Hydrangea hydrangeoides (Siebold & Zucc.) Bernd Schulz — Japanese hydrangea vine

Introduction

The Japanese hydrangea vine, Hydrangea hydrangeoides, is a deciduous, aerial-rooting woody climber of temperate forests in Japan and the southern Kuril Islands, with related island populations on Jeju and Ulleungdo in Korea. It is the characteristic Japanese representative of Section Schizophragma and remains much better known in horticulture under its older name Schizophragma hydrangeoides. Its coarsely serrated leaves provide one of the easiest distinctions from most mainland members of the section, while the sterile flowers surrounding each inflorescence usually possess a single conspicuous enlarged white sepal. This contrasts especially with the superficially similar H. petiolaris, whose sterile flowers normally have about four enlarged sepals (Liu & Zhu 2011; Sutton 2025).

The species is also one of the best-studied climbing hydrangeas from the standpoint of growth architecture. Seedlings and juvenile plants develop horizontal stems along the forest floor, rooting as they spread. Erect shoots arise from these creeping axes and may encounter tree trunks or rocks, after which rapidly extending shoots attach by aerial roots and ascend the support. Experiments on wild Japanese plants have shown both positive and negative phototropic behavior: shorter shoots tend toward light, while longer searching shoots can grow toward darker areas, a pattern interpreted as helping the plant locate the shaded bases of potential supports. Once established above the forest floor, shorter non-rooting lateral shoots carry much of the foliage and flowering growth (Kato et al. 2012, 2014).

This developmental complexity also contributes to taxonomic confusion. Leaves on creeping, searching, ordinary, and flowering shoots can differ considerably, making individual specimens appear more distinct than whole plants actually are. Historical Japanese taxonomy consequently produced several names for particular expressions of the species, including Schizophragma hydrangeoides var. concolor, var. formosum, and var. molle, later often reduced to formae. Current Kew treatment places all of these beneath H. hydrangeoides var. hydrangeoides.

Tree TSAR agrees that much of this variation does not warrant formal rank, but it does not treat every named expression identically. The historical concolor phenotype receives a separate Concolor Form page because it represents a recognizable foliage character with an unusually strong horticultural identity. The formosum and molle concepts remain ordinary synonymized variation because neither presently shows comparable geographic, evolutionary, or horticultural coherence.

Geographic variation presents a different problem. Kew currently recognizes three named island varieties in addition to the Japanese–Kuril autonym: var. taquetii on Jeju, var. ullungdoensis on Ulleungdo, and var. yakushimensis on Yakushima. Tree TSAR accepts this structure only in part. The Jeju population is retained formally as var. taquetii, and the morphologically distinctive Yakushima population is provisionally retained as var. yakushimensis. The Ulleungdo population is instead treated as the editorial Ulleungdo Form because its proposed morphological distinctions overlap typical Japanese material and the limited available molecular evidence associates it more closely with Japanese populations than with the differentiated Jeju material (Joung et al. 2010; Shaw 2012; Sutton 2025).

These treatments should not be interpreted as implying four equivalent subdivisions within the species. They deliberately record different evidentiary situations. Var. taquetii represents a relatively persuasive island morphogeographic unit; var. yakushimensis is a provisional formal hypothesis awaiting broader sampling; the Ulleungdo Form preserves an identifiable island population without overstating its differentiation; and the Concolor Form preserves biologically modest but horticulturally significant variation within ordinary Japanese populations.

The species itself was described by Siebold and Zuccarini as Schizophragma hydrangeoides in 1837. Its apparently redundant modern epithet—effectively “the Hydrangea that resembles Hydrangea”—is a historical artifact of that original generic placement. Bernd Schulz later transferred it into broad Hydrangea. Kew now accepts H. hydrangeoides with a native range from the southern Kurils through Japan to the two Korean island populations.

Horticultural and Agricultural Uses

Hydrangea hydrangeoides is one of the major ornamental root-climbing hydrangeas. It has been grown in Western gardens since the nineteenth century, although early records are complicated by repeated confusion with H. petiolaris. Many plants labelled Schizophragma hydrangeoides in Europe and North America proved to be the latter species, and the historical record of introductions is consequently difficult to reconstruct with confidence. Modern plants are readily separated in flower by the single enlarged sepal of the sterile flowers and in leaf by careful comparison of morphology (Sutton 2025).

The species is shade tolerant and well suited to climbing trees, walls, and other substantial supports, although flowering generally improves as upper growth reaches stronger light. Its developmental ability to spread across the forest floor before locating a support helps explain why young garden plants may spend considerable time establishing horizontally before beginning vigorous vertical growth.

