Trailing begonia's leaves and stems will wilt and die within a few days of the first killing frost. Dig around the plant with a spade. Carefully lift the entire plant, leaving a small amount of soil on the bulb's roots.
Cut the foliage and all but 1 inch of stem. Place the bulb in a dark, cool location. Allow it to dry for two to three weeks.
Remove the soil from the cured bulb. Cut off the roots and stem remnant. Dust the bulb with fungicide powder. Ensure complete coverage by pouring the fungicide powder into a paper bag. Add the bulb and shake the bag well.
Cover the dusted bulb in vermiculite or dry peat moss. Store it at 40 to 50 degrees Fahrenheit. Replant it when the soil warms in spring.