Big beefy dahlias with their dinner-plate-sized flowers are darlings of the garden from summer through the first autumn frosts. Although many gardeners treat dahlias as disposable annuals, it’s easy to store them over winter – and save money – for another display the following year. It’s simply a matter of digging up the tubers and roots after the first fall frost.
George Rebersky, Jr., of Worth, Ill., grows dozens of dahlias, which he replants each year. “You want to keep your dahlia flowers on the plants as long as possible,” he says. But after the first fall frost hits, you cut the stalk of the plant just a few inches above the soil. “You must retain part of the main stalk when you are cutting and dividing the tubers,” he says. Dahlia tubers are like potatoes; They should have “eyes” that will sprout new shoots come spring. He carefully digs up the tuber clumps and washes off the soil. Alternatively, place them in a cardboard box to allow the soil to dry so that it can be brushed off.
After his tuber clumps are thoroughly dry, Rebersky dusts them with powdered sulfur, which is available at garden centers. “The sulfur wards off bacteria and mold and gives you a higher success rate for a good tuber in spring,” he says. He places the tubers in small plastic storage bags with a handful of wood shavings, which you can buy in small bags in a pet department or pet store. “The bags can be layered in a large storage container with wood shavings between the layers to keep moisture away from the bags.”
Some gardeners also have luck simply placing the dried tubers in vermiculite. They check the roots periodically and sprinkle or mist water on them to keep the vermiculite barely moist if the tubers show any signs of shriveling. To identify the hundreds of dahlias that he grows each year, Corny Morgan of Palos Heights uses a permanent ink marker to write the name on each dried tuber.
Chicagoland Gardening editor Carolyn Ulrich has had luck overwintering container-grown dahlias that she stores in her basement. “The ones that have come back for me when kept in soil are ‘Bishop of Llandalf’, ‘Andries’ Orange’ and ‘Prince Noir’,” Ulrich says. This past winter, however, she stored dahlia tubers in bags in the basement but only ‘Prince Noir’ survived. “I sprinkled the tubers several times, and they appear to have rotted.”
From October through late April, store the tubers in an area such as a crawl space, attic or the inside corner of an attached garage that stays between 38 to 50 F from October through late April. A cool temperature is critical to maintaining dormancy. “When I’ve grown and overwintered container-grown plants in the basement, the ‘Andries’ Orange’ sometimes broke dormancy in February, and that was a problem,” Ulrich explains. “That meant I ended up with a 2-foot plant in my living room by mid-April and didn’t know what to do with it while it was waiting to go outside. I just kept it close to the window and hoped for the best. It survived, and with such a head start it was blooming outdoors in June.”
Liz Omura of Cantigny Gardens in Wheaton overwinters dahlia tubers in open-sided plastic crates that offer air circulation. “We store them loose – no soil – and the crates are typically stacked 10 to 12 high.” The crates are kept in a hoop house that stays about 40 F all winter. “We pull them out as the tubers begin to sprout and then plant them directly in the garden.” Although some tubers don’t make it, Omura says that she has had great success overwintering ‘Mystic Illusion Yellow’ using this method for several years.
When spring rolls around, you’ll be ready for another dahlia display. “May 15 is the golden day when the frost is typically gone and it’s time to plant the tubers in the ground,” Rebersky says.
• Use a fork to carefully dig the tubers out of the ground after the first fall frost has withered the leaves.
• Discard any tubers that do not have “eyes.”
• Leave the roots and tubers in a dry cool place for several days.
• Brush off the dried soil.
• Store tubers in bags with or without vermiculite or wood shavings.
• Check periodically that tubers haven’t dried out. Mist shriveled tubers lightly with water and return them to storage.
• Plant tubers in pots or in the ground in mid-May.
Nina A. Koziol teaches horticulture and garden design classes at The Chicago Botanic Garden and The Morton Arboretum. She is a horticulturist with a degree in education and environmental science. firstname.lastname@example.org