Begonia: Greenhouse Plants
W – warm, minimum of 13°C (55°F)
This is a large group of plants containing some 350 species and many hybrids. It is important to know which of the three main types ofyou have in order to understand its cultivation. Both fibrous and rhizomatous types, although growing adequately in a peat-based , will do better in a John Innes modified by the addition of extra peat and grit. They do not like a sticky, poorly drained compost. The surface of the compost should be allowed to dry out between waterings, especially in winter with the rhizomatous types. Both prefer a temperature of 15°C (60°F) but can survive well at 10°C (50°F). A combination of either being too hot or too cold with overwatering will result in the plant collapsing.
These include the well-known Begonia semperflorens which are mostly grown as bedding plants and would hardly merit space in a greenhouse collection. However, there are many interesting species which are easy to grow. B. fuchsioides from Mexico can reach 1 m (3 ft) in height and has delicate small leaves and many pink shell-like flowers. B. haageana from Brazil reaches a shrubby 60 cm (2 ft) or more and has large hairy leaves which are purple on the undersides. Bunches of rose-pink flowers are produced virtually all year round. B. metallica from Bahia can grow to 1.2 m (4 ft) and is distinguished by the metallic sheen on the surfaces of the leaves which are reddish beneath. Flowers are white and pink during summer and autumn. B. venosa is a great favourite of mine. It has incredibly silvery scales which give the leaves a downy appearance and lacy stipules, which are structures that appear to bind the leaf to the stem. The flower is white. This plant from Brazil needs good light to prevent it from losing its silvery appearance and compact growth. Propagation of these is mostly by softwood shootwhich root easily and can even be done in water.
These are easily recognised as you can see the rhizomes, or modified stems, lying on the surface of the compost. They use these to spread the plant into a large clump. It is characteristic that if they become potbound they will go into decline because there is no space for the new growth at the tip of the rhizome to move into. Well-known examples include B. Rex and B. masoniana, the Iron Cross Begonia, which are grown for their highly decorative leaves. It is said that leaving their rather insignificant flowers on will diminish the beauty of the leaves. I have not found this to be true. They do, however, produce very small, poor leaves after being in the same pot for several years. It is possible to divide the plant into several rhizomes, each with its own roots which can then be potted separately. Both these plants can be propagated by. There are many other rhizomatous species but few that are commonly available. The exception is B. manicata, a favourite of parks and gardens for its reliability as a winter display plant for the greenhouse. It is a tall-growing worth growing for its profusely borne sprays of tiny pink flowers.
Sections of stem about 5 cm (2 in) long taken as rhizome cuttings in June will give small flowering plants by early spring and good sized display plants for the winter after that. This method of propagation is preferable for these as there is often very little material other than rhizome to choose. Some pieces may even have root attached and they are simply laid on to some cutting compost and just nestled into theso that they are half buried. Keep them warm and moist (not too moist in the compost or they will rot) and new roots and shoots will be made. Tip cuttings can be used provided the material is available.
These immediately call to mind the huge hybrid bedding and showwhich are dormant in winter and are started off in February or March by placing the tubers hollow side uppermost, half submerged in moist peat in good light and at 13°C (55°F). After three or four weeks or when some growth can be seen the tubers can be potted and grown on either for display or for bedding. For really large blooms it is necessary to remove the two smaller female flowers that appear either side of the showier male flower. The plants will die down in autumn and can be left dry in their pots until early spring. It is most important to keep them warm at 10-13°C (50-55°F) during their dormant period. In order to bulk up favourite colours the tuber can be cut into two or more portions in spring, provided each has a growing point (usually visible as little red knobbles on the tuber). Cut surfaces should be treated with a fungicidal powder and then the pieces can be brought into growth as normal.
There are two other groups of tuberous begonia. There are those that are really fibrous in practical terms but have been derived from tuberous parents and have a tuberous ‘look’ about them. The Lorraine begonias come into this category, which are very useful winter flowerers from seed sown in spring. However, they must be grown warm in peat-based compost to do well. Their pretty shell-pink flowers are very welcome in the dark months. There are various other hybrids to be bought or grown; however they are not really worth saving from year to year and are best discarded after flowering. The other group are permanent members of the collection. These are the tuberous species such as the quite commonly grown B. sutherlandii with its delicate leaves and pretty little orange flowers. It is distinguishable by its habit of producing small tubers in the axils of the leaves which can be used for propagation. The plant will die down in autumn and go dormant during winter when it needs to be kept dry. It is important to label these dormant plants. I have known many occasions when they were thrown away as a dead plant.
It is not difficult to germinate begonia seed although as it is so small it needs careful handling. Use peat compost lightly pressed and soaked. Surface sow, cover with cling film and germinate at 21°C (70°F) in light.