A biological species definition, based in zoology, like the one applied to humans (Homo sapiens) are of limited use for a plant genus like Hosta. Biological species are defined as populations of similar individuals, alike in structural and functional characteristics, which in nature breed only with each other, and which have a common heritage (i.e., they are genetically closely related). Hosta species can and do interbreed (in most cases) resulting in populations that are intergrading (interspecific) hybrid swarms. Only geographic isolation will keep Hosta species from hybridizing in nature. For this reason, the biological definition is rarely applied to the genus Hosta.
A legal species definition is now mostly used by botanists. In the legal definition species are considered accepted, fundamental units of study in plant taxonomy (the practice of classifying plants). To keep order in the in the world of classifying plants, taxonomists came up with a set of rules called the International Code of Botanical Nomenclature (ICBN). Within the ICBN a plant species is defined as the principal basic rank called a species. A number of species make up a genus, and each succeeding higher rank is so nested into the next higher rank, genera into family, families into order, orders into class, classes into division and all of these combined make up and are nested into the plant kingdom. The ICBN allows species to be further subdivided into subspecies, varieties and forms. When the term “species” is used in AHS literature and articles, it is always the legal definition, unless otherwise noted. A species name is always presented in italics and consists of a genus name and a species epithet, as in Hosta ventricosa or H. ventricosa. The word “species” is used for both the singular and plural.
A cultivar definition is now universally used in horticulture. A cultivar is an assemblage of plants that have been selected for a particular attribute or combination of attributes and that is clearly distinct, uniform and stable in these characteristics and that when propagated by appropriate means retains these characteristics. Because Hosta cultivars do not come true from seed, they must be vegetatively propagated (i.e., by division or tissue culture), so are usually clones of the original, selected plant. Cultivar names are always written in Roman letters enclosed in single quotes, like Hosta ‘August Moon’ or H. ‘August Moon’. While cultivars can originate in the wild as sports, they are not considered cultivars until they are selected for garden use and given a cultivar name in accordance with the International Code of Nomenclature for Cultivated Plants (ICNCP).
W. George Schmid
Hosta Hill - Tucker Georgia USA