Many begonias are either wrongly named or not named at all, especially in garden centres.
There are standard ways of writing all plant names. With true species, all of which were originally wild plants, the rule is easy. The genus gets a capital letter but the specific name starts with a low case (small) letter, for example, Begonia luxurians. Note that it is normal to put the scientific name into italics when in print – not necessary on a label on a pot.
Even when the plant is named after a person as in Begonia sutherlandii or the specific name still starts with a small letter. With Begonia rex the rule still applies though it is perfectly acceptable to talk about the Rex group of Begonias! As there are so many of these around without names the term Rex-cultorum is often used.
Many of the “species and cultivars” are not true species but hybrids. The rules for presenting their names are different. The cultivar name starts with a capital whether the plant is named after a person or not and it is not put into italics, for example, Begonia Little Brother Montgomery or B. Tiger Paws.
It has been traditional to put single inverted commas round the cultivar name when in print eg Begonia ‘Sugar Candy’ though these marks are sometimes omitted today.
The situation becomes more confusing with hybrids which were given Latinized names which make them sound like true species. For example Begonia ‘Thurstonii’ is a hybrid between two true species; other examples include B. ‘Erythrophylla’ and B. ‘Margaritacea’.
Begonia x tuberhybrida is sometimes used to cover all the large flowered hybrids though these are usually just referred to by their individual cultivar names. Where a named variety of a species exists the convention is to write it in the form Begonia dregei ‘Glasgow’.
However, if it is a subspecies – a distinctive form that occurs naturally in the wild – it is all in italics as in Begonia masoniana subspecies maculata, a form of the Iron Cross Begonia with a distinctive dark rim to its leaves. Any species should have only one accepted name but there are many examples where a plant has a commonly used alternative name, often because the botanists have given it a new scientific name but the previous one is still used by many growers. One example is Begonia scharffii which for years was known as B. haageana. The two names are often put on the label with the word syn. (synonym) alongside the previous name.
Credits / references:
Stanley R.D. da Prato