This section is from the book "Commercial Gardening Vol2", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
These originated from B. boliviensis, B. Pearcei, B. rosoeflora, B. Clarkei, B.
Veitchi, and B. Davisi, the first-named having been introduced from Bolivia in 1857, the last-named in 1876. Of these B. Pearcei is the only one having yellow flowers, the others being rose or crimson. In all cases the flowers are small and more or less drooping, and quite insignificant in comparison with those of the modern florists' Begonia, many of which have flowers from 6 to 9 in. across. Magnificent shades of colour have also been evolved by hybridization, selection, and cross fertilization, and there are now all colours, varying from the purest of whites through pink and purple to the deepest scarlet and crimson; while clear-yellow, primrose, and orange shades also abound. There are also single- and double-flowered varieties (figs. 264, 265), some with plain edges, others crimped and frilled, and others again with crests on the face of the petals.
Fig. 264. - Tuberous Begonia, single.
And all this wonderful work has been accomplished practically within the past forty or fifty years. Some thirty years ago varieties called Weltoni-ensis and Saundersi, both with pink flowers, were grown largely for market, but they have long since been superseded by improved strains which are sturdier in habit and more free in blossom.
For bedding out during the summer months tuberous Begonias are particularly well suited, and as their original parents above-mentioned came from an elevation of 11,000 to 13,000 ft., they are as hardy as the Dahlia.
They are ornamental in foliage, the large somewhat irregularly lobed and coarsely toothed leaves being larger on one side of the midrib than the other, and of a deep glossy green. On the whole they are more easily raised and more quickly grown than the Zonal Pelargonium, and they possess the great advantage that the tubers can be stored away in frostproof lofts or cellars during the winter months until wanted to start into growth in the spring.
The simplest method of raising a stock of tuberous Begonias is from seed. This is small and like grains of brown dust. It should be sown in January or February, on the surface of a light sandy compost in a temperature of 70° F. by night and 75° to 80° F. by day. The little seedlings are pricked out when the seed leaves are well developed, with a pointed stick into a similar compost, being spaced out about J in. apart. They are carefully watered and protected from strong sunshine until established, and they grow rapidly in the temperature mentioned. When large enough to handle easily, the young plants are then transferred singly to 3-in. pots, using a compost of good turfy loam, to which may be added about one-third of leaf mould or well-rotted manure from a hotbed or mushroom bed, and a good dash of coarse sand. A similar temperature may be still maintained, and the plants should be watered carefully and sprinkled overhead daily to encourage rapid growth. By the middle or end of April they will have attained a fair size, and the temperature may then be gradually reduced, and more air and light given to harden the plants off for sale in May and June. Seedlings the first year even will develop tubers about 1 in. in diameter. Any plant not sold may be planted out for stock in some part of the garden, and the tubers will serve to produce an early crop the following season.
Fig. 265. - Tuberous Begonia, double.
A great trade is done by nurserymen in the tubers every year, and some special varieties obtain high prices. Where plants are raised annually from seeds, it is always possible that a particularly charming variety may appear, and may be worth while propagating specially from cuttings and division of the tubers.