Nomenclature is the naming of plants. The name that identifies one plant and distinguishes it from the hundreds of thousands of others in the world is its botanical name. The botanical name is expressed in Latin and is recognized internationally. Plant taxonomists, governed by the International Rules of Botanical Nomenclature, assign the names based on where a plant fits within the kingdom’s classification system. The unique botanical name given to a plant is in two parts, termed a Latin binomial. The first part is taken from the genus in which it is classified. The second part is called the epithet, and the two parts together identify the species. While a genus name may be applied to several closely related types of plants, such as Pinus (the pines) or Cornus (the dogwoods), the epithet eliminates all other types but one and creates the unique species. Thus, Pinus sylvestris, Pinus resinosa, Pinus nigra, and Pinus strobus are distinctly different plants, as are Cornus florida and Cornus mas.
NOTE: Many texts and individuals use the word species to mean epithet as well as using the term to refer to the complete binomial.
It is not uncommon for plants to have two names, the botanical name and a common name by which it is known within a country or region of a country. Examples are Euonymus alatus, which some know as Winged Euonymus and others as Burning Bush, and Acer saccharum, known as Sugar Maple to most Americans, but as Hard Rock Maple in parts of New England. As familiar as they are, common names can be misleading and are not wisely used when there is a need to be specific about a plant. As examples, the Douglas Fir is not a fir. Neither is the White Cedar a cedar.
As unique as the Latin binomial is in identifying a plant, the nuances of Nature and the artificially induced differences created by horticulturists and plant breeders often require additional words to fully describe a specific plant. The genus and epithet names of a plant may be followed by a variety or cultivar (cultivated variety) name. Though there are additional subclassifications, such as subspecies and form, it is usually enough to know just the varieties and cultivars of a species to be certain of their identity.
Varieties are plants within a single species that are distinctively different from others of the species. They are capable of breeding true to form and owe their variance to natural origins. The Thornless Honeylocust exemplifies a true variety, differing from other Honeylocusts by its lack of prickly thorns. Its botanical name is properly written as Gleditsia triacanthos var. inermis. The var identifies it as a variety of the species. It is not italicized, unlike the genus, epithet, and variety names. As an example, here is how the tree with the common name thornless honey – locust would be classified by taxonomists:
Division: Tracheophyta Class: Angiospermopsida Order: Rosales
Family: Leguminosae Genus: Gleditsia Species: triacanthos Variety: inermis
Cultivars are varieties that are selected and sustained by people who value some distinctive attribute of the plant and propagate it in large numbers, usually by vegetative means. Were that propagation to cease, the cultivar would soon disappear. Some cultivars are patented by the person who first discovers it. The plant patent gives that person exclusive rights to propagate and sell the cultivar to greenhouses and nurseries and others who wish to grow it. Patented cultivars often cost more than nonpatented varieties. Correctly written, a cultivar’s botanical name appears as either Viburnum opulus cv. Sterile or Viburnum opulus “Sterile.” Note where italics are and are not used.
When two different species within one genus are successfully crossbred, the resulting species is a hybrid of the two. The name of the new species retains the name of the genus, but drops the epithets of the two parent plants and is given a single new epithet. Its status as a hybrid is represented by an x. For example, Epimedium xyoungianum, common name Young’s Barrenwort.