For a truly beautiful display of flowers, try some of these. Hybrids are brilliantly handsome in color and form, and produce large flowers sometimes 8 to 10 inches across when properly cultivated. Colors include white, yellow, apricot through pink, salmon to red, and picotee forms. Flowers may be single, double, crested, ruffled, frilled; flower forms resemble carnations, roses, rosebuds, and camellias.
Start tubers in March in a mixture of leafmold and sand or in milled sphagnum moss, as soon as tiny pink shoots appear in the indentation in the tuber. Settle tuber into mixture until entire tuber is covered and shoots are just at the surface of the soil. When roots are well established, transplant to pots in a light, fibrous, well-drained soil mixture. (Summer-flowering tuberous begonias are one- sided plants, that is, the flowers always face in the direction in which the leaves point.) Water well and place in a section of the greenhouse where plants will get filtered sunlight. Bright sun burns blossoms; too much shade makes the plants leggy. Take care not to allow water to collect in the concave top of the tuber from which the stems grow; this often causes tubers to rot.
If, after blooming for a while during the summer, your tuberous begonias seem suddenly to go into a decline, keep them on the dry side for about three weeks. This will induce new growth. Fertilize to strengthen this new growth and remove old stems. The new blooming period will carry through till autumn. This summer hiatus in blooming can be avoided if you remove the first few flower buds produced by the plant, giving it a chance to build a sufficiently strong root system to sustain it during the exhausting flowering period. The first flowers are usually small and of poor quality, anyway, so unless you just can't wait to see the flowers, do remove the first buds to allow the plant to build into a specimen able to support the large flowers.
When plants start dying off in the fall, withhold water and let the pots dry out completely. Remove all roots and dead foliage and store tubers in dry sphagnum moss or dry sand at a temperature of 40 to 50 F until new growth is evident. Then the growing procedure may be started again.
Tuberous begonias may be started from seed in February for summer or fall flowering. The extremely fine seed should be broadcast on dampened milled sphagnum moss, preferably in a seed flat. Cover with glass but nothing else light is essential to germination, as is a night temperature of 65 to 70 F. At no time during germination must the medium dry out, hence the use of sterile sphagnum moss to prevent damp-off from too much moisture. When seedlings are large enough to handle, pot them in a mixture of leafmold, garden loam, and well-rotted manure, and treat the same as newly started tubers.
'A. L. Berry', double yellow; 'Ballerina', a group of double-ruffled hybrids in pink, yellow, apricot; 'Red Triumph', ruffled frilled ruby red; 'Santa Maria', ruffled pure white; 'Charrnain', double pink; 'Flambeau', double orange-scarlet; 'Mandarin', double salmon-orange; 'Stars and Stripes', rose-colored camellia-type flowers blotched with white.