Humidity is the enemy
If you decide to save tender summer bulbs over the winter, wait until the foliage dies back for the year. Then, gently dig up the bulb or tuber and shake the soil off. If the bulb seems firm and healthy, cut back some of the roots and let it dry in the sun for a day or two. Put the bulb in a paper bag with some peat moss, label it and put it where it will stay dry and cool, but not freeze, such as a cool basement or garage.
The great enemy of stored bulbs is humidity. If they don't stay dry, they may rot. "They should be firm, like an onion," says Lizzie Holmberg, manager of Lizzie's Garden in Naperville. That's why they should never be stored in plastic bags or boxes, which could trap moisture.
Some gardeners dust bulbs with a fungicide or sulfur before storing them to help forestall problems.
Check the bulbs from time to time over the winter, and discard any that have succumbed.