How to identify your peony species?

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
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
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
2A. Flowers usually 2-4 in a cyme, more or less pendent; disk fleshy, enveloping only the base of the carpels; carpels always glabrous: subsect. Delavayanae
2B. Flowers solitary, erect; disk leathery, enveloping carpels halfway or completely until mid- anthesis; carpels tomentose or glabrous: subsect. Vaginatae

3A. Carpels usually 2-5(-7); follicles less than 4 cm long, 1.5 cm in diameter; petals, filaments and stigmas often not purely yellow: P. delavayi

3B. Carpels nearly always single, rarely 2; follicles 4.7-7 cm long, 2-3.3 cm in diameter; petals, filaments and stigmas always yellow: P. ludlowii

3A. Carpels glabrous, 2-5; disk enveloping carpels halfway or up to the base of the styles until mid-anthesis; lower leaves decompound with leaflets (29-)33-63 in number, all lobed
4A. Carpels almost always 5, rarely 4 or 3; disk enveloping carpels halfway at anthesis; leaflets 35-63 in number, elliptic to narrowly rhomboid: P. decomposita
4B. Carpels mostly 3, less often 4 or 2, rarely 5; disk enveloping carpels up to base of styles at anthesis; leaflets (19-)25-35(-39) in number, rhomboid to nearly orbicular: P. rotundiloba
3B. Carpels densely lanate or tomentose, 5(-7); disk completely enveloping carpels until mid-anthesis; lower leaves biternate, biternate-pinnate or ternate-bipinnate; leaflets usually fewer than 20(-33) in number, if more at least some of them entire
4A. Lower leaves biternate; leaflets 9 in number, very occasionally 11 or 15 in P. jishanensis
5A. Leaflets ovate or ovate-orbicular, mostly entire, often reddish above; petals often with a reddish blotch at the base: P. qiui
5B. Leaflets oval, ovate or nearly orbicular, mostly or all lobed; green above; petals without a blotch at the base
 6A. Leaflets oval or ovate, terminal leaflets 3- or 5-cleft, with additional 1 to several lobes, lateral leaflets mostly 2- or 3-lobed, less frequently entire; lobes acute at apex; leaves glabrous on lower surface; sepals all caudate or mucronate: P. cathayana
6B. Leaflets ovate-orbicular to orbicular, all 3-cleft; segments lobed, acute to rounded at apex; leaves villose along veins on lower surface; sepals all rounded at apex: P. jishanensis
 4B. Lower leaves ternate-pinnate, ternate-bipinnate or biternate-pinnate; leaflets more than 9 in number, usually oval to lanceolate, mostly entire, less frequently oval-orbicular and mostly lobed
5A. Lower leaves ternate-pinnate; leaflets no more than 15 in number, ovate to ovate-lanceolate, mostly entire; petals white, rarely pale pink, without a blotch: P. ostii
5B. Lower leaves ternate-bipinnate (rarely biternate-pinnate); leaflets (17-)19-33 in number, lanceolate or ovate-lanceolate and mostly entire, or ovate to ovate-orbicular and mostly lobed; petals white, rarely red, always with a large, dark purple blotch at the base: P. rockii

3A. Lower leaves biternate; leaf segments and final lobes 55-110 in number, acute to rounded at apex; carpels usually 5, rarely 3, 4 or 6 in number; sepals exceeding petals: P. brownii

3B. Lower leaves ternate; leaf segments and final lobes 30-78 in number, mostly acute at apex; carpels usually 3, rarely 2 or 4; sepals slightly smaller than, or subequal to, petals: P. californica

3A. Flowers several to a stem, less frequently solitary but with 1-2 undeveloped flower buds at axils, rarely really solitary; sepals mostly caudate at apex; leaves usually with bristles along veins above: subsect. Albiflorae
3B. Flowers always solitary and terminal; sepals mostly rounded at apex; leaves glabrous or with bristles along veins above.
4A. Lateral roots always tuberous; leaflets or leaf segments of lower leaves mostly more than 20 in number, rarely fewer; leaves covered with bristles along veins above, or, if glabrous above, nearly always villose beneath: subsect. Paeonia
4B. All roots carrot-shaped; leaflets or leaf segments of lower leaves numbering up to 21 (only in P. broteri up to 32 and in P. clusii 23—95); leaves always glabrous above: subsect. Foliolatae
5A. Leaves cartilaginous and denticulate on margin, pubescent along veins or glabrous beneath; carpels glabrous, very rarely hairy: P. lactiflora
5B. Leaves smooth on margin, glabrous beneath; carpels glabrous or hairy
6A. Leaflets or leaf segments of lower leaves 70-100 in number: P. anomala
6B. Leaflets or leaf segments of lower leaves fewer than 30 in number
7A. Carpels 1, rarely 2 in number, mostly tomentose, less frequently glabrous; flowers often several per stem: P. emodi
7B. Carpels mostly 2 or 3 in number, rarely 4, always glabrous; flowers nearly always solitary, rarely 2 in number, but sometimes with 1-2 undeveloped flower buds at axils: P. sterniana
5A. Stems mostly hirsute; sepals mostly hairy on abaxial side; leaves always glabrous or rarely villose at base above, more or less villose beneath
6A. Leaflets or leaf segments of lower leaves mostly more than 20, very rarely as few as 11 in number, linear-elliptic or lanceolate; sepals hispidulous or glabrous on abaxial side: P. officinalis
6B. Leaflets or leaf segments of lower leaves mostly less than 20, occasionally up to 32 in number; elliptic, oblong or ovate-lanceolate; sepals densely villose on abaxial side
7A. Flowers rose to red; anthers yellow; leaflets or leaf segments of lower leaves 11-25, occasionally up to 32 in number: P. arietina
7B. Flowers dark purple; anthers orange; leaflets or leaf segments of lower leaves 9-15, occasionally up to 25 in number: P. parnassica
5B. Stems glabrous; sepals always glabrous; leaves mostly with bristles along veins above
6A. Leaflets or leaf segments always dentate-lobed, with lobes less than 1 cm long; stigmas yellow or pale pink: P. peregrina
6B. Leaflets or leaf segments entire or rarely deeply lobed; stigmas red
7A. Leaflets or leaf segments of lower leaves 19-45 in number; leaves hispidulous beneath: P. saueri
7B. Leaflets or leaf segments of lower leaves more than 70 in number; leaves always glabrous beneath
8A. Leaflets or leaf segments of lower leaves 130-340 in number, mostly filiform, 0.5—8 mm wide: P. tenuifolia
8B. Leaflets or leaf segments of lower leaves 70-100 in number, linear, 4-18 mm wide: P. intermedia
5A. Leaflets or leaf segments of lower leaves 23-95 in number, linear to ovate: P. clusii
5B. Leaflets or leaf segments of lower leaves numbering less than 21 (up to 32 in P. broteri), broad elliptic to obovate
6A. Leaflets or leaf segments acuminate to caudate-acuminate at apex; carpels densely brown-papillose or hispidulous, rarely glabrous: P. mairei
6B. Leaflets or leaf segments acute to mucronate at apex; carpels tomentose or glabrous
7A. Lower leaves with 9 or fewer leaflets; carpels always glabrous
8A. Carpels 3-10, mostly 4-6 in number; plants purple-red and glabrous throughout: P. cambessedesii
8B. Carpels mostly 2 or 3, rarely 1, 4 or 5 in number; plants usually green; leaves more or less hirsute beneath: P. obovata
7B. Lower leaves usually with 10-15 leaflets or segments, less frequently 9 or more; carpels mostly tomentose
8A. Carpels 1, less frequently 2 in number, nearly always glabrous, very occasionally sparsely hairy; follicles columnar, 4-5.4 cm long: P. algeriensis
8B. Carpels (1-)2-4 in number, rarely more, mostly tomentose; follicles long-ovoid or ellipsoid, up to 4 cm long
9A. Styles 1.5-3.5 mm long; leaflets or leaf segments of lower leaves 11-14, rarely up to 17 in number, densely or sparsely villose beneath; carpels always glabrous: P. kesrouanensis
9B. Styles usually absent (only in P. corsica 1.5-3 mm long); leaflets/leaf segments of lower leaves 9-20 in number, glabrous, sparsely hispid, puberulent or villose beneath; carpels tomentose or glabrous
10A. Carpels always lanate or tomentose with hairs 2-3 mm long; styles absent
11A. Leaflets usually entire, leaflets or leaf segments of lower leaves 9, rarely 10, very occasionally 11 in number, usually ovate, rounded, nearly truncate with a mucro, or rounded, less frequently acute at apex: P. daurica
11B. Leaflets at least some segmented, leaflets or leaf segments of lower leaves usually numbering 10 or more, very occasionally 9, usually acute at apex
12A. Leaflets or leaf segments of lower leaves mostly (11-)15-21, rarely up to 32 in number, 4-10(-15) cm long, 1.5-5(-6.5) cm wide; leaves always glabrous; hairs on carpels 2 mm long: P. broteri
12B. Leaflets or leaf segments of lower leaves mostly (9-)11-15, rarely up to 21 in number, 9-18 cm long, 4.5-9 cm wide; leaves sparsely hispid or glabrous; hairs on carpels 3 mm long: P. mascula
10B. Carpels glabrous or tomentose with hairs 1.5 mm long; styles present or absent
11A. Carpels tomentose, rarely glabrous; leaflets or leaf segments of lower leaves usually 9, rarely up to 20 in number, holosericeous beneath; styles 1.5-3 mm long: P. corsica
11B. Carpels glabrous, very occasionally sparsely hairy; leaflets or leaf segments of lower leaves 10-15 in number, glabrous, very occasionally sparsely hairy (hairy leaves always accompanied by hairy carpels); styles lacking: P. coriacea

Source: “Key to species.” In: Hong De-Yuan. “Peonies of the World. Polymorphism and diversity.” Royal Botanic Gardens: Kew, 2011, pp. XII-XV.

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