The tuberous begonia in the garden or container is one of many hybrids developed by crossing species originally found in the Andes. As a tropical plant, it is not frost-tolerant and won't survive the winter in cold climates.
Although tuberous begonias are available in many cultivars and come in varieties including upright or trailing plants and single and double blooms, they are not to be confused with the fibrous rooted wax begonia, which is considered an annual plant.
As the temperatures fall, the tuberous begonia will begin to show signs of stress. Using a garden fork, gently lift the plant and tuber from the soil. Cut back the stem to a point just above the soil line. Leave the soil ball around the tuber and roots in place, and bring the plant into a cool, dry storage area.
Allow the tuber, soil and root mass to dry completely and to cure for two to three weeks in a cool area. Do not water. When the soil has dried and crumbled, shake the tuber and then remove any of the remaining planting medium from the tuber and roots.
Remove any dried stalk or root material that is left on the tuber with a clean, sharp knife. Examine the tuber for any spots that could be indications of rot and cut them out. Properly dispose of the discarded plant material, being careful that any debris that shows signs of pests or disease not be introduced to the garden or compost.
Place the bulbs in a paper bag and add a fungicide, sometimes called bulb dust. Follow the directions on the package of the specific type of fungicide used, as they may differ.
Place the treated tubers of the begonia in a dry paper bag. Store them in a dry, cool location that is safe from frost until spring. Many rodents will eat tubers, so extra precaution should be taken if the storage area is prone to attracting pests.
The tubers can be removed from storage as early as February and planted indoors to give them an early start. Plant tuberous begonias in a well-drained container using about 2 inches of peat or sphagnum moss, with the concave side up. Cover with more moss and dampen with a sprinkle of water. Place the container in a bright place that maintains a temperature of between 60 and 70 degrees Fahrenheit. Sprinkle more water as the moss dries out. When the shoots appear, they will use more water, but be careful not to allow water to accumulate. The tuberous begonia can be transplanted to a container when the shoots are 4 inches high and all danger of frost has passed.