Large flowered begonias are usually grown from tubers, but they can be grown from seed and they will flower about eight months after sowing if sufficient heat is available. There are many other kinds of begonias with fibrous roots, and some with rhizomes. Typical of this type are the B. rex cultivars with multi-colored leaves. These will grow well along the edge of a greenhouse path, partly shaded by the staging. The most popular fibrous rooted begonias are those grouped under B. semperflorens, with green, bronze or variegated foliage, and white, pink or red flowers. Most have single flowers, but there are a few fully double varieties, chiefly grown as pot plants, while the single flowered kinds are used for summer bedding. There are many others, some with silver spots on the foliage, many of which make good house plants.
Growing from Seed
Sow seed of tuberous double varieties and B. semperflorens in January or early February, in soilless compost or general seed compost. Sow in a clay half pot or pan, and when using a soilless compost it should not be firmed down. Before sowing water the compost. Sprinkle the seed on the surface of the compost and do not cover it with additional compost, or with paper to exclude light. Place the pan in a heated propagator, and maintain a temperature of 60°-70°F (15°-21°C). The seed should germinate in 8-14 days, and the seedlings should be pricked out as soon as possible, into a similar compost, about 1 in. apart. Return them to the propagator if possible and, after a further 4-6 weeks, the seedlings should be touching each other, and they will then need spacing 2 in. apart. Return to the propagator again for a few weeks, but if this is not practicable, keep them as warm as possible and if it is dull or cool they will need very little water. By mid-May the seedlings should be hardened off in a frame prior to planting out in early June. They like some peat or similar material in the soil, and they can be planted in shade or full sun.
Seedlings to be grown in pots in the greenhouse should be potted up into 3-in. pots in early May, in part general potting compost, mixed with part of soilless compost. After a few weeks the plants will need potting on into 5-in. pots, and for these and all larger pots use 2 parts loam, 1 part peatmoss, 2 part decayed cow manure, 1 part sand, plus a 4-in. pot of bone meal to every bushel of mix. The B. semperflorens varieties will flower in 5-in. pots but the tuberous doubles will need potting on into 6 1/2- 7 1/2–in. pots. Clay pots are preferable as there is less danger of over-watering; if you use plastic pots add an extra part of coarse sand to the potting mix. In a greenhouse plant will need some shading from bright sunshine.
Begonias need cool conditions to produce sturdy plants with good blooms, and once all danger of frost is over the ventilators should be left open day and night to allow maximum circulation of air.
Wherever you grow tuberous double begonias, it is best to remove the first flower buds. This allows the plants to become established before the strain of flowering and will result in larger and better-formed flowers. Three blooms usually develop on each flowering stem, and with pot-grown plants the two outer blooms should be pinched off, leaving the larger double male bloom to open. The two outer blooms are usually females, and are always singles. The female blooms have a light green, three-winged seed capsule immediately behind the petals, but this is missing from all male blooms.
When in bloom tuberous double begonias will need a weekly feed with a weak tomato-type fertilizer, high in potash. Stop feeding in early September. Plants will need less water as the cooler weather arrives, but it is vital that the compost should be kept moist and no attempt should be made to force the plants into dormancy by with holding water. As top growth ceases moisture is needed at the roots to help build up a good tuber to survive the winter dormancy. Winter Treatment Any green leaves retained by the end of October can then be removed, but the stems must not be forcibly pulled off. Stop watering as soon as all green leaves have dropped, or been removed, and the stems should drop off within a few days. The tubers can then be taken from the pots and cleaned up, but be careful not to damage the ‘eyes’ (axillary buds) on the top of the tuber as next year’s growth will start from these. Store the tubers in dry peat in a frost-proof place until the following spring when they can be restarted. Plants in the garden should be lifted in early October with a ball of soil round the roots, placed on the floor of the greenhouse until all stems have dropped, then stored in dry peat.
Recommended Begonia Varieties
Tuberous doubles: ‘Roy Hartley’; ‘Diana Wynyard’ and ‘Avalanche’, white; ‘Crown Prince’, crimson; ‘Guardsman’, red; ‘Harlequin’, white ground picotee; ‘Midas’, yellow. Of the hundreds of different varieties of B. semperflorens ‘Muse Rose’ and ‘Color Queen:, are well worth growing.