The novice gardener regularly encounters the word “cultivar” without necessarily understanding its meaning. Yet it’s so simple.
A cultivar is a plant discovered or developed by a human. The word simply means “cultivated variety” (culti + var). It could result from a deliberate cross between two other varieties, as in the case of the apple ‘Emperor’ above, therefore a hybrid, or it could be a mutation found on a plant or a seedling with qualities (bigger flowers, differently shaped leaves, etc.) than the original species. Usually we write the cultivar name in between single quotation marks and in Roman letters. ‘Osiris Fantaisie’, for example, is a cultivar of Ligularia dentata.
Also, to avoid confusion, the rule is that the cultivar name must always be derived from a spoken language such as French, English or Japanese, rather than a dead language like Latin or ancient Greek. The botanical name of the plant, on the contrary, is always written in italics and does derive from ancient Latin or Greek. There are still a few old cultivars with Latin-based cultivar names (‘Rubra’, ‘Variegata’, etc.), but these are exceptions rather than the rule.
A Cultivar Name Is International
Take the name Vitis vinifera ‘Petit Rouge’, a grape vine, as an example. Vitis vinifera, italicized, is the botanical name for all European grapes. ‘Petit Rouge’, between single quotation marks and in Roman letters, is the cultivar name. This name helps distinguish it from the hundreds of other cultivars of European grape.
Note too that you never translate a cultivar name: it’s like a proper name. Frenchman Laurent Roy doesn’t become Larry King when he travels to England and Ludwig König while he’s in Germany: it remains the same wherever he goes, all over the world. So it would be incorrect to change the name of ‘Petit Rouge’ to ‘Little Red’ when you grow it an English-speaking country.
You’d like more examples? Here are a few. The first part, in italics, is the species name and the last part, in single quotes and Roman type, is the cultivar name:
Acer platanoides ‘Crimson King’
Hosta ‘Sum and Substance’
Rudbeckia fulgida sullivantii ‘Goldsturm’
Solanum lycopersicum ’Sweet 100’.