Hydrangea acuminata var. yakushimensis
Yakushima sour hydrangea
Synonyms
- Hydrangea acuminata subsp. yakushimensis
On this page
Introduction
Yakushima sour hydrangea, Hydrangea acuminata var. yakushimensis, is a rare variety confined to the island of Yakushima in southern Japan. It is one of the most geographically and ecologically distinctive members of Section Macrophyllae, occurring primarily on wet cliffs beside mountain streams at approximately 1520–1750 m. An exceptional lower-elevation population has also been documented along the Miyanoura River at about 575 m. (Hirota et al. 2022)
The variety differs strikingly from ordinary H. acuminata in several characters. Its leaves are comparatively glossy and nearly glabrous above except along the veins, possess more numerous and generally longer marginal teeth, and are associated with broader infructescences. Its capsules are especially diagnostic, only about 2.2–2.7 mm long, compared with substantially longer fruits in ordinary H. acuminata, var. australis, and H. minamitanii. Its flowers are characteristically blue, further distinguishing it from the pink- or white-flowered H. minamitanii that occupies somewhat analogous stream-cliff habitats on Kyushu.
The plant was discovered as a distinctive lineage during studies of western Japanese mountain hydrangeas and was formally described in 2022 as H. acuminata subsp. yakushimensis. Nuclear MIG-seq data placed it close to the southern-Kyushu australis lineage, a relationship that parallels several other plant groups in which Yakushima endemics have particularly close relatives in southern Kyushu. (Hirota et al. 2022)
More comprehensive genomic sampling subsequently showed that ordinary H. acuminata, southern-Kyushu australis, and Yakushima yakushimensis form a single larger evolutionary lineage. Within it, however, the Yakushima plants remain a strongly supported local group. Murakami et al. (2024) therefore retained their taxonomic recognition while reducing the rank from subspecies to H. acuminata var. yakushimensis.
The variety illustrates the exceptional evolutionary role of Yakushima. The island's steep topography, extreme rainfall, elevational gradients, and geographic isolation have generated or preserved numerous endemic plants. Yakushima sour hydrangea appears to represent a young peripheral derivative of the western Japanese H. acuminata lineage that has acquired both morphological differentiation and a specialized stream-cliff ecology without becoming fully separated at species level.
Horticultural and Agricultural Uses
Yakushima sour hydrangea is primarily of botanical and conservation interest rather than established horticultural importance. Its glossy foliage, blue flowers, compact mountain habit, and strongly serrate leaves nevertheless give it considerable ornamental potential for specialist collections.
Its highly specific native ecology suggests that cultivation should not be inferred simply from general hydrangea requirements. Wild plants occupy persistently moist stream cliffs in a cool, exceptionally wet island climate. Ex situ collections intended for conservation should therefore prioritize accurate provenance and cultivation methods that avoid both drought and prolonged stagnant conditions.
The variety also has value as research germplasm. Comparison with ordinary H. acuminata and var. australis offers an unusually clear natural system for investigating the early stages of island differentiation in a woody ornamental genus.
Conservation Concerns
Yakushima sour hydrangea is one of the clearest conservation priorities in Section Macrophyllae. When the taxon was described, its known population was estimated at more than 50 but fewer than 250 mature individuals, and Hirota et al. (2022) assessed it as Endangered under IUCN criterion D because of its very small population.
Japan's current fifth national Red List independently lists the taxon as Endangered (EN) under its original subspecific name. The national recognition is particularly significant because the taxon had not appeared on the preceding 2020 list; its formal description allowed it to be assessed explicitly.
Its vulnerability is compounded by ecological specialization. Most known plants occupy narrow stream-cliff habitats at high elevations, so a small number of localized disturbances could affect a substantial fraction of the entire population. Climate change may also alter cloud, rainfall, or stream regimes in Yakushima's mountains, although species-specific responses have not yet been quantified.
Ex situ collections can provide valuable insurance, but they should sample more than one wild population whenever legally and biologically appropriate. A single cultivated clone would preserve the appearance of the taxon without adequately representing its remaining genetic diversity.
Additional Information
- Plants of the World Online — Hydrangea acuminata var. yakushimensis (opens in a new tab): Published varietal combination and nomenclatural information; POWO currently treats the variety as a synonym of the earlier subspecies.
- iNaturalist — Yakushima Hydrangea acuminata observations (opens in a new tab): Photographic records of the rare Yakushima lineage.
- Japan Ministry of the Environment — Fifth Red List (opens in a new tab): National listing of Yakushima sour hydrangea as Endangered.
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)
Ministry of the Environment, Japan (2025) Fifth Red List: Vascular plants. Ministry of the Environment, Tokyo. https://www.env.go.jp/nature/kisho/5th-rl-2025-book/01-5thredlist-vascularplant.pdf (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)