It is not easy to clearly distinguish the Petunia genus from that of Calibrachoa. Plant breeders have created a vast assortment of varieties from their hybridization and selection programs but seed companies and especially young plant propagators tend to mystify the taxonomy of the new hybrids. In part this is driven by marketing considerations to create and promote brand names rather than botanical names.

With regard to Calibrachoa there appears to be relatively little taxonomic information in circulation. For those readers interested, the following paper makes a good start. "Leaf Structure and Taxonomy of Petunia and Calibrachoa (Solanaceae)"; REIS, Claudia dos, SAJO, Maria das Graças and STEHMANN, João Renato; Brazilian Archives of Biology and Technology; Volume 45, no.1. Print ISSN 1516-8913 page 59-66.

The authors studied the leaf anatomy of sixteen species of Calibrachoa and eight species of Petunia to separate the two genera and to charcaterize the various Calibrachoa species. Both belong to the Solanaceae family, important for countless ornamental, medicinal and nutritious species. Petunia and Calibrachoa are both native of South America. Petunia has a ploidy (chromosome) level n=7 and a seed coat formed by cells with wavy walls. Most species are herbaceous and annual. Garden hybrids originate from crosses between Petunia axillaris x P. integrifolia. Calibrachoa has a ploidy level of n=9 and the cell walls that make up the seed coat are straight. Calibrachoa species are predominantly sub-arbustive and perennial and ornamental varieties are derived essentially from Calibrachoa parviflora but the large number of species demonstrate considerable variation, especially in vegetative charcateristics, and provide breeders with a lot of future scope. The paper provides a full bibliography and list of species. Hyperlink:http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516 89132002000100010&lng=en&nrm=iso