Most growers of double Begonias use a loam based compost such as John Innes. This is also suitable for the small flowered tuberous and semi-tuberous types. Many growers prefer a more open compost for rhizomatous plants, many of which come from forest floors, banks or even tree branches where they grow in humid conditions but in a very free drained material. This explains the apparent paradox that plants that thrive on high humidity detest water logging. Over watering, especially in winter, is the most common cause of death with most begonias. Even when a plant survives over watering it may perform poorly for some time due to damaged roots. Composts based on peat or peat substitutes are popular; many growers add perlite to improve drainage.
Feeding should not be overdone. Established plants can befit from a high potash fertiliser such as tomato feed applied at half strength when plants are actively growing.
Temperature should be as uniform as possible though with some difference between night and day. As a rough rule, small leafed plants tolerate cool conditions better than large leafed. Plants with silver, red or other colours in their leave tend to be more delicate than those that are largely green with more chlorophyll.
Most of these plants prefer a minimum temperature of 55-60 F. This can happily rise to 85 F during the day. Very high temperatures can inhibit growth. As much natural light as possible is needed in winter. As the sun’s power increases in spring some shade is necessary to prevent scorching. Begonias grown in very bright light produce small, discoloured leaves. Bright filtered light in the summer is ideal and promotes healthy growth and deep leaf colour.
Many begonias are shallow rooted. Half pots are ideal but can be hard to find in plastic – cactus growers at major shows are a useful source. Over potting can cause problems when combined with over watering. The tall growing cane and some shrub types are often grown in clay pots to reduce the risk of toppling over.
Stanley R.D. da Prato