Hydrangea grosseserrata Engl. — Yakushima hydrangea

Introduction

Yakushima hydrangea, Hydrangea grosseserrata, is a small woody shrub centered on the extraordinarily species-rich island of Yakushima in southern Japan. It belongs to Section Kawagoeanae, the East Asian lineage separated from the old broad Section Chinenses by Yang et al. (2024). Tree TSAR accepts H. grosseserrata at species rank, departing from POWO and several Japanese floristic treatments that recognize it as H. kawagoeana var. grosseserrata.

The plant is generally about 1 m tall, often much branched, with reddish-brown young stems and opposite leaves that are oblong to oblong-lanceolate, tapering into a long apex. The upper portion of the blade bears conspicuous coarse, sharp teeth. Terminal cymes contain numerous pale-yellow fertile flowers and usually only a few enlarged yellowish-white marginal flowers, whose four or five petaloid sepals can themselves be irregularly toothed. Japanese cultivated and floristic accounts emphasize its narrow foliage and strongly serrate appearance as characteristic features.

Its closest demonstrated relative is H. kawagoeana. Hirota et al. (2022) sampled grosseserrata from two Yakushima localities and kawagoeana from Suwanose-jima. Genome-wide MIG-seq data measured an FST of 0.395 between the two, while ITS recovered them together with 91% bootstrap support. Thus their relationship is close, but their genomic differentiation is comparable to that observed among accepted species elsewhere in the same assemblage. (Hirota et al. 2022)

Tree TSAR therefore interprets the Yakushima population as an island lineage that has progressed beyond ordinary geographic variation within H. kawagoeana. The evidence is not yet exhaustive: population-scale sampling from Yakushima, the Tokaras, and adjacent Ryukyu islands would provide a more rigorous test of gene flow and reciprocal lineage cohesion. For now, however, the combination of genome-wide differentiation, geography, and morphological distinction makes species rank preferable to varietal rank.

Horticultural and Agricultural Uses

Yakushima hydrangea remains a specialist ornamental rather than a mainstream garden species. It is represented in Japanese botanical collections, where its finely toothed foliage and small lacecap inflorescences distinguish it from the much larger garden hydrangeas. The Kobe Municipal Arboretum treats it as a wild species from Yakushima and highlights its white marginal flowers and pale-yellow fertile flowers.

Its strongest horticultural value is likely to be in collections emphasizing wild hydrangea diversity, Japanese island endemics, and genetically documented provenance. Because H. grosseserrata has frequently been treated as a variety of H. kawagoeana, cultivated accessions should be checked against origin data rather than identified solely from horticultural labels.

Conservation Concerns

Under the narrow Tree TSAR concept, H. grosseserrata is essentially a Yakushima island lineage. POWO maps its varietal equivalent specifically to Yakushima, and Japanese forestry surveys identify Yakushima hydrangea as a plant of distributional conservation significance.

It is not currently highlighted as nationally threatened in the same manner as some of Yakushima's much rarer endemic plants, but its restricted geographic extent makes preservation of multiple wild populations important. Island endemics can retain substantial population structure over short distances, and the existing genomic study included only two grosseserrata samples. Conservation collections should therefore retain collection locality and avoid treating all Yakushima material as genetically interchangeable.

The principal research need is a population-genomic study explicitly designed around the grosseserrata–kawagoeana boundary. Evidence of persistent genomic clustering despite geographic proximity would further strengthen species status; extensive contemporary admixture could instead support reconsideration of rank.

Additional Information

Plants of the World Online — Hydrangea kawagoeana var. grosseserrata (opens in a new tab): Current backbone treatment and complete historical combination list; Tree TSAR differs by accepting H. grosseserrata as a species.

Kobe Municipal Arboretum — Yakushima hydrangea (opens in a new tab): Japanese horticultural and morphological account.

iNaturalist — search for Hydrangea grosseserrata (opens in a new tab): Photographic observations; records may follow alternative taxonomic treatments under H. kawagoeana.

References and Further Reading

Hirota SK, Yahara T, Fuse K, Sato H, Tagane S, Fujii S, Minamitani T, Suyama Y (2022) Molecular phylogeny and taxonomy of the Hydrangea serrata complex (Hydrangeaceae) in western Japan, including a new subspecies of H. acuminata from Yakushima. PhytoKeys 188: 49–71. https://doi.org/10.3897/phytokeys.188.64259 (opens in a new tab)

Iwatsuki K, Boufford DE, Ohba H (eds.) (2001) Flora of Japan, vol. IIb. Kodansha, Tokyo. https://www.kspub.co.jp/book/detail/1546059.html (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)