Cultivar diversity is now considerable. ‘Moonlight’, with strongly silvered foliage and green venation, is among the most widely cultivated selections and is historically associated with the concolor concept. ‘Roseum’ is notable for pink sterile sepals, while newer selections such as ‘Trarose’ and its descendants emphasize colored sterile flowers or precocious flowering. These horticultural selections should remain distinct from botanical varieties and Tree TSAR Forms: a cultivar may arise from a particular wild expression without being taxonomically equivalent to it.

The Korean and Yakushima populations have also entered cultivation through documented wild collections, making horticultural collections potentially valuable repositories of geographical diversity. Collection numbers should be retained wherever possible because the taxonomy of the island populations remains incompletely resolved.

Conservation Concerns

No formal global IUCN assessment was located for Hydrangea hydrangeoides. The species spans much of Japan, reaches the southern Kuril Islands, and includes populations on Jeju and Ulleungdo. Kew's Angiosperm Extinction Risk Predictions currently classifies it as predicted not threatened with confident model support, although that result is a predictive screening assessment rather than a formal Red List evaluation (Bachman et al. 2024; Royal Botanic Gardens, Kew 2026).

Species-wide security does not automatically imply equal security of its geographically structured components. Jeju, Ulleungdo, and Yakushima each support morphologically or taxonomically noteworthy populations, and even the poorly investigated southern Kuril populations could contain distinct genetic diversity. Conservation sampling should therefore represent the species geographically rather than assuming that abundant populations on the Japanese main islands capture its full evolutionary variation.

The uncertainty surrounding infraspecific boundaries also provides a strong argument for conserving precise locality data. Population genomics encompassing Hokkaido, Honshu, Shikoku, Kyushu, Yakushima, Jeju, Ulleungdo, and the southern Kurils could simultaneously clarify taxonomy, biogeographic history, and conservation priorities.

Additional Information

Plants of the World Online — Hydrangea hydrangeoides (opens in a new tab): Plants of the World Online — current accepted nomenclature, native range, and recognized infraspecific taxa.

Trees and Shrubs Online — Hydrangea hydrangeoides (opens in a new tab): Trees and Shrubs Online — detailed account of morphology, climbing biology, cultivation, cultivars, and geographic variation.

References and Further Reading

Bachman SP, Brown MJM, Leão TCC, Nic Lughadha EM, Walker BE (2024) Extinction risk predictions for the world's flowering plants to support their conservation. New Phytologist 242(2): 797–808. https://doi.org/10.1111/nph.19592 (opens in a new tab)

Joung YH, Suh JK, Lee NS, Eum SM, Choi I-Y, Roh MS (2010) Identification of Hydrangeaceae accessions of wild origin from Jeju, Korea, using molecular markers. Plant Genetic Resources 8(3): 235–241. https://doi.org/10.1017/S1479262110000286 (opens in a new tab)

Kato S, Kanematsu T, Kawakubo N, Komiyama A (2012) Positive and negative phototropism in Schizophragma hydrangeoides and Parthenocissus tricuspidata. Japanese Journal of Forest Environment 54(1): 1–5. https://doi.org/10.18922/jjfe.54.1_1 (opens in a new tab)

Kato S, Morito H, Hanaoka S, Komiyama A (2014) Relationship between distribution of erect shoots in two root-climbing lianas Schizophragma hydrangeoides and Hydrangea petiolaris (Saxifragaceae) and light environment on the forest floor. Japanese Journal of Forest Environment 56(1): 49–54. https://doi.org/10.18922/jjfe.56.1_49 (opens in a new tab)

Liu W, Zhu X-Y (2011) Leaf epidermal characters and taxonomic revision of Schizophragma and Pileostegia (Hydrangeaceae). Botanical Journal of the Linnean Society 165(3): 285–314. https://doi.org/10.1111/j.1095-8339.2010.01101.x (opens in a new tab)

Shaw JMH (2012) Climbing hydrangeas and their relatives. The Plantsman, New Series 11(4): 226–231. https://www.rhs.org.uk/about-us/pdfs/publications/the-plant-review/general-pdfs/index-for-2012.pdf (opens in a new tab)

Sutton J (2025) Hydrangea hydrangeoides. Trees and Shrubs Online. International Dendrology Society. https://www.treesandshrubsonline.org/articles/hydrangea/hydrangea-hydrangeoides/ (opens in a new tab)