This has been treated at times as a monospecific genus or a number of different species within the genus. It is extremely variable and many subordinate taxa have been described (e.g. TROPICOS lists 199 synonyms for this name). TROPICOS lists this taxon as P. communis Trin. by which name it was formerly known in the UK. There is a degree of uncertainty over the taxonomic status of this species, some authorities consider that there is only species worldwide (e.g. T.A. Cope pers. comm.), however, a number of taxa are generally recognized (e.g. The Plant List). As a consequence of this uncertainty, it is very difficult to establish the true distribution of the various taxa. This assessment uses a combination of the World Checklist of Selected Plant Families and the Invasive Species Compendium (CABI International) to derive a country list, however, it is not certain that this list is comprehensive.