Hosta naming: Wherefore art thou rohdeifolia?
I name thee:
H. x Tardiana 'Halcyon'
As with all living things, hostas are classified according to strict (but not permanent) rules of nomenclature. This taxonomy has changed through the years, as new specimens were brought from the far east to (usually) Europe. Hostas were first named Funkia and classified along with Hemerocallis. Later they were given their present genus name—honoring Nicolaus Thomas Host, an Austrian botanist—and (currently and debatably) live in the Agavoideae subfamily of Asparagaceae, if you can believe it!
For classification and naming of hostas, the breakdown is as follows:
Species These plants are found in the wild in their native habitats, where most grow from seed. Currently, over 40 species have been identified, with more being discovered as botanists travel further through China and Korea. For a full list of species and species forms (see below), click here. Wild species can vary significantly from place to place, but will be consistent according to the major identifying characteristics: number of veins, style of flowers, color of pollen, etc. Note that no significant wild populations of variegated hostas exist; wild hosta species are all green. Example: Hosta kikutii. Both names are in italics; Hosta (the genus) can be abbreviated as H. Note that "species," whether singular or plural, always ends in "s" ("specie" is an old word meaning "coin"). Note also that the plural of "hosta" is "hostas."
Species form or variant Where a significant difference between wild populations exists, it may be designated as a form or variant. For example, some populations of H. clausa have flowers that open, and are classified as H. clausa var. normalis. Others have flowers that do not open and are classified as H. clausa var. clausa. Interbred species variants will be true to seed. In addition to the number of species given above, there are maybe 20 identified variants. Example: Hosta kikutii f. leuconata or H. kikutii var. yakusimiensis.
Species selections (cultivars) If a plant of a particular species is selected based on desirable characteristics or mutation, it can be named as a cultivar (see below). Strictly speaking, these plants are all then clones of the mother plant, although seed strain cultivars (like H. sieboldiana 'Elegans') are known. The plant can be identified only by its cultivar name, but should be more properly identified with its species as well (as H. montana 'On Stage'). There is no distinction between such cultivars found in the wild or in the garden or nursery. Cultivars names are shown in single quotation marks. And don't get me started on trade names (which fortunately do not badly contaminate the hosta world—yet); trade names are properly shown in ALL CAPS.
Specioids (cultivars) This is not an official term, but is useful in distinguishing some historical hosta names. As hostas were gathered, particularly from Japan, no distinction was made as to whether the plant was collected from the wild or from a cultivated garden. Since European botanists assumed, erroneously, that all these plants were "natural" species, they were given species names. Later investigation revealed that these "species" were in fact hybrids (although the parentage of many is unknown), found only in gardens. So specioids are Latin-type names that have been "demoted" to cultivar status (see below). This includes many popular classes including 'Undulata', 'Tokudama', 'Lancifolia' and 'Fortunei' and less-popular plants like 'Tardiflora' and 'Helonioides'. Specioids generally do not reproduce true to seed; in fact many of these are effectively sterile, a good indicator that they are not true species.
Specioid selections (cultivars) Again, not an official term, but useful in sorting out names like 'Tokudama Flavociricinalis'. (Note that current nomenclature rules do not allow using these Latin-type names for new cultivars, but we are stuck with these older names.)
Grex This term is used for a complex (family of plants) resulting from a cross between two species. Orchid people use this all the time, but for hostas, it is really only applied to the famous cross between H. 'Sieboldiana' and H. 'Tardiflora' (both once considered as species) to give the Hosta x Tardiana complex. (Note that an "x" is also used for describing the parentage of hybrid cultivars, sometimes with complicated mathematics-style notation to show generations of crosses.)
Cultivars This applies to all hosta hybrids and their mutations/sports, as well as the species selections as noted above. Strictly speaking, all plants of a named cultivar come from the same mother plant, whether it was a seedling or a mutation/sport of another plant. One current database of hostas lists over 11,000 identified cultivars! Of these, over 6000 have been registered with the American Hosta Society, which is the international authority for the genus.
On cultivar names
It's not quite a free-for-all, but pretty much anything goes as long as it is pronounceable, not confusing, and not obscene. (I understand there was some discussion about allowing H. 'Derriere' a few years ago.) One notable hosta grower worked the system to try to secure both the first and last names on any alphabetized list: see the Hosta Library to see what you think. Names can be short and sweet, like 'May', or rather long, like 'A Many-Splendored Thing'.
If you browse the Hosta Library or the registered hosta pages, you'll come across strings of hostas with the same "first" name: Abba, Lakeside, Rosedale, Xanadu... These are generally from hybridizers who wanted to "mark" their hostas to clearly indicate who was responsible for them. Other marks are more subtle, like the many hostas from Mildred Seaver which incorporate "sea" (or attributes of the sea) into their names, or the national parks series from Olga Petryszyn.
You can easily build a "theme" garden on many themes by choosing the hosta names. Don Rawson has compiled a great number of these in his Hosta Lists pages.