Hydrangea acuminata var. australis
Southern sour hydrangea
Synonyms
- Hydrangea acuminata subsp. australis ()
- Hydrangea serrata var. australis
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Introduction
Southern sour hydrangea, Hydrangea acuminata var. australis, is a geographically differentiated variety endemic to southern Kyushu. Its range is centered on southern Kagoshima and Miyazaki prefectures, particularly the Ōsumi and Satsuma peninsulas, where it occurs chiefly at lower elevations along disturbed margins of evergreen forest and Cryptomeria plantations. Populations also extend into mountain environments; genetically identified material has been collected from a volcanic cliff at approximately 1700 m on Mount Karakuni. (Hirota et al. 2022)
The variety differs from ordinary H. acuminata most consistently in its foliage. Leaves tend to be larger and broader, often exceeding about 6.2 cm in width, with more numerous marginal teeth and relatively dense curled hairs on the lower surface, especially along the veins. Typical var. acuminata generally has narrower leaves with fewer teeth and sparser, non-curled pubescence. These characters are useful but not absolute, and individual plants near the geographic transition can approach the morphology of the widespread variety.
Molecular evidence confirms that the southern plants are differentiated without supporting separation as an independent species. Hirota et al. (2022) initially treated the lineage as H. acuminata subsp. australis because MIG-seq recovered it as a strongly differentiated group. Their sampling also found a plant on Mount Karakuni that grouped genetically with australis while resembling ordinary acuminata morphologically, suggesting either introgression or gradual intergradation near the boundary.
Broader population sampling subsequently placed australis, ordinary acuminata, and the Yakushima yakushimensis lineage together within a single major genomic clade. Each of the southern and Yakushima groups remained strongly supported locally, but neither was separated from the larger H. acuminata lineage at the level expected of an independent species. Murakami et al. (2024) consequently recombined the southern plant as H. acuminata var. australis.
The variety is also biogeographically important because southern Kyushu forms an evolutionary bridge between widespread western Japanese H. acuminata and the still more isolated Yakushima variety. The close relationship among these three geographic components illustrates how repeated climatic shifts and island-mainland separation during the Pleistocene helped structure Section Macrophyllae. (Murakami et al. 2024)
Horticultural and Agricultural Uses
Southern sour hydrangea is little known as a distinct entity in mainstream horticulture. Its broad leaves, dense lower-surface pubescence, colored lacecap flowers, and southern provenance could nevertheless make it valuable to specialist collections, particularly those emphasizing wild Japanese hydrangeas.
The variety's greatest horticultural and breeding significance may lie in its environmental provenance. Southern Kyushu populations experience warmer conditions than the widespread northern portion of H. acuminata, while some populations also extend into exposed volcanic mountain habitats. Wild-source material may therefore contain useful variation in heat tolerance, moisