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Chicago Master Gardener

A blog for Chicago Master Gardeners providing information on volunteer opportunities, training, workshops and resources.

Overwintering Flowering Bulbs

Posted by Ron Wolford -

When summer draws to a close and it's time to say goodbye to summer flowering 'bulbs' such as cannas, gladiolus, dahlia and tuberose begonia, why not consider giving them a reprieve and store the underground parts (tubers and corms, rhizomes) over the winter so you can include them back into the garden next year, said a University of Illinois Extension horticulture educator.

"Because these are classified as tender bulbs they need to be brought in right after a light frost has blackened their foliage," said Greg Stack. "In the Midwest this can be any time from mid-October on depending on where you live. This is the time of the year when gardeners are very attentive to the weather forecasts and make their local weatherman the most important person on the local news channel."

Storing summer-flowering bulbs is not difficult and can be very successful as long as some simple steps are taken to ensure the bulbs go into storage properly. Basically the steps include proper digging, curing or air drying, and storage under proper conditions of temperatures and air circulation.

"Cannas are spectacular plants for both their foliage and flowers so they are worth saving," he said. "These are among the easiest to store and the way they multiply ensures you will have a lot more the second year than the first year to replant in the garden."

After the first frost blackens the foliage, cut back the stems to about six inches. Carefully dig the rhizome clump out of the ground and leave the soil attached. Try to avoid cutting or injuring the rhizomes if possible. Allow them to air dry for a few hours in the sun.

"This air drying helps to callus over wounds that might have occurred in the digging process," Stack explained.

Put the clumps into crates or boxes that have good ventilation. If there is not a lot of soil attached, cover the rhizomes with peat moss. Place the crate in a basement, crawlspace or other dark, well-ventilated space where the temperature is around 50 degrees. Check on them occasionally through the winter and if the rhizomes show signs of shriveling, moisten the peat slightly. In the spring the rhizomes can be cut apart, potted and started indoors about six weeks before the last frost in the spring.

Dahlias form tuberous roots that are saved from one season to the next. After frost has blackened the stems, carefully dig the tubers.

"They will look like very fat "thumbs" connected to a central point," he said. "Cut the stalk down to about four inches and allow the clump to air dry for a day or two. Again this will help callus over any injuries that occurred during digging.

"Carefully brush away any soil. Do not wash or scrub the tubers. Place the tuberous roots in a well-ventilated box and use peat moss, sawdust or similar material to cover the roots and keep them from shriveling."

Dahlias can be stored at 40 to 50 degrees. A basement or crawl space works well. Check on them periodically to ensure they are not drying out. If needed, add a little moisture to keep the tubers plump. In the spring divide the clumps, making sure to include an eye or bud that is attached to each tuber close to where they were joined in the clump.

Gladiolus corms can be dug about six weeks after they finish flowering or when the tops start to turn slightly yellow. After digging, wash off the soil and cut the tops to within an inch of the corm. Leave the corms outdoors in the sun for a few days and then move them to a light, airy place. Spread them out and allow them to cure for two to three weeks.

"After they are dry, remove the old corm located under the new corm by twisting it off," he said. "Do not remove the papery husk from the corm. Place the corms in an open flat or in onion bags or nylon stockings. Store at 40 to 50 degrees in a well-ventilated area. The small cormels (baby gladiolus corms) can also be saved for future planting. Keep in mind, however, it may take two to three seasons before they will produce blooming-size corms."

Tuberose begonias should be dug before any frost hits them. Dig them with the stems attached and allow them to air dry. Remove the dry stem. Tubers can then be stored in flats or containers with dry sand, peat moss, or vermiculite.

While bulbs are in storage, check on them periodically over the winter. Slightly dampen the peat moss if bulbs show signs of shriveling or drying out. Also, if any of the bulbs show signs of decay or other soft rots, remove them immediately.

"Overwintering your summer-flowering tender bulbs properly ensures that you will have plants to put back into next season's garden and may also have extras to share with your gardening friends," Stack said.

Source: Greg Stack, Extension Educator, Horticulture

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