How to keep dahlias blooming in the garden (2024)

This year I planted about 40 dahlia plants in my front yard. They bloomed very well. Will they come back next year? Why they are called biennial? And I read that dahlias have tubers. Can we save them and plant them next spring?

— Meghana Valame, Palatine

Dahlias are native to Mexico and cannot survive a Chicago winter in the garden. But because these tender perennials grow from underground tubers, they can survive periods of dormancy, making it possible to grow them in climates with frosts like those in most of the United States.

You can dig up the tubers in fall, store them over the winter and plant them again next spring.

Dahlias are not considered to be biennial. A biennial plant is a flowering plant that takes two years to complete its biological life cycle. In the first year, the plant grows roots, leaves and stems, usually staying low to the ground, and then goes dormant during the winter. In the second year, the plant grows taller, flowers and produces seeds before dying.

Dahlias, on the other hand, are perennials. In their native warm climate, they re-sprout from their underground tubers to bloom each year. That will not happen in Chicago because our cold winter soil would kill the tubers. (Hence, the term "tender perennial.")

They are valued for their bright late-summer and autumn flowers.

But plants that produce such show-stopping flowers have demands.

A full-sun site is best for dahlias, though they may tolerate some afternoon shade. They prefer rich and well-drained soil that has been amended with compost. It is important to keep the soil evenly moist, but not excessively wet, or the tubers may rot.

Dahlias are fast growers and heavy feeders, so provide supplemental fertilizer during the growing season. Stop fertilizing in the middle of August.

Avoid windy sites to prevent stems from being broken. Tall varieties will require staking wherever you place them in your garden. Give the big varieties plenty of space to grow.

Tubers can be planted directly into the ground after May 15, when the air and soil have warmed. However, be sure that the danger of frost has passed; in cold springs you may want to delay planting for a week or two. Plant each tuber 2 to 3 inches deep with 1 inch of soil covering the sprouting tip.

In late fall, after a frost has blackened the plants, very carefully dig up the tubers. To avoid injuring the tubers, lift them gently with a pitchfork rather than stabbing into the soil with a shovel. Leave a few inches of stalk attached to each tuber, but sift off all the dirt and trim back the roots.

Hang the tubers upside down to dry for a few days. Then cut off the stalks and store the tubers in cardboard or wooden boxes lined with newspaper, sawdust, perlite, wood shavings or peat moss. Air circulation is important. Tuck a label in with each tuber.

Check the tubers monthly for signs of shriveling, blackening or softness. If the tubers are shriveled, moisten the storage medium very slightly. If a tuber is soft, throw it out.

Tim Johnson is director of horticulture for the Chicago Botanic Garden in Glencoe (chicagobotanic.org). Send questions to: Gardening Q&A, Sunday, Chicago Tribune, 435 N. Michigan Ave., Chicago, IL 60611-4041; email to

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