Numerous begonias can be grown in heated, but the most popular are the tuberous-rooted kinds with their large double . There are also the tuberous pendulous begonias, seen at their best in hanging baskets.
Tuberous begonias can be grown from dry tubers or seed. Unless one has a heated propagating frame, where a temperature of 18 to 2°C. (65 to 70°F.) can be maintained, though, tubers are the best proposition.
Seed sown in January or February will produce flowering plants by mid-summer. The very small seed needs careful sowing in pots or pans filled with seed compost, and should be covered with fine sand rather than compost. After germination, it is best to supply water by-holding the pot or pan in a bucket of water until the moisture seeps through to the surface.
Dormant tubers started into growth in March will make flowering plants by late June. These tubers should be pressed hollow side uppermost into moist peat and coarse sand and placed in a warm part of the greenhouse with shade from strong sunshine. Light spraying overhead is appreciated, but overwatering must be avoided, especially in cold weather. When the growths are a few inches high, pot the plants up in 5-in. pots using John Innes No. 2 Potting Compost. Place the tubers half way down the pots, and cover with about ½ in. of compost. Later, when the plants are established they can be topdressed with more compost so that eventually the tubers are about 2 in. below the surface.
When the seeds have germinated, the seedlings must be pricked out into boxes, 1-1/2 in. apart. These are small and difficult to handle and a forked stick is helpful to transfer them to the dibber holes in the box. The tiny plants need a temperature of 18°C. (65°F.) and shade from strong sunshine. When large enough, the seedlings should be moved into 3-in. pots using John Innes No. 1 Potting Compost and be treated in the same way as plants raised from tubers.
The final move for seedling begonias is into 5- or 6-in. pots using John Innes No. 2 Potting Compost. Goodis essential.
As the plants develop, support them with small canes and raffia. Pay particular attention to the flower stems, as the blooms are rather heavy.
The first flowers to appear should be removed so that the plants make good growth before the flowers open. Later on it is wise to do more disbudding. It will be noticed that the flowers usually appear in threes. In addition to the male double flower there will be two female flowers on either side. These are single and less spectacular and should be pinched out, leaving the double flowers to open to their full size.
Botrytis. The grey mould fungus disease, can be troublesome on begonias. It often makes an appearance in cold, damp weather or in an insufficiently ventilated greenhouse. The fungus may gain entry to the plant through broken leaf stalks. Any withered leaves should be removed with a sharp knife close to the stem, and if the disease appears it pays to dust the infected area immediately with flowers of sulphur.
Watering and Feeding
Begonias need to be watered carefully. Water should be given sparingly after potting and until new roots are made into the fresh compost. Each plant must be treated individually and when thein the pot is beginning to dry out the pot should be filled with water to the rim. Water must not be given again until the soil shows signs of dryness once more.
As the plants become established in their final pots, feed with liquid fertiliser at intervals of 7 to 10 days.
Tuberous begonias must not be dried off too quickly. I like to keep the plants growing for as long as possible into the autumn, but when the foliage shows signs of yellowing, watering can be reduced and the plants laid on their sides under the greenhouse staging. When the stems have withered completely, remove the tubers from their pots and shake out the old soil. Then dust with flowers of sulphur and store in boxes of dry peat, sand or old potting soil in a dry, frost-free place until it is time to start them into growth again in the early spring.