University of Illinois Extension

Protecting Flowering Bulbs in Winter

If you want to save those glorious summer flowering bulbs, it is important to have a good winter storage site, said Martha Smith, U of I Extension horticulture educator. “Throughout central Illinois, our growing climate withstands winter temperatures of -10 to -20°F,” explained Smith. “This puts us in Zone 5 on the USDA Plant Hardiness Zone Map. Horticultural plants planted in our area are able to survive these winter extremes.

“Unfortunately, many of the glorious summer flowering bulbs such as cannas, dahlia and tuberous begonia cannot survive here. In order for us to enjoy their splendor, extra attention and care in the fall is needed. They need to be dug-up and stored. Our winter soil temperatures are too cold and summer flowering bulbs left in the ground won’t survive.”

Among the options for winter storage, a garage attached to your home or a deep cellar will keep the temperatures between 35-40°F. This is critical if the bulbs are to survive. At warmer temperatures, the bulbs may begin to sprout or rot. At colder temperatures, they can freeze. Before storing, check all bulbs and discard any diseased or damaged ones.

“Cannas should be cut back within two to three inches of the soil after the first frost,” she said. “Dig the tuberous roots with a spading fork taking care not to damage the fleshy root systems. Place them in a frost-free location, allowing them to dry out. This may take several weeks. Bulbs are sufficiently dried when the stem easily breaks away from the roots. Cannas do not require covering in the winter. Often they are laid upside down in flats of dried peat moss.”

Tuberous begonias should be dug when the leaves turn yellow in the late summer or early fall. Cut tops back to within a couple of inches of the tubers. Dry in a warm location for two to three weeks. Store tubers between two-to-three-inch thick layers of vermiculite, peat moss, sawdust or wood shavings.

“Check tubers throughout winter. If shriveling occurs, lightly moisten packing material. If roots appear, move to a cooler, drier location,” said Smith.

After a light frost, dahlias should be cut back to four to six inches above the soil. “Choose a sunny day when the soil is not too wet or too dry,” she said. “Lift the tubers with a long-tined fork. Insert it one-foot away from the bush on all sides. Then gently pry up the clump so the tubers will not be ripped off. Shake most of the soil off and leave them in the sun for several hours to dry.”

Do not allow the tubers to freeze, she added. If still damp, you can lay them on screens in a protected area for a day. Leave undivided, packing them carefully in flats, boxes or bushel baskets and cover them with vermiculite, peat moss, sawdust or wood shavings.

“Check dahlia tubers throughout the winter,” said Smith. “If shriveling occurs, moisten packing material lightly. Do not let tubers completely dry out.”

“Don’t confuse these types of bulbs with the spring flowering tulips, daffodils and crocus,” she warned. “These spring bloomers stay in the ground all year. Summer bloomers may need more attention but you will be rewarded all summer long.”