Bull. Natl. Mus. Nat. Sci., Ser. B, 40(4), pp. 139–144, November 22, 2014

Observations on Variation in the Ornamental Flowers ofHydrangea macrophylla (Thunb.) Ser. (Hydrangeaceae)
Hideaki Ohba1* and Shinobu Akiyama2
1

Department of Botany, the University Museum, the University of Tokyo,
Hongo 7–3–1, Tokyo 113–0033, Japan2
Department of Botany, National Museum of Nature and Science,
Amakubo 4–1–1, Tsukuba, Ibaraki 305–0005, Japan* E-mail: ohba@um.u-tokyo.ac.jp(Received 14 August 2014; accepted 24 September 2014)

Abstract Variation in the ornamental ﬂowers of Hydrangea macrophylla (Thunb.) Ser. in wild
populations at Jogasaki on the east coast of the Izu Peninsula, Honshu, Japan, is reported. Formamacrophylla, which was known only in cultivation, was found growing in the wild. Two forms
with peculiar features are described as new forms, f. isobue and f. isonotaki.
Key words : Hydrangea, Izu, Japanese ﬂora, ornamental ﬂowers.

Introduction
Hydrangea macrophylla (Thunb.) Ser. f. macrophylla has been cultivated as an ornamental
plants since at least the Edo period in Japan andabroad. Hydrangea macrophylla f. normalis (E.
H.Wilson) H.Hara, also cultivated for a long time,occurs naturally on the Boso, Miura, and Izu peninsulas on the Paciﬁc Ocean side of central Honshu, and also in the Izu Islands and the Volcano
Islands, south of Tokyo. Siebold (1828) andSiebold and Zuccarini (1839) described several
species allied to H. macrophylla cultivated inJapan. Variations in ﬂowers and leaves have been
known in the plants under cultivation since
Sieboldʼs time, but the range of variation in wild
populations of f. normalis is extremely narrow(Sugimoto, 1926; Ohba, 2001). Recently, a number of forms showing diversity in the sepals of theornamental ﬂowers have been named as cultivars
by Mallet (2008, 2009), Arai (2012), and others.
Mallet visited the Izu Peninsula and collectedwild plants of H. macrophylla at Jogasaki withthe cooperation of Mr. Hirasawa (www.izu.fm/hydrangea/j-izu.html). Mallet (2008) mentionedthat some cultivars named by her were growing
wild at Jogasaki. Various forms of H. macrophylla occur there on steep rocky slopes undertrees and on cliffs near the sea (Hirasawa, www.
izu.fm/hydrangea/j-izu.html).
Thus, thinking that Hydrangea macrophylla isa species with a wide range of variation in thesepals of the ornamental ﬂowers, we observed
variation in the populations in the Boso and Izu
peninsulas in 2013 and 2014.
Results and Discussion
A large number of wild populations of H. macrophylla occur near the sea on the Boso and Izu
peninsulas mainly in sunny place (Fig. 1: A, B)
and rarely in shady place (Fig. 1: C). The populations in the Boso Peninsula are rather constantand stable in the shape of their ornamental ﬂowers, but not in color. The color of the sepals of theornamental ﬂowers are white, pink or pale blue,
but we found no ﬂowers with deep blue sepals.
In the Izu Peninsula we encountered several
individuals with features represented by f. macrophylla and also double sepaled forms at the140

Hideaki Ohba and Shinobu Akiyama

Fig. 1. A. Hydrangea macrophylla f. normalis in sunny place near the sea on the Boso Peninsula with Pueraria
lobata. B. Hydrangea macrophylla f. normalis in sunny place near the sea on the Boso Peninsula. C. Hydrangea macrophylla f. normalis in shady place on the Boso Peninsula. D. Hydrangea macrophylla f. macrophylla(lower left, purple ﬂowers) at the margin of a coastal forest at Jogasaki on the Izu Peninsula.

margin of a coastal forest at Jogasaki (Figs. 1: D,2: A, B). Moreover, we observed a considerable
number of individuals with peculiar features inplants transplanted from Jogasaki to Hirasawaʼsnursery (Fig. 2: C–F). Our ﬁndings indicate thatthe sepals of the ornamental ﬂowers are
extremely variable in wild populations on the Izu
Peninsula, but not so on the Boso Peninsula. The
difference between the two peninsulas is signiﬁcant. On mountain sides on the Izu Peninsula are
two additional species of Hydrangea, H. serrata(Thunb.) Ser. (var. serrata and var. angustata(Franch. & Sav.) H.Ohba) and H. luteovenosa
Koidz. Their ﬂowering times overlap with that ofH. macrophylla. On the Boso Peninsula, however, there are no other species of Hydrangea innatural situations near the sea. The populationsof H. macrophylla on the Izu Peninsula might
possibly receive genetic affects from the two

sympatric species of Hydrangea, but populationsof H. macrophylla on the Boso Peninsula are
hardly likely to receive genetic inﬂuence fromother species. We considered this to be one of thereasons that H. macrophylla on the Izu Peninsula
shows rather wide variations.
The variation in the sepals of the ornamental
ﬂowers found in wild is worth recognizing taxonomically at the infraspeciﬁc taxa. In this paper
two new forms with peculiar feature in the sepals
on the ornamental ﬂowers are described.
Number of sepals
Hydrangea macrophylla is known to usually
have four petaloid sepals in ornamental ﬂowers(Ohba, 2001), although as cultivars some plants
have more than four petaloid sepals, i.e. double
sepaled ﬂowers. Previously, no wild plants withdouble sepaled ﬂowers are known, but recently

Variation of the Ornamental Flowers of Hydrangea macrophylla141

Fig. 2. Wild forms of Hydrangea macrophylla in the Izu Peninsula. A, E and F. Forma macrophylla. B and C.
Forma isobue. D. Forma isonotaki.

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Hideaki Ohba and Shinobu Akiyama

Fig. 3. Hydrangea macrophylla (Thunb.) Ser. f. isobue H.Ohba & S.Akiyama (H. Ohba and S. Akiyama 2014139,TI-holotype).

Variation of the Ornamental Flowers of Hydrangea macrophylla

143

Fig. 4. Hydrangea macrophylla (Thunb.) Ser. f. isonotaki H.Ohba & S.Akiyama (H. Ohba and S. Akiyama
2014146, TI-holotype).

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Hideaki Ohba and Shinobu Akiyama

double sepaled forms were found on the Izu Peninsula and on Izu-oshima Island.
A plant with deep blue sepals, discovered in
the wild at Jogasaki, was named ʻIso-bueʼ as acultivar by Hirasawa (www.izu.fm/hydrangea/
j-izu.html), and another cultivar, perhaps derived
from it, was named ʻShamrockʼ by Mallet (2008).
These names, however, have no formal taxonomic status.
Hydrangea macrophylla (Thunb.) Ser. f. isobue
H.Ohba & S.Akiyama, f. nov.
[Figs. 2: B, C, 3]
Petaloid sepals of ornamental ﬂowers 12–14,
in 3 whorls, deep blue, widely rhombic-elliptic(outer ones) to rhombic-elliptic (inner ones).
Type: Japan. Honshu. Shizuoka Pref., Ito-shi,Yawatano, cultivated (origin: Jogasaki) (H. Ohba
& S. Akiyama 2014139 [Fig. 2: C], 21 June
2014, TI-holotype, TNS-isotype); Jogasaki, atthe margin of the coastal forest (H. Ohba
2013002 [Fig. 2: B], 12 June 2013, TI-paratype).
The epithet, tentatively named by Hirasawa,comes from a Japanese, Isobue, which means thewhistle-like heavily breathing of ﬁshing woman
diver after diving. A cultivar, ʻShamrock,ʼ namedby Mallet (2008), may have the same origin.
Color of sepals
In wild populations of H. macrophylla the petaloid sepals are white and usually slightly purplish(Ohba, 2001). A plant whose sepals of ornamental
ﬂowers are deep blue with a single white stripe on
the Izu Peninsula was named as the cultivar, ʻIsono-taki,ʼ by Hirasawa (www.izu.fm/hydrangea/j-izu.
html). Some plants with white striped sepals ofornamental ﬂowers were also found at Jogasaki andtransplanted in Hirasawaʼs nursery.
Hydrangea macrophylla (Thunb.) Ser. f. isonotaki H.Ohba & S.Akiyama, f. nov.
[Figs. 2: D, 4]
Petaloid sepals of ornamental ﬂowers usually4, bluish purple with white stripes.
Type: Japan. Honshu. Shizuoka Pref., Ito-shi,Yawatano, cultivated (origin: Jogasaki) (H. Ohba& S. Akiyama 2014146, 21 June 2014, TI-holotype, TNS-isotype).
The epithet, tentatively named by Hirasawa,comes from a Japanese, Iso-no-taki, meaning awaterfall by the sea near the original place of thisplant. This is compared to the color pattern withwhite strips on a bluish purple ground of thesepals of ornamental ﬂowers.
Specimens examined of wild plants of Hydrangea macrophylla (Thunb.) Ser. f. macrophylla
Japan. Honshu. Shizuoka Pref., Ito-shi, Jogasaki, at the margin of the coastal forest (H. Ohba
2013001, 12 June 2013, TI) (Fig. 2: A); Yawatano,cultivated (origin: Jogasaki) (H. Ohba & S. Akiyama 2014126, 2014133, 2014135 [Fig. 2: E],2014143 [Fig. 2: F], 21 June 2014, TI, TNS).
Acknowledgments
We are deeply indebted to Mr. Tetsu Hirasawaand Mr. Taka-aki Sugimoto for permitting us tostudy their valuable collections and for giving us
information. We greatly appreciate Dr. David E.
Bouffordʼs, Harvard University Herbaria, criticalreading of the manuscript.
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