Tuberous begonias, or Begonia x tuberhybrida, are winter-dormant plants. When growing in bright shade and warm conditions, they become upright, bushy plants with fleshy leaves and stems. Their flowers, mostly double-formed with extra petals, appear in summer as long as weather doesn’t get too hot or humid. Tuberous begonias were created through numerous genetic crosses of several begonias native to the warm and low-humidity summer areas from the Andes Mountains, where only short, light frosts typically occur.
The tuberous begonia grows from an small but swollen underground stem known as a tuber. Depending on temperature and day length, these plants always go dormant in the fall and winter months. The tubers persist in a relatively dry to barely moist soil in cool temperatures and then resprout in spring once soils warm. Leaves and stems grow first; by summer, many buds open to reveal large flowers, about 12 weeks after tubers first sprouted. If pollinated, a flower may yield tiny seeds that germinate in 10 to 21 days.
Once the plant naturally withers after the flowering season or the first light fall frost partially kills above-ground leaves and stems, dig up the tubers. The process is the same whether outdoors in the ground or in small containers as houseplants. Clip away the stems so a length 3 to 5 inches remains above the tubers, and let them dry and shrivel on their own. Crumble away any clinging soil and allow the tubers to air dry at indoor room temperature out of direct sunlight. Do not wash the tubers.
Once the tubers dry and are cleared of stem and root debris, they must endure a dark, cool dormancy. Some gardeners sprinkle the tubers with a powder fungicide. Place the tubers in a perforated plastic bag filled with dry sphagnum moss, sawdust or vermiculite particles. Bury the tubers in the fill. The temperature range for the dormant tubers needs to remain between 40 and 50 F. Sustain them in dormancy as needed between eight and 20 weeks.
If tuberous begonia tubers are left in soil in containers over winter when dormant, it’s vital to maintain the soil at a chilly temperature but never reaching freezing. Wet soil or storage bag medium encourages fungal rot. Check on tubers every two to three weeks and promptly remove any tubers that are soft or show signs of black or white rot or mold.
After the prolonged chilling dormancy, plant the largest, firm and plump tubers in well-drained but moist potting soil that’s between 60 and 65 degrees Fahrenheit, although standard home conditions of 70 F suffice. When planting, orient the hollow side of the tubers upward in the soil. A general guideline is to plant tubers indoors in pots or trays two months before the last spring frost date in your area.