The process of overwintering begonias depends on the climate, but in cold areas where frost is common, it is important to bring them in before the first frost. In warmer areas, it is often possible to overwinter the plants outdoors by moving them to a dry area.
In cold climates, move the plants to a well-ventilated area, and stop watering them. Within a few weeks, they become dormant, and the stem and leaves are easy to remove from the tuber. Remove the tuber from the potting soil, wrap it in a brown paper bag, and store it in a cool area that does not freeze. Place each tuber in a separate bag.
In warmer climates, moisture from rain is the biggest problem in overwintering. The changes in the available light signal the begonia to begin its dormant stage. Once the leaves wither, remove any remaining leaves or stems from the tuber. Remove it from the bed, and store like tubers in colder climates.
For begonias in warmer areas that grow in pots, turn the pots on their sides to prevent watering. Since frost is not a danger, simply leave them in the pots. When it's time for them to grow again, set the pots upright, and begin watering the plants.