Tuberous begonias, one of three types of begonias, are summer and fall blooming perennials known for their large, brightly-hued, camellia-shaped flowers. Native to the tropics of South America and Africa, tuberous begonias thrive in warm, humid conditions and are hardy only in USDA zones 9 through 11. In cooler climes they are dug and overwintered indoors. Begonia tubers can be forced to propagate by dividing stem and tuber cuttings and will propagate on their own by seeding and by naturalization in the landscape. Even under the best circumstances, begonias can be finicky to propagate and the failure rate can be quite high. Patience is essential when trying to multiply begonia tubers.
Dig up and divide your begonia tubers in the late spring or late summer while the plant is in its active growth phase. Brush off excess soil and make clean cuts along the tuber, making sure that there is at least one, and better yet two or three stems sprouting from each cut section.
Plant the tuber piece and stems in a nursery pot filled with moist, sterile potting mix. Place the tuber and stems at the same soil depth at which it was previously planted. Keep the soil moist but not wet. Place two small bamboo stakes or wood dowels on either side of the pot. Cover the pot with a clean, clear plastic bag and either tie off or tuck the ends of the bag under the pot to secure and to create a greenhouse effect.
Keep the humidity around the cuttings high for a two- to four-week period, during which they will root in the potting mix. Place in a location where they will receive only bright, indirect light. Begin to acclimatize the cuttings after you see some new growth or after one month has passed. Open the bag 1/4-inch more each day to mix fresh air with the greenhouse environment.
Place the cutting-filled pots outdoors when temperatures are mild and in the desirable natural range for begonias. Transplant the rooted tuber cuttings into the soil after at least a month of being placed outdoors. Staged changes in the growing environment will reduce the stress on the plant, prevent shock, and raise the chances of a successful tuber propagation.