Tuberous begonias are considered to be tender bulbs. They grow best wherever they find temperatures of 58 to 72F., indoors or out. Consequently, their tubers must be dug up and stored over the winter in all but the mildest climates. Only in the southernmost regions of Florida and Texas (zone 10) do they grow outdoors year round.
Locating And Planting
Begonia tubers come in various sizes, from 1-1/4 to 3 inches or more in diamater. The largest tubers produce the largest plants and the most flowers. Plant tuberous begonias outdoors in the spring, when the tubers begin to show pink buds. Locate them in a lightly shaded spot after all danger of frost has passed. Like most bulb plants, tuberous begonias do not do well in wet, clayey soils. If this is a problem, improve the drainage by building begonia beds higher than the normal soil surface. Add lots of organic matter (chopped leaves, peat moss, sand) to the soil. It should be slightly acidic (pH 6.5). If possible, loosen the soil 1 foot deep when preparing the bed for planting.
Relatively short northern growing seasons dictate that tuberous begonias be started indoors well before frost danger is past outdoors. Purchase already grown potted plants from the garden center in the spring for earliest bloom. Plant them outdoors in May. Otherwise, start tubers in pots yourself in February or March on a windowsill or under lights. Since they will be in pots only for a short time, 3 inch deep trays that have holes for drainage are suitable. Place the tubers, hollow side up (where the buds will emerge), 2 inches apart in potting soil. Cover them lightly with a thin layer of the soil mix. Water them to moisten them, but avoid soaking them. Put these containers aside in an area that maintains 65°F. temperature. When sprouts appear, move them into an area with strong light. When the sprouts are an inch or two high, move each tuber to a 4 inch pot of its own. Provide the burgeoning plants lots of light to prevent their becoming spindly.
When all danger of frost is past, transplant the tuberous begonia plants from their small pots into the garden. Plant tubers 10 inches deep and 18 inches apart. Measure the planting depth from the bottom of the hole. At the end of the growing season, before frost hits them, dig up these outdoor plants. Store them, as described below. Or pot them for indoor display. Once potted and placed in a sunny, humid (40% to 60%) spot indoors or under lights, they'll bloom all winter.