To preserve tuberous begonias over winter, reduce watering in early autumn. After the foliage dies and the soil around the plant is dry, dig the plant up, and cut back the stems. Allow the soil to remain on the root ball, and place the root ball in a cool, dry area, such as a garage, for several weeks to cure. After curing, remove excess growth from each tuber. Store the prepared tubers in boxes until spring.
The tuber is typically located where the stem of the plant meets the soil. After locating the tuber, carefully cut the roots and stems off, as these parts may rot while the plant is in storage. Inspect each tuber carefully, and discard any that have pests or shows signs of disease.
To further protect the tubers from disease, dust them in fungicide before storing them. Store the tubers in cardboard boxes filled with peat moss or sawdust until spring, or store the tubers in a single layer on wire mesh trays. Do not expose the tubers to freezing temperatures.
In spring, plant the tubers in small containers, and place the containers in a sunny location indoors until the danger of frost passes. When new growth appears, water and fertilize the begonias as usual.