Botanists divide the many species of begonias into 66 sections. For practical purposes growers use far fewer.
The Scottish Begonia Society uses a modified version of the American Begonia Society system. This is based on the way the plants grow.
This is the biggest group. Rhizomes look like thick roots at the soil surface but are really stems. Many modern cultivars such as B. ‘Tiger Paws’ are hybrids of Begonia bowerae discovered in Mexico in the 1940s. These tend to be small leaved plants which tolerate lower temperatures than many bigger leaved begonias and are less likely to contract mildew.
Most rhizomatous begonias crawl along the ground but some such as B. ‘Little Brother Montgomery’ are tall, upright plants which at first glance look as though they should be in a different section.
Some types of rhizomatous begonia have their own sections at Ayr Show. These include the small leaved Mac Macintyre hybrids and the larger and more delicate Rex-cultorum group.
These include the ancestors of today’s hybrid show doubles and pendulas which are dealt it in detail elsewhere. Not many of these plants are seen on the show bench though B boliviensis is becoming popular again – its drooping habit was used in developing modern pendulas.
This plant is best thought of as semi-tuberous and not dried off in winter. The small flowered B. sutherlandii from South Africa does die back completely. Its hybrid pink offspring is better kept growing.
Fibrous rooted begonias
All begonias have fibrous roots but the term is used by growers for those that do not also have a tuber or rhizome. These plants are sub-divided into cane types, such as the tall ‘Lucerna’, and shrub-like, such as B scharffii.
The Semperflorens group, now such popular and reliable bedding plants, originate from the small, shrubby B. cucullata. A few of these plants such as B. kellermanii are adapted to drier conditions than most begonias and can take more direct light.
The American Begonia Society has a separate category for scandent or trailing species such as B. convolvulacea. In Scotland these would be exhibited under the catch all class for any begonias.
Credits / references:
Stanley R.D. da Prato