Hydrangea yayeyamensis Koidz. — Nansei hydrangea

Introduction

Nansei hydrangea, Hydrangea yayeyamensis, is an evergreen or nearly evergreen shrub of the southern Ryukyu Islands and one of the most geographically distinctive members of Section Kawagoeanae. POWO accepts it as a species native to the Nansei-shoto, while older Japanese treatments commonly submerged it within H. chinensis as a variety or form. Its synonymy preserves that history through H. chinensis var. yayeyamensis, H. chinensis f. yayeyamensis, and the separately named var. koidzumiana.

The type history itself reflects the old Taiwan–Ryukyu confusion. Koidzumi described H. yayeyamensis in 1929 from Yaeyama material; subsequent revisionary work placed it within H. chinensis, and Ohba & Akiyama later recombined it as Hortensia chinensis var. yayeyamensis. Their type review confirms the Yaeyama origin of the name while also documenting Taiwanese material that had historically been mixed into the concept.

Modern phylogenetic evidence changes the interpretation considerably. Yang et al. (2024) sampled material under the name H. chinensis var. yayeyamensis and recovered it not with residual Section Chinenses, but in the lineage containing H. kawagoeana, H. luteovenosa, H. macrosepala, H. chinensis var. lobbii, and H. scandens var. grosseserrata. This result was one of the observations that led the authors to establish Section Kawagoeanae. Tree TSAR therefore follows the modern species-level treatment of H. yayeyamensis rather than returning it to an infraspecific position beneath H. chinensis.

Field descriptions from the Yaeyama Islands portray a well-branched evergreen shrub about 1–2 m tall, with opposite elliptic to elliptic-oblanceolate leaves approximately 7–11 cm long. The upper leaf margins are coarsely serrate, while hairs are concentrated along the veins beneath. Terminal inflorescences bear white to pale-yellow enlarged marginal flowers with usually three to five petaloid sepals. The plant occurs in somewhat moist mountain forest and along streams on Ishigaki and Iriomote.

That combination of subtropical evergreen habit, Yaeyama geography, and molecular separation from residual Chinenses makes H. yayeyamensis an important southern endpoint in the Japanese Kawagoeanae story.

Horticultural and Agricultural Uses

Nansei hydrangea appears to be uncommon in mainstream horticulture. Its subtropical evergreen habit and relatively large glossy foliage suggest considerable ornamental potential, especially in mild climates, but there is too little securely documented cultivated material to generalize about cold hardiness or garden performance.

Its most important horticultural value at present is as wild genetic material from the southern extreme of the Japanese section. Collections with precise Yaeyama provenance could help illuminate the evolution of evergreen foliage, subtropical adaptation, and flowering phenology within Kawagoeanae. Such material should be maintained separately from Taiwanese plants historically labelled as H. chinensis var. yayeyamensis, since the nomenclatural history contains evidence of geographic mixing.

Conservation Concerns

Hydrangea yayeyamensis has a much narrower distribution than H. scandens or H. luteovenosa. POWO restricts its native range to the Nansei Islands, and Japanese field accounts identify it specifically from Ishigaki and Iriomote.

I did not locate a current formal global assessment that would justify assigning a threat category here. Its island restriction nevertheless makes population-level documentation worthwhile, particularly because habitat condition and population connectivity on individual Yaeyama islands could matter greatly even when the species remains locally frequent.

Future nuclear sampling should also determine whether Ishigaki and Iriomote populations constitute one freely exchanging lineage or preserve meaningful island differentiation. That would be especially valuable given the strong role of island isolation elsewhere in Section Kawagoeanae.

Additional Information

Plants of the World Online — Hydrangea yayeyamensis (opens in a new tab): Plants of the World Online accepts H. yayeyamensis at species rank and documents its five historical synonyms.

Yang et al. (2024) (opens in a new tab): The modern sectional placement is established by Yang et al. (2024), whose expanded sampling moved the former H. chinensis var. yayeyamensis into Section Kawagoeanae.

References and Further Reading

Ohba H, Akiyama S (2017) Notes on the genus Hortensia (Hydrangeaceae). Journal of Japanese Botany 92(4): 245–247. https://doi.org/10.51033/jjapbot.92_4_10803 (opens in a new tab)

Yang X, Xue T, Gao T, Liang Y, Smets EF, Gadagkar SR, Yu S (2024) An updated infrageneric classification based on phylogenomics and character evolution in Hydrangea (Hydrangeaceae). Taxon 73(2): 503–518. https://doi.org/10.1002/tax.13158 (opens in a new tab)