The naming of cultivars derived from crosses between two or more different species, belonging to the same or different genera, involves rather special problems. By "derived from" I mean not only the first generation from a species cross, but all subsequent generations and back-crosses with the original parents or with members of the first or later generations. Any cultivar which is the progeny of a species cross, however remote, comes, for the purpose of the Code, under the heading of a hybrid and its naming is subject to definite rules. The full name of a "hybrid cultivar" must be regarded as consisting of three distinct parts: (1) the name of the genus (or "hybrid genus" if a hybrid between two or more genera is concerned); (2) a "collective" name or phrase covering all the progeny resulting from the particular species-cross concerned; and (3) a cultivar-name for the particular form (cultivar) under consideration. In the name Viburnum × bodnantense 'Dawn,' Viburnum is the generic name, × bodnantense is the collective name for all progeny of the cross V. fragans × V. grandiflorum, and 'Dawn' is the cultivar-name for a particular seedling of this parentage. It is essential always to bear in mind these three distinct parts of the name of a garden hybrid, even if, as it often done, one or other of the parts is omitted in actual usage; the three parts broadly correspond, of course, to the generic name, specific epithet, and cultivar-name of a non-hybrid cultivar (see above).
Let us consider these three parts in turn and see what types of name can be used in each part in actual practice—and how this affects the naming of new "hybrid cultivars."
The first part, the generic name, presents no difficulties, except in the case of new hybrids between two or more genera. Names of such "hybrid genera" are usually "manufactured" from a combination of the names of the parent genera (e.g. × Heucherella, from Heuchera and Tiarella); in the case of hybrids between more than two genera, however, where a "combination" name would be unwieldy, it is permissible to make a new name by adding the termination ara to the name of a person connected with the plant concerned (e.g. × Sanderara for a tri-generic orchid hybrid). Before making a new "hybrid generic" name, a botanist should be consulted, as a Latin description in proper, botanical form must be provided. It will have been noticed that a multiplication sign is placed in front of the names of "hybrid genera," but after the generic name in the case of hybrids within a single genus; further details on this point are given later.
The second part of the full name of a hybrid is a more tricky business. It is, as I have said, a collective designation for all the progeny of the particular cross concerned, and it may take one or all of three possible forms:—
(1) If the parentage is known, a bare formula consisting of the names of the parents, in alphabetical order, connected with a multiplication sign, e.g. Lewisia cotyledon × rediviva.
(2) A name in Latin form (corresponding to a non-hybrid specific epithet), preceded by a multiplication sign, e.g. Viburnum × bodnantense.
(3) A vernacular phrase containing the word "hybrid," "cross," "grex" (Latin for flock or group), or other similar word, making evident the collective nature of the phrase, e.g. Lilium 'Bellingham Hybrids.'
Why do we have to have these three different forms for the second part of the full name, and when, to put it briefly, should we use which?
I must admit that practice on this particular point is as yet by no means fixed, and no doubt it will be modified considerably during the next few years, as more horticulturists try to apply the Code to the groups in which they are interested.
In order to understand the alternatives available, it will be helpful, I think, to consider first the third part of the full name, as the use of this, and of the second part, are very closely connected. The third part, as I have said, is a cultivar-name of a particular form of the hybrid concerned, and, in the first place, it is extremely important to realize that every cultivated hybrid that is considered worth naming at all should receive a cultivar-name from the outset of its "career," even if, at the time, only one form is known, or is considered worth naming. To take an actual example, at the time when the new hybrid Viburnum × bodnantense was described in the Botanical Magazine, only one form, or clone, of it had been "put into circulation," and yet that single clone was given a cultivar-name, 'Dawn.' The object of doing this was to be able, in the future, to refer by name to this particular clone and so avoid confusion with any later, and possibly inferior, forms of the same cross that might be produced. In the absence of the name 'Dawn,' less desirable clones could legitimately be passed off under the collective name V. × bodnantense without the acquirer realising that he was not getting the original and superior form.