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The flowers normally appear in groups of three - one large male flower in between two smaller females. Multiflora types produce many stalks, leaves, and flowers are the simplest to raise. Grandifloras grow taller with more brittle stems which require staking. Hanging begonias are usually more difficult to grow.
The size of the tuber determines the number of flowers that will be produced. For display pots, choose 1½ to 2 inch tubers to plant in 10 to 12 inch pots; the plants will grow 15 to 20 inches tall with 5 to 8 blooms. Smaller tubers are best for smaller containers or for mass plantings in a garden bed.
Start tuberous begonias in flats in March. Set outside only after there is no danger of frost. Plant the tuber with the concave side up, and cover with ½ inch of soil mix. Most of the roots grow from the top rather than the base of the tuber. A good starting mixture consists of a 1:1 peat moss and sand mixture. A ½ gallon milk carton with one side removed is a good starter flat. The plants prefer temperatures from 65° F to 70° F and a moist, but not soggy, soil.
As soon as the tuber develops two leaves the same size, it is ready to pot. Use a rich, loose, friable soil, such as 3:1 mixture of a standard potting soil and additional humus. Set the tuber at the same soil level in was before; adding more soil on top can cause stem rot. Settle the soil by gently thumping the pot rather than firming the soil by hand. Stake if the plant will have single stalk. If the tubers are planted in an outdoor bed, remember that the dart-shaped leaves point in the direction the flower will face. Place the tubers 16 to 18 inches apart in the ground, and stake the plants as soon as they get top-heavy with blossoms.
It is important that the young potted plants get cool night temperature (55° F to 60° F) and enough light for strong growth. Spring sunlight or artificial lights are fine. If the light source is a window, rotate the plants every two or three days.
Once placed outside, tuberous begonias prefer light shade with brief sun, at least 50% humidity, and good air circulation. Some eastern Washington gardeners may find these plants difficult to grow because of the region's hot, dry summers. Too much sun can cause leaf burn and bud drop; over watering may also cause bud drop. If the leaves curl under, the plant is overfed; if they look pale, they need more fertilizer. For larger blooms and sturdier plants, fertilize weekly with a formulation high in phosphate and potash starting after the flower buds appear and show color. Foliar feeding is also beneficial.
Within a week after the first light frost touches the begonias and turns their leaves yellow, it is time to lift the tubers and let them dry. Do not remove the stems immediately. Wait until the leaves and stems have withered enough to be remove without injuring the tuber. Then store for the winter in dry peat moss, sand, or vermiculite at 55° F.
