Hydrangea serrata (Thunb.) Ser. — Mountain hydrangea

Introduction

Mountain hydrangea, Hydrangea serrata, is a deciduous woodland shrub centered on the mountains of Japan and Korea and one of the principal evolutionary lineages of Section Macrophyllae. In the revised Tree TSAR treatment it is more narrowly circumscribed than in many twentieth-century floras: the western Japanese populations are separated as H. acuminata, the eastern-Kyushu H. minamitanii is accepted as a species, and the northern Japanese H. yesoensis is likewise removed to species rank. The resulting H. serrata remains the principal eastern-Honshu mountain lineage, with Korean populations provisionally retained pending denser comparative genomic sampling.

Compared with H. macrophylla, mountain hydrangea is usually a smaller and more delicate shrub, often about 1–2 m tall, with thinner leaves that commonly bear hairs and with smaller lacecap inflorescences. Its characteristic habitat is humid mountain woodland rather than exposed maritime forest. Murakami et al. (2024) recovered the eastern Japanese H. serrata lineage as part of one of the two deep branches of Section Macrophyllae, distinct from the clade containing H. macrophylla and H. yesoensis.

The species is nevertheless not genetically isolated from all of its relatives. Where its distribution approaches that of H. macrophylla on the Izu and Bōsō peninsulas, natural hybridization occurs. Uemachi et al. (2026) found evidence of interspecific gene exchange in many peninsula populations, in contrast to the relatively limited hybridization detected on the sampled Izu Islands. The result helps explain why single nuclear or plastid loci have sometimes blurred the distinction between these species even though genome-wide SNP analyses recover coherent major lineages.

Tree TSAR retains H. serrata var. angustata as a recognizable local variety from the Izu Peninsula. Murakami et al. found no clear genome-wide separation between angustata and nearby eastern serrata and formally synonymized it, but the variety remains morphologically distinctive through its narrow lanceolate leaves and geographically concentrated distribution. Under the Tree TSAR framework, this combination of morphology and geography without strong genomic independence is better represented at varietal rank than either species rank or complete taxonomic disappearance.

Mountain hydrangea has also accumulated an enormous nomenclatural and horticultural history. POWO currently places 55 synonyms beneath var. serrata, although the Tree TSAR revision transfers the buergeri lineage to H. acuminata on the basis of the modern type-informed genomic treatment. The remaining names include numerous flower-color and double-flowered forms, as well as the historically famous sitsitan–stellata complex.

The Korean component of the species deserves particular caution. Names such as H. coreana have historically been treated within H. serrata, but the comprehensive 2024 genomic study was designed around Japanese populations and therefore does not independently demonstrate that Korean material belongs to exactly the same evolutionary unit. Tree TSAR retains those populations provisionally within H. serrata rather than creating an unsupported Korean segregate, with dense Korean–Japanese nuclear sampling established as the review trigger.

Horticultural and Agricultural Uses

Mountain hydrangea has contributed extensively to ornamental horticulture. Compared with H. macrophylla, cultivars tend to be smaller, finer-textured, earlier flowering, and more cold hardy, with lacecap inflorescences ranging through white, pink, red, purple, and blue. Numerous selections have entered breeding programs aimed at combining the refined habit and hardiness of H. serrata with the larger flowers and broad horticultural adaptability of H. macrophylla.

The species also carries an important Japanese cultural and biochemical history through amacha, or sweet hydrangea tea. Certain mountain-hydrangea plants accumulate phyllodulcin, an intensely sweet dihydroisocoumarin. Selected material has long been harvested and processed to produce the sweet infusion associated particularly with celebrations of the Buddha's birth in Japan. Chemical analysis of amacha has confirmed phyllodulcin as one of its characteristic sweet constituents.

The sweet-leaf phenotype should not, however, be converted automatically into a botanical taxon. Murakami et al. specifically note that sweetness was not unique to the historical var. thunbergii and that sweet plants had repeatedly been confused and cultivated under that name. Tree TSAR therefore treats amacha as an important biochemical and horticultural selection phenomenon within the broader H. serrata lineage rather than as evidence requiring another accepted variety.

Conservation Concerns

Mountain hydrangea remains geographically widespread relative to several other members of Section Macrophyllae, and Kew's automated model predicts the species to be not threatened. No evidence presently supports treating the species as globally imperiled.

Regional populations can nevertheless be conservation concerns. Japanese prefectural assessments list ordinary mountain hydrangea as Near Threatened in Miyagi and Vulnerable in Kagoshima under the historical var. serrata concept. Those records need to be interpreted cautiously because the taxonomy of western populations has changed substantially: some plants historically counted as serrata now belong to H. acuminata or other segregated taxa.

Conservation should therefore emphasize accurately identified, geographically documented wild populations rather than the apparent security created by horticultural abundance. The Korean portion of the range is particularly important for future sampling because its relationship to the genomically revised Japanese species concept remains incompletely tested.

Additional Information

Plants of the World Online — Hydrangea serrata (opens in a new tab): Current international backbone treatment, distribution, and nomenclature; several Tree TSAR rank decisions differ from this broader treatment.

Trees and Shrubs Online — Hydrangea serrata (opens in a new tab): Extensive account of horticultural diversity and the long-running taxonomic difficulties within the complex.

iNaturalist — Hydrangea serrata observations (opens in a new tab): Useful photographic records, but identifications should be interpreted in light of the revised acuminata, minamitanii, and yesoensis circumscriptions.

References and Further Reading

Murakami S, Ito T, Uemachi T, Fujii S, Matsuo A, Suyama Y, Maki M (2024) Phylogenetic relationships and divergence time of Hydrangea sect. Macrophyllae (Hydrangeaceae) revealed by genome-wide SNPs. Plant Systematics and Evolution 310(4): 27. https://doi.org/10.1007/s00606-024-01902-5 (opens in a new tab)

Tachibana Y, Hashimoto Y, Hagiwara Y, Konishi T, Kurokawa N (1974) The quantitative analysis of phyllodulcin in “Amacha” (sweet hydrangea) by means of thin-layer chromatography. Yakugaku Zasshi 94(9): 1167–1169. https://doi.org/10.1248/yakushi1947.94.9_1167 (opens in a new tab)

Uemachi T, Mizuhara Y, Deguchi K, Shinjo Y, Kajino E, Ohba H (2014) Phylogenetic relationship of Hydrangea macrophylla (Thunb.) Ser. and H. serrata (Thunb.) Ser. evaluated using RAPD markers and plastid DNA sequences. Journal of the Japanese Society for Horticultural Science 83(2): 163–171. https://doi.org/10.2503/jjshs1.CH-092 (opens in a new tab)

Uemachi T, Tojo N, Hayashi Y, Kinoshita M, Shibata A, Kondo Y, Iwasawa H (2026) Genetic conservation of endemic Hydrangea macrophylla: Assessing natural hybridization with H. serrata using ITS sequences of nuclear ribosomal DNA. The Horticulture Journal 95(2): 285–297. https://doi.org/10.2503/hortj.SZD-094 (opens in a new tab)