Hydrangea yesoensis Koidz. — Hime hydrangea

Introduction

Hime hydrangea, Hydrangea yesoensis, is a large-leaved northern Japanese woodland hydrangea that Tree TSAR restores to species rank after decades of treatment as H. serrata var. yesoensis. It ranges from Hokkaido south through northern Honshu, reaching approximately Fukushima, Niigata, northern Nagano, and Toyama prefectures under the comprehensive revision of Murakami et al. (2024). Its traditional Japanese name is Ezo-ajisai, reflecting its northern distribution, while Tree TSAR retains the established common name Hime hydrangea.

Morphologically, H. yesoensis has long stood apart from ordinary mountain hydrangea. Plants tend to be larger, with broader and larger leaves, larger showy marginal flowers, and larger fruits than the typical H. serrata assemblage. Earlier classifications nevertheless treated these differences as northern variation within H. serrata.

Genome-wide SNP evidence demonstrates that interpretation to be inadequate. Murakami et al. sampled the entire Japanese Section Macrophyllae complex and recovered yesoensis as a fully supported monophyletic lineage outside the principal H. serrata clade. Instead, it formed one of two fully supported lineages within the deeper branch that also contains H. macrophylla. The species-level split between yesoensis and macrophylla lies within a clade whose crown age was estimated at approximately 1.23 million years.

This relationship is biogeographically striking. H. macrophylla is a comparatively large, thick-leaved shrub adapted to warm Pacific coastal environments; H. yesoensis is a northern temperate-forest plant extending into Hokkaido. Their sister-lineage relationship therefore records a deep evolutionary history that is no longer obvious from present-day ecology. It also shows why simply ranking hydrangeas according to superficial similarity can be misleading: the northern plant formerly buried within serrata is more closely associated at the deepest genomic level with bigleaf hydrangea than with ordinary mountain hydrangea.

Murakami et al. therefore reinstated H. yesoensis Koidz. and explicitly concluded that it should no longer be treated as an infraspecific taxon of H. serrata. POWO has not yet adopted that revision and continues to accept H. serrata var. yesoensis, citing principally the older Flora of Japan treatment and type revision. Tree TSAR follows the newer genome-wide evidence.

The nomenclature is correspondingly rich. Twenty-four names are currently gathered under the POWO varietal concept, including H. yesoensis itself, H. serrata ssp. yesoensis, H. macrophylla f. yesoensis, the historical belzonii complex, and names derived from Viburnum cuspidatum. Tree TSAR transfers that historical complex to the species H. yesoensis, but one caveat remains: the all-showy cultivated cuspidata entity was not sampled by Murakami et al. They therefore adopted the species name yesoensis provisionally with respect to that cultivated problem.

Horticultural and Agricultural Uses

Hime hydrangea has long been cultivated, frequently under names associated with H. serrata, megacarpa, cuspidata, or yesoensis. Its larger leaves and inflorescences give it a bolder garden presence than ordinary mountain hydrangea, while its northern distribution suggests useful cold-hardiness potential.

The cuspidata problem is particularly relevant horticulturally. Historical material bearing all-showy flowers has been interpreted as a cultivated expression of the northern lineage, but modern genomic work has not yet tested authentic cultivated cuspidata against wild H. yesoensis. Until that is done, Tree TSAR should avoid treating every horticultural plant bearing a cuspidata label as a genetically verified representative of the species.

Wild-source northern material could prove valuable in breeding programs seeking greater cold tolerance, larger foliage, or novel combinations with H. macrophylla. Because the two are close genomic relatives despite their ecological contrast, their relationship may be particularly informative for understanding how climatic adaptation evolved within the section.

Conservation Concerns

Hydrangea yesoensis occupies a broad portion of northern Japan rather than a narrowly restricted endemic range, and current Japanese databases do not identify it as nationally threatened. A prefectural conservation listing in Okayama concerns marginal occurrences under the older varietal name and does not imply widespread decline throughout the northern range.

The more important conservation issue is taxonomic recalibration. Much of the available occurrence, horticultural, and conservation information was accumulated under H. serrata var. yesoensis. Once the lineage is recognized as a species, records should be reassessed to determine which genuinely represent H. yesoensis and which belong to neighboring mountain hydrangeas.

Its newly clarified phylogenetic position also increases the value of geographically representative germplasm. Conservation collections should sample across Hokkaido and northern Honshu rather than relying on one cultivated clone, particularly because population structure within the species has received much less attention than its species-level distinction.

Additional Information

Plants of the World Online — Hydrangea serrata var. yesoensis (opens in a new tab): Current backbone treatment and 24-name synonym complex. Tree TSAR differs by restoring H. yesoensis at species rank.

Murakami et al. 2024 — genome-wide revision of Section Macrophyllae (opens in a new tab): Primary evidence supporting species rank.

iNaturalist — search for Hydrangea yesoensis (opens in a new tab): Photographic records; many observations remain filed under H. serrata var. yesoensis.

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)

Ohba H, Akiyama S (2013) A revision of the species of Hydrangea (Hydrangeaceae) described by Siebold and Zuccarini, part 1. Bulletin of the National Museum of Nature and Science, Series B (Botany) 39(4): 173–194. https://www.kahaku.go.jp/albums/abm.php?d=4704&f=abm00001838.pdf&n=BNMNS_B39-4_173-194.pdf (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)

Wilson EH (1923) The hortensias Hydrangea macrophylla DC. and Hydrangea serrata DC. Journal of the Arnold Arboretum 4(4): 233–246. https://doi.org/10.5962/p.317979 (opens in a new tab)