International Poplar Commission

Rules for botanical taxa

The general framework is set by the International Code of Botanical Nomenclature, also known as the St Louis Code, which was approved by the Sixteenth International Botanical Congress held in St Louis, Missouri, July-August 1999 and published in 2000 (W. Greuter Ed., Regnum Vegetabile 138. Koeltz Scientific Books. Koenigstein, Germany).

The St Louis Code sets rules for the nomenclature of taxa of all sorts of plants. A selection of the rules most closely related to the process of registration of Populus cultivars is reported here.

1. Publication. The name of a taxon (e.g. of a species) must be published validly before use. The breeder proposing the registration of a new cultivar must make sure that he/she is using correct botanical names for the identification of the cultivar and its parentage. Reference to largely accepted classifications is strongly recommended (e.g. FAO. 1980. Poplars and Willows. Rome.; Zsuffa L. 1976. Proc. Can. Tree Impr. Ass. (28-30 Aug. 1973). pp. 107-123; Houtzagers G. 1937. Het Geslacht Populus in verbund met zijn Beteekenis voor de Houtteelt. Veenman & Zonen. Wageningen).

2. A genus may be subdivided into sections, which is the case for the genus Populus. The name of the section should be written in italics, with a capital initial letter. If the "type" referred to in the publication of the section name is the same as that used for the genus, the section takes up the same name of the genus (autonym). This was the case for P. alba L. that was used as a type for the genus Populus but also for white poplars and aspens which should thus be included in a section named Populus rather than Leuce. If a section must be mentioned in a botanical name, it should be placed in parentheses between the names of genus and species. Example: Populus (Sect. Aigeiros) fremontii Wats.

3. A "priority rule" states that, among names of a same taxon of the level of family or below, validly published after 1 May 1753, the earliest to be published has priority. According to this rule, P. deltoides Marsh. should replace P. deltoides Bartr. in use.

4. Infraspecific taxa (subspecies, variety), if any, shall be written in italics with small initial letter and preceded by the corresponding abbreviation (subsp., var.). Example: P. nigra var. thevestina. Names such as typica are not allowed: if the type used for the description of the species is the same as that used for a variety, the variety takes up the species name (autonym). Example: P. nigra var. nigra, not P. nigra var. typica.

5. Orthography. The following rule applies to taxon names but it is strongly recommended that it be followed also in cultivar epithets: diacritical signs are to be avoided; accents are omitted. Transliteration into the Latin alphabet (plus the letters W, Y and K) is mandatory for epithets derived from languages with different alphabets (e.g. Greek, Cyrillic, Chinese, Arabic, ...).

6. Nomenclature of hybrids. Whenever possible, hybrids should be designated by a formula specifying parentage. Parents are separated by the multiplication sign × or the small letter x if the former is not available. The order of the species may be alphabetical or, when the sex of parents is relevant, with the female first. Example: P. simonii × P. nigra means a P. simonii mother and a P. nigra father.

If a formula becomes impractical a hybrid taxon may receive a name; for poplars the case is relatively frequent for interspecific hybrids. The hybrid status is indicated by placing the multiplication sign x attached to the left of the taxon name. Example: P. ×interamericana. If the multiplication sign × is not available, it may be substituted by the small letter x, in which case a space is left between the x and the taxon name: Example: P. x interamericana.

The rules about valid publication of a description and priority apply to names of hybrid species. P. x euramericana for intermediate forms between P. deltoides and P. nigra is subject to criticism by botanists on the basis of priority: P. x canadensis should be used instead.

The name of a hybrid taxon between two or more known species applies to all intermediate forms, as far as the contribution of the parent taxa is recognized: F₁ hybrids, but also F₂, backcrosses etc.