Paeonia: classification and overview of peony species.

The latest and most widely accepted scientific classification (2021) was made by Hong De-Yuan.1 It revises his previous classification2, which was based on morphology, by adding insights from genetic analysis. Although this will be the most widely accepted classification, some scientists still disagree with some of his findings.3 The reason for this is twofold: morphological characters in paeonia are not discrete but rather continuous and paeonia shows large genetic variability.4

There are two other alternative classifications also shown here. The classification by Don Smith is based largely on genetics and groups the species according to their historical relationships.5 The last classification is our own grouping of species and includes some ‘other’ species not accepted or mentioned by Hong De-Yuan. The reason why we consider them separate species are listed with the individual species descriptions themselves. Our own classification will be a ‘dynamic’ classification as revisions deemed reasonable will be made from time to time to keep it up-to-date with new insights and/or discoveries.

Remark: still under construction!

Source: Hong, De-Yuan. “Peonies of the world. Part III: Phylogeny and evolution.” Kew Publishing: Royal Botanic Gardens Kew; 2021, pp. 215-222

The infrageneric subdivision of paeonia consists of: 2 subgenera and 7 sections with 34 species in all.
1A. Shrubs. Subgenus 1. Moutan 
2A. Flowers 2-4 in a cyme; disk fleshy, annular, enveloping only base of carpels; carpels always glabrous. Section 1A. Delavayanae 
2B. Flowers solitary and terminal; disk leathery or half-leathery, enveloping only carpels by half or entirely; carpels tomentose or glabrous. Section 1B. Moutan 
1B. Herbs. Subgenus 2. Paeonia
3A. Flowers 2-4 in a cyme or solitary but often with one or two undeveloped flower buds in axils; roots carrot-shaped. Section 2A. Albiflorae 
3B. Flowers solitary and terminal; roots fusiform-thickened or carrot-shaped.
4A. Leaves mostly with bristles along veins above; leaflets finely segmented; leaflets plus segments (9-)21-340 in number; roots fusiform-thickened. Section 2B. Paeonia
4B. Leaves always glabrous above; roots carrot-shaped or slightly fusiform-thickened.
5A. Petals nearly as long as sepals (slighly longer or shorter); stems often branched; roots slighly fusiform-thickened. Section 2E. Onaepia
5B. Petals much longer (>2 times) than sepals; stems simple
6A. Lower leaves always biternate, with 9 leaflets; leaflets entire; carpels mostly 2 or 3, rarely 1, 4 or 5. Section 2C. Obovatae
6B. Lower leaves biternate, with 9, rarely 8 or 7 leaflets; leaflets segmented or entire; leaflets plus segments 9-95, very rarely 8 or 7; carpels 2-5, rarely 6, 1, 7-10. Section 2D Corallinae 
Subgenus 1. Moutan
Shrubs. Flowers in a cyme, or solitary and terminal on a branch. Disk fleshy, annular, dentate, or leathery and enveloping carpels half to entirely. The subgenus consists of nine wild species all endemic to China and one cultivated hybrid species, Paeonia x suffruticosa, which is now cultivated almost worldwide. Two sections are easily recognized as follows.
Section 1A. Delavayanae
Shrubs up to 3.5 m tall. Flowers 2-4 in a cyme. Disk fleshy, dentate, annular, enveloping only the base of the carpels; carpels 1-5 in number.
Included species (2):
Paeonia ludlowii
Paeonia delavayi
Distribution: endemic to SW China: SE Tibet, W Sichuan and Yunnan.
Section 1B. Moutan
Shrubs up to 2 m tall. Flowers solitary on top of branches, not in a cyme. Disk leathery, enveloping carpels by half or entirely until mid-anthesis. Carpels mostly 5, less often 3 or 4 in number.
Included species (7):
Paeonia decomposita
Paeonia rotundiloba
Paeonia rockii
Paeonia ostii
Paeonia jishanensis
Paeonia qiui
Paeonia cathayana
Distribution: endemic to C China: N & W Sichuan, SE Gansu, Shaanxi, Hubei, Henan, SW Shanxi and C Anhui
Subgenus 2. Paeonia
Section 2A. Albiflorae
Roots more or less carrot-shaped. Leaves glabrous or with bristles along veins on the upper surface. Flowers usually several per stem, in a cyme, rarely solitary, or solitary but often with undeveloped (sterile) flower buds at axils; sepals mostly caudate at the apex. Almost all diploid with 2n=10 (P. emodi with a tetraploid population, 2n=20, in S Xizang (Tibet).
Included species (5):
Paeonia anomala
Paeonia emodi
Paeonia lactiflora
Paeonia sterniana
Paeonia veitchii
Distribution: all the species in Paeonia sect. Albiflorae are found in Asia, but with the range of P. anomala extending into north-eastern Europe (Kola of Russia)
Section 2B. Paeonia
Lateral roots fusiform or tuberous. Lower leaves biternate or triternate; leaflets nearly always segmented with leaflets/leaf segments (9-)21-340 in number; leaves mostly with bristles along veins above. Flowers solitary and terminal; sepals mostly rounded at apex. Mostly tetraploid (diploid only in P. intermedia and P. tenuifolia).
Included species (7)
Paeonia arietina
Paeonia intermedia
Paeonia officinalis
Paeonia parnassica
Paeonia peregrina
Paeonia saueri
Paeonia tenuifolia
Distribution: from Middle Asia to the northern Mediterranean.
Section 2C. Obovatae
Roots carrot-shaped. Lower leaves biternate; leaflets 9, entire, or segmented, forming 13-24 leaflets/segments, always glabrous above. Flowers always solitary and terminal; sepals mostly rounded at the apex, but sometimes one caudate; carpels mostly 2 or 3, rarely 1, 4 or 5, glabrous or yellow papillate to hispidulous. One diploid and one tetraploid.
Included species (2)
Paeonia obovata
Paeonia mairei
Distribution: E Asia: Jpan, Korean peninsula, the Far East of Russia, and NE, E, C and SW China, with its westernmost part in the eastern edge of the Pan-Himalaya.
Section 2D. Corallinae
Roots carrot-shaped. Lower leaves biternate; leaflets/leaf segments usually numbering 9 or more but fewer than 21 (up to 32 in P. broteri and 95 in P. clusii); leaves always glabrous above. Flowers always solitary and terminal; sepals mostly rounded at the apex. Both diploid and tetraploid with 2n=10 and 2n=20.
Included species (9)
Paeonia algeriensis
Paeonia broteri
Paeonia cambessedesii
Paeonia clusii
Paeonia coriacea
Paeonia corsica
Paeonia daurica
Paeonia kesrouanensis
Paeonia mascula
Distribution: S Europe, NW Africa and W Asia (From the western Mediterranean to Iran).
Section 2E. Onaepia
Perennials. Lateral roots slightly fusiform. Stems often branched. Lower leaves ternate or biternate. Flowers solitary and 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.
Included species (2)
Paeonia brownii
Paeonia californica
Distribution: the section is endemic to western (Pacific) North America: the USA and Mexico.
1a. Shrubs; disk fleshy and annular, enveloping only base of carpels or half-leathery, enveloping half to whole carpels until mid-anthesis (Subgenus Moutan)
2a. Flowers usually 2 – 4 in a cyme, more or less pendent; disk fleshy, enveloping only base of carpels; carpels always glabrous (Section Delavayanae)
3a. Carpels nearly always single, rarely 2; follicles 4.7 – 7 cm long, 2 – 3.3 cm in diameter; petals, filaments and stigmas always purely yellow Paeonia ludlowii
3b. Carpels usually 2 – 5 (- 7); follicles less than 4 cm long, 1.5 cm in diameter; petals, filaments and stigmas often not purely yellow Paeonia delavayi
2b. Flowers solitary, upright; disk half-leathery, enveloping carpels halfway or completly until mid-anthesis; carpels tomentose or glabrous (Section Moutan)
4a. Carpels glabrous, 2 – 5 in number; disk enveloping carpels halfway or up to base of styles until mid-anthesis; lower leaves decompound with (20 -) 25 – 54 (- 71) leaflets; leaflets all lobed.
5a. Carpels 5, very occasionally 4, in number; disk enveloping ovary up to halfway at anthesis; leaflets of lower leaves (35 -) 37 – 54 (- 71) in number, terminal leaflet elliptic or narrow-rhomboid, with length/width ratio (1.46 -) 1.62 – 2.18 (- 2.55) Paeonia decomposita
5b. Carpels mostly 3, less often 4, 2 or 5 in number; disk enveloping whole ovary at anthesis; leaflets of lower leaves (20 -) 25 – 37 (-49) in number, terminal leaflet orbicular or wide-rhomboid, with length/width ratio (1.02 -) 1.03 – 1.57 (- 2.20) Paeonia rotundiloba
4b. Carpels densely lanate or tomentose, 5 (- 7) in number; disk completely enveloping carpels until mid-anthesis; lower leaves biternate, biternate-pinnate or ternate-bipinnate, with leaflets usually fewer than 20 (- 33), if more, at least some of them entire
6a. Lower leaves with 9 leaflets; leaflets ovate or ovate-orbicular, only terminal leaflets usually 3-lobed, often reddish above; petals often with a reddish blotch at base Paeonia qiui
6b. Lower leaves with more than 9 leaflets, if 9, then leaflets mostly lobed; leaflets green above; petals without a blotch, or with a large, dark purple blotch at base
7a. Lower leaves with 11-33 leaflets; leaflets usually ovate to lanceolate, mostly entire, less frequently ovate-orbicular and mostly lobed
8a. Lower leaves with 11-15 leaflets; leaflets ovate tot ovate-lanceolate, mostly entire; petals white, rarely pale pink, without a blotch Paeonia ostii
8b. Lower leaves with (17 -) 19 -33 leaflets; leaflets 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 base Paeonia rockii
7b. Lower leaves with 9 (occasionally 11 or 15 in P. jishanensis) leaflets; leaflets ovate, ovate or nearly orbicular, mostly or all lobed
9a. Leaflets 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; leaf blade glabrous on lower surface; sepals all caudate or mucronate Paeonia cathayana
9b. Leaflets ovate-orbicular to orbicular, all 3-cleft; segments lobed, acute to rounded at apex; leaf blade villose along veins on lower surface; sepals all rounded at apex Paeonia jishanensis
1b. Herbs; disk fleshy and short, enveloping only base of carpels (Subgenus Paeonia)
10a. Petals nearly equal in size to, or smaller than, sepals; disk dentate, almost interrupted; lower leaves ternate or biternate, with 3 or 9 leaflets; lateral root slightly fusiform. (Section Onaepia)
11a. Lower leaves biternate, with 55 – 110 leaf segments and final lobes; segment or lobes acute to rounded at apex; carpels usually 5, rarely 3, 4 or 6 in number; sepals exceeding petals Paeonia brownii
11b. Lower leaves ternate, with 30 – 78 leaf segments and final lobes; segments or lobes mostly acute at apex; carpels usually 3, rarely 2 or 4 in number; sepals slightly smaller than, or subequal to, petals Paeonia californica
10b. Petals much larger than sepals; disk annular, waved or flat; lower leaves biternate or triternate, with 9 or more leaflets; lateral roots carrot-shaped or fusiform
12a. Flowers 2 – 4 in a cyme, less frequently solitary but with 1 – 2 undeveloped flower buds at axils, rarely simply solitary; sepals mostly caudate at apex; leaf blade usually with bristles along veins above (Section Albiflorae)
13a. Leaf blade cartilaginous and denticulate on margin, pubescent along veins or glabrous beneath; carpels glabrous, very rarely hairy Paeonia lactiflora
13b. Leaf blade smooth on margin, glabrous beneath; carpels glabrous or hairy
14a. Lower leaves with 70 – 140 leaflets/leaf segments; petals red, rarely pale rose, very occasionally white
15a. Plants (60 -) 66 – 103 (- 120) cm tall; flowers usually simply solitary, less frequently solitary but with 1 or 2 undeveloped flower buds, very rarely 2; carpels mostly 5, less frequently 4, rarely 3 Paeonia anomala
15b. Plants (24 -) 34 – 59 (-71) cm tall; flowers mostly 2 or 3, less frequently 4 or solitary but with 1 or 2 undeveloped flower buds, occasionally simply solitary; carpels mostly 3 or 2, rarely 4 or 1 Paeonia veitchii
14b. Lower leaves with fewer than 30 leaflets or leaf segments; petals white, rarely pale rose
16a. Carpels 1, rarely 2 in number, mostly tomentose, less frequently glabrous; flowers often several per stem Paeonia emodi
16b. 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 Paeonia sterniana
12b. Flowers always solitary and terminal; sepals mostly rounded at apex; leaf blade glabrous or with bristles along veins above
17a. Tap roots not thickened, lateral roots always fusiform; lower leaves with mostly more than 20, rarely fewer, leaflets or leaf segments; leaf blade covered with bristles along veins above, or, if glabrous above, nearly always villose beneath (Section Paeonia)
18a. Stems mostly hirsute; sepals mostly hairy on abaxial side; leaflet blade always glabrous or rarely villose at base above, more or less villose beneath
19a. Lower leaves with mostly more than 20, very rarely as few as 11, leaflets or leaf segments; leaflet blade linear-elliptic or lanceolate; sepals hispidulous or glabrous on abaxial side Paeonia officinalis
19b. Lower leaves with mostly fewer than 20, occasionally up to 32,leaflets or leaf segments; leaflet blade elliptic, oblong or ovate-lanceolate; sepals densely villose on abaxial side
20a. Flowers rose to red; anthers yellow; lower leaves with 11 – 25, occasionally up to 32, leaflets or leaf segments Paeonia arietina
20b. Flowers dark purple; anthers orange; lower leaves with 9 – 15, occasionally up to 25, leaflets or leaf segments. Paeonia parnassica
18b. Stems glabrous; sepals always glabrous; leaflet blade mostly with bristles along veins above
21a. Leaflets or leaf segments always dentate-lobed, with lobes less than 1 cm long; stigmas yellow or pale pink Paeonia peregrina
21b. Leaflets or leaf segments entire or rarely deeply lobed; stigmas red
22a. Lower leaves with 19 – 45 leaflets or leaf segments; leaf blade hispidulous beneath Paeonia saueri
22b. Lower leaves with more than 70 leaflets or leaf segments; leaf blade always glabrous beneath
23a. Leaflets or leaf segments of lower leaves 130 – 340 in number, mostly filiform, 0.5 – 8 mm wide Paeonia tenuifolia
23b. Leaflets or leaf segments of lower leaves 70 – 100 in number, linear, 4 – 18 mm wide Paeonia intermedia
17b. Tap roots carrot-shaped; lower leaves with no more than 24 (only in P. broteri up to 32 and in P. clusii 23 – 95) leaflets or leaf segments; leaf blade always glabrous above
24a. Leaflets 9 or 13 – 24 (including leaf segments); carpels mostly 2 or 3, rarely 1, 4, or 5 (Section Obovatae)
25a. Leaflets 9, entire; carpels glabrous Paeonia obovata
25b. Leaflets mostly segmented, forming 13 – 24 leaflets or segments; carpels yellow papillate to hispidulous, rarely glabrous Paeonia mairei
24b. Leaflets 9, rarely 8 or 7, or leaflets/leaf segments up to 95; carpels 2 – 5, rarely 6, 1, 7 – 10 (Section Corallinae)
26a. Leaflets plus leaf segments of lower leaves 23 – 95 in number, linear to ovate Paeonia clusii
26b. Leaflets plus leaf segments of lower leaves less than 21 (up to 32 in P. broteri), broad elliptic to obovate
27a. Lower leaves with 9 or fewer leaflets; carpels always glabrous Paeonia cambessedesii
27b. Lower leaves with usually 10 – 15, less frequently 9 or more, leaflets plus leaf segments; carpels mostly tomentose
28a. Carpels 1, less frequently 2, nearly always glabrous, very occasionally sparsely hairy; follicles columnar, 4 – 5.4 cm long Paeonia algeriensis
28b. Carpels (1 -) 2 – 4, rarely more, tomentose or glabrous; follicles long-ovoid or ellipsoid, up to 4 cm long
Less indentation here for readability purposes! All that follows should be a level below 28b.29a. Styles 1.5 – 3.5 mm long; lower leaves with 11 – 14, rarely up to 17, leaflets or leaf segments; leaf blade densely or sparsely villose beneath; carpels always glabrous Paeonia kesrouanensis
29b. Styles usually absent (only in P. corsica 1.5 – 3 mm long); lower leaves with 9 – 20 leaflets or leaf segments; leaf blade glabrous, sparsely hispid, puberulent or villose beneath; carpels tomentose or glabrous
30a. Carpels always lanate or tomentose with hairs 2 – 3 mm long; styles absent
31a. Leaflets usually entire; lower leaves with 9, rarely 10, very occasionally 11, leaflets; leaflet blade usually ovate, rounded, nearly truncate with a mucro, or rounded, less frequently acute at apex Paeonia daurica
31b. Leaflets at least some segmented; lower leaves with usually 10 or more, very occasionally 9, leaflets or leaf segments; leaflet blade usually acute at apex
32a. Lower leaves with mostly (11 -) 15 – 21, rarely up to 32, leaflets or leaf segments; leaflets 4 – 10 (- 15) cm long, 1.5 – 5 (- 6.5) cm wide, always glabrous; carpels with hairs 2 mm long Paeonia broteri
32b. Lower leaves with mostly (9 -) 11 – 15, rarely up to 21 leaflets or leaf segments; leaflets 9 – 18 cm long, 4.5 – 9 cm wide, sparsely hispid or glabrous; carpels with hairs 3 mm long Paeonia mascula
30b. Carpels glabrous or tomentose with hairs 1.5 mm long; styles present or absent
33a. Carpels tomentose, rarely glabrous; lower leaves with usually 9, rarely up to 20 leaflets/leaf segments; leaf blade holosericeous beneath; styles 1.5 – 3 mm long Paeonia corsica
33b. Carpels glabrous, very occasionally sparsely hairy; lower leaves with 10 – 15 leaflets/leaf segments; leaflet blade glabrous, very occasionally sparsely hairy (hairy leaves always accompanied by hairy carpels); styles lacking Paeonia coriacea
Source: Hong, De-Yuan. “Peonies of the world. Part III: Phylogeny and evolution.” Kew Publishing: Royal Botanic Gardens Kew; 2021, pp. 219-222
Paeonia classificationClade 1Clade 2
Non-hybrid speciestenuifoliaintermediaveitchiiExtinct #1lactifloraExtinct #2mairei
Hybrid intra-clade species
peregrina1/42/41/4
officinalis3/82/83/8
arietina, humilis, parnassica1/21/2
anomala3/41/4
emodi1/21/2
Hybrid inter-clade species
wittmanniana1/41/41/41/4
obovata1/41/42/4
clusii, mascula group1/21/2
banatica1/41/42/4
sterniana1/41/42/4
cambessedesii, corsica1/21/2

We are far from concluding this, so whilst it is unfinished, here are some of the (sub)species unrecognized by Hong:

Paeonia wendelboi
Paeonia archibaldii
Paeonia flavescens
Paeonia arietina ssp arasicola
Paeonia hybrida
Paeonia sandrae
Paeonia turcica

A/ P. jishanensis B/ P. decomposita C/ P. rockii D/ P. ostii E/ P. qiui F/ P. delavayi G/ P. ludlowii H/ P. lactiflora I./ P. mairei J/ P. sterniana K/ P. anomala L/ P. obovata. Image from: Zhang, X., Zhai, Y., Yuan, J. et al. "New insights into Paeoniaceae used as medicinal plants in China." Scientific Reports, 2019, vol 9, article 18469, p. 2
Footnotes:
  1. Hong, De-Yuan. “Peonies of the world. Part III: Phylogeny and evolution.” Kew Publishing: Royal Botanic Gardens Kew; 2021, pp. 215-222[]
  2. Hong De-Yuan. “Peonies of the World. Polymorphism and diversity.” Royal Botanic Gardens: Kew, 2011, pp. XII-XV[]
  3. Punina, E.O.; Machs, E.M.; Krapivskaya, E.E.; Kim, E.S.; Mordak, E.V. Myakoshina Y.A. & A.V. Rodionov. “Interspecific hybridization in the genus Paeonia (Paeoniaceae): Polymorphic sites in transcribed spacers of the 45S  rRNA genes as indicators of natural and artificial peony hybrids.” In: Russian Journal of Genetics, 2012, vol 48, nr 7, pp. 684-697[]
  4. Degtjareva, G.V.; Efimov, S.V.; Terentieva, E.I.; Samigullin, T.H. & C.M. Vallejo-Roman. “The problem of species in the genus Paeonia.” In: Skortsovia, 2020, vol 6, nr 2, pp. 26-27[]
  5. Smith, Donald R. “A new ‘unified’ classification for the genus paeonia”. In: American Peony Society Bulletin, 2021, nr  398, pp. 25-40, and nr 399, pp. 33-37[]

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