Though they're not technically bulbs, begonias grow from underground tubers and must be treated like bulbs when the plant dies back every winter. Proper curing and storage is necessary to keep the tubers viable and ensure a healthy plant the next year. Simply leaving the begonias outdoors in the ground will freeze them to death or cause the tubers to rot due to excess moisture in the soil.
Dig out the begonia plants after the above-ground foliage has wilted due to frost. Some gardeners choose to use a gardening fork instead of a spade to help reduce the risk of damaging the underground tubers.
Brush off any dirt and lay the tubers on a towel indoors. Let the tubers dry for several days. The tuber is sufficiently dry when any attached stems break off easily, according to the American Begonia Society.
Put the tuber in a paper bag, with one tuber per bag. Loosely crumple the top of the bag to close it.
Stack the bagged tubers in a cardboard box. Store the box in a cool and dark area, like a basement, until you replant them the following spring.