This genus exhibits, perhaps more acutely than any other, the taxonomic difficulty which arise from long-established cultivation, hybridization and selection, and subsequent escape and naturarilisation. For many populations, especially in S. France and N.W. Italy, it is impossible to say with confidence whether they are native or naturalized; and nearly all the early binomials are based on cultivated plants of which the origin was only guessed at. This not only gives rise to nomenclatural problems, but it means also that the geographical background for specific delimination is confused and defective.
Cytological investigations has helped greatly in establishing the hybrid nature of taxa formely regarded as independent species. Of the innummerable hybrids in cultivation a large number have become naturalized, at least temporarily, in a few locaties (espacially in places were plants of the genus are grown on a commercial scale). Only those hybrids which are long-established, widespread, not too variable, and of agreed parentage are included below.
Species and images:
