Hydrangea acuminata Siebold & Zucc. — Sour hydrangea

Introduction

Sour hydrangea, Hydrangea acuminata, is a deciduous woodland shrub of western Japan and one of the principal species of Section Macrophyllae. Its native range extends through western Honshu, Shikoku, and northern Kyushu, where it commonly occupies moist forest margins, stream valleys, slopes, and cliffs. The species typically bears relatively small lacecap inflorescences whose enlarged marginal flowers are pink, purple, or pale blue; white-flowered populations are unusual but have been documented. (Hirota et al. 2022; Murakami et al. 2024)

For much of the twentieth century, these western Japanese plants were incorporated into a very broadly defined Hydrangea serrata, usually as var. acuminata. Genome-wide studies have demonstrated that the relationship is deeper than ordinary varietal differentiation. Hirota et al. (2022) recovered western Japanese acuminata as a distinct nuclear lineage, and the much larger population study of Murakami et al. (2024) confirmed that H. acuminata belongs to a strongly supported western branch separate from eastern H. serrata. That branch also contains the geographically differentiated southern-Kyushu var. australis and Yakushima var. yakushimensis.

The larger study also clarified the internal structure of H. acuminata. Western Honshu, Shikoku, and northern-Kyushu populations form the widespread var. acuminata, while southern Kyushu and Yakushima contain recognizable local lineages. All three nevertheless remain nested within one broader evolutionary unit. Murakami et al. (2024) therefore treated australis and yakushimensis as varieties rather than subspecies, recognizing their geographic and morphological distinctiveness without implying the deeper independence expected of separate species.

This interpretation also resolves an older nomenclatural problem. The historical Hydrangea acuminata var. buergeri and related buergeri combinations are based on Kyushu material belonging to the ordinary H. acuminata lineage rather than to H. serrata. The type-based treatment of the western lineage therefore brings the buergeri name complex back into the synonymy of H. acuminata. (Murakami et al. 2024)

Sour hydrangea is readily distinguished from the more localized H. minamitanii where the two approach one another in Kyushu. H. acuminata normally has at least some leaf pubescence and commonly grows on soil along forest margins and streams, whereas H. minamitanii has nearly glabrous leaves and shoots and is particularly associated with wet stream cliffs. In places where the two occur within about 100 m of one another, no obvious morphological intermediates were observed during the detailed field study of Hirota et al. (2022).

Horticultural and Agricultural Uses

Hydrangea acuminata is far less prominent horticulturally than H. macrophylla, but its compact stature, fine-textured foliage, colored lacecap flowers, and woodland adaptation make it an attractive ornamental species. Plants historically distributed in horticulture under names such as H. serrata var. acuminata have sometimes been selected for especially colorful flowers or compact habit, although cultivated naming has not always tracked modern botanical circumscription.

The species also represents an important source of wild germplasm within Section Macrophyllae. Its broad west-Japanese distribution spans different climatic and geographic settings, while its southern and insular varieties demonstrate ongoing regional differentiation. Maintaining wild-source accessions from Honshu, Shikoku, and northern Kyushu separately would therefore preserve considerably more evolutionary information than maintaining the species through a few horticultural clones.

Its close relationship with var. australis and var. yakushimensis also makes the species useful for studying how geographic isolation within the Japanese archipelago produces morphological differentiation before full species separation. That evolutionary continuum may ultimately prove useful to breeding programs interested in flower color, foliage, climatic adaptation, or compact architecture.

Conservation Concerns

The widespread var. acuminata is not presently identified as a nationally threatened Japanese plant. Its range extends across several major islands and encompasses a variety of forest and streamside habitats. The species nevertheless should not be regarded as conservation-neutral simply because it is geographically broader than its two recognized varieties.

Population-genomic work indicates geographic structure within the species, particularly among Kyushu populations. Preserving wild populations across western Honshu, Shikoku, and Kyushu therefore protects both current diversity and the evolutionary context from which the more localized southern and Yakushima varieties emerged.

Taxonomic precision is also important when interpreting historical records. Older conservation databases frequently recorded these plants under H. serrata var. acuminata. Modern occurrence and conservation data should therefore be evaluated in light of the now better-supported distinction between eastern H. serrata and western H. acuminata.

Additional Information

Plants of the World Online — Hydrangea acuminata (opens in a new tab): Current nomenclature and synonymy for the species.

iNaturalist — Hydrangea acuminata observations (opens in a new tab): Photographic occurrence records; older identifications may appear under H. serrata.

Murakami et al. 2024 — Section Macrophyllae genome-wide revision (opens in a new tab): Comprehensive population sampling supporting the modern species circumscription.

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)

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)