Subdivision of the genus Paeonia((Hong, De-Yuan. “Peonies of the World. Taxonomy and phytogeography.” Kew: Royal Botanic Gardens, 2010, pp. 50-55.))
Paeonia divides naturally into three groups, as treated by Lynch (1890), Stebbins (1939) and Stern (1946). Morphologically, the three are easily recognised (see the following key to sections). This division is now supported by molecular trees based on sequences of internal transcribed spacers (ITS) (nrDNA), the nuclear Adh gene family, mat K (a coding region of cpDNA) and trn H (an intergenic spacer of cpDNA). All show that the genus Paeonia comprises three major groups: 1) woody peonies; 2) herbaceous peonies in the New World; and 3) herbaceous peonies in the Old World (Sang et al., 1995, 1997, 1997). Sectional rank was proposed for these groups by de Candolle (1824) and accepted by Salm-Dyck (1834), Stern (1946), and Uspenskaya (1987); subgeneric rank was proposed by Seringe (1849) and accepted by Lynch (1890), Stebbins (1939) and Halda (1997, 2004). Since the genus Paeonia comprises only 32 species according to our account, subdivision into three infrageneric levels, including either subgenera or series, as recognised by Uspenskaya (1987) (section, subsection and series) or by Halda (2004) (subgenus, section and subsection), is too ﬁne a distinction to support. Therefore, we here adopt a two-ranked system for Paeonia with the two lower ranks of section and subsection. At the sectional level, we recognise three sections in Paeonia: sect. Moutan DC. (1824), sect. Onaepia Lindl. (1839) and sect. Paeonia.
Key to sections in Paeonia
1a. Shrubs; disk leathery and halfway to wholly enveloping carpels until mid—anthesis, or fleshy and short, enveloping only the base of carpels: sect. Moutan DC.
1b. Herbaceous perennials; disk fleshy and short, enveloping only base of carpels
2a. Petals nearly equal in size to or smaller than sepals; disk dentate, almost interrupted; lower leaves ternate or biternate, with leaflets 3 or 9, lateral roots slightly fusiform: sect. Onaepia Lindl.
2b. Petals much larger than sepals; disk annular, waved or flat; lower leaves biternate or triternate, with leaflets 9 or more; lateral roots carrot—shaped, fusiform or tuberous: sect. Paeonia
Paeonia sect. Onaepia
Perennials. Lateral roots slightly fusifonn. Stems often branched. Lower leaves ternate or biternate. Flowers often several, terminal on main stem and branches; petals nearly equal in size to, or smaller than, sepals; disk dentate, almost interrupted, enveloping the base of the carpels until mid-anthesis; carpels 2—5 in number, always glabrous. Diploid with 2n = 10.
Paeonia sect. Onaepia is endemic to western (Paciﬁc) North America in the USA and Mexico