sure to do this BEFORE you get a frost so you can still see the flowers. Save only the varieties that really impressed you.
2. CUT BACK. After the first hard frost, cut back all the stems to within 4” of the ground. If your dahlias are in containers, skip down to number 6.
3. DIG. Dig up and remove the entire root ball. Start digging at least a foot away from the stem. The root ball may have a diameter of 12” to 30” and may be 18” deep. Go slow and be gentle as the tubers are extremely brittle. Damaged tubers are likely to decay.
4. DRY. Let the clumps air dry for a day or two, protected from direct sun and frost. Tubers may be divided at this point, or you can wait and do it in the spring. Either way, be sure to label the tubers by name. If you plan to divide in the spring, there's no need to wash soil off the clumps. Just store the entire root ball as it came out of the ground. If you want to divide the tubers now (it saves on space if that's an issue) see below for instructions.
5. PACK. The root balls can be stored in one of several ways. You can plant them in 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 large black plastic trash bags. Don't seal the bags, just loosely gather the top so moisture stays in, but there's still some air circulation.
6. 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.
7. CHECK. Check on your dahlias periodically through the winter. If conditions are too moist, there may be some mushy tubers. Remove them and reduce the moisture level. If the tubers are wrinkled and dry, mist them or add some damp growing mix to help them rehydrate.
When spring arrives, go through all of your saved dahlias and discard any tubers that look like they didn’t make it. Then it's time to start dividing the root balls into manageable clumps.
If you haven't done this before, the key is to make sure each division includes several sprouts or growth eyes. These occur in a very specific location, where the tuber meets the stem. To be on the safe side, you can simply divide the root ball into several clumps, leaving some of last year’s stem attached to each clump.
To learn more, watch our bulb expert Hans Langeveld in this video: How to Lift and Store Dahlias. For more information about growing, you may be interested in: All About Dahlias, How to Pinch and Stake Dahlias, How to Plant Dahlias (video) and Dahlias: 8 Great Looks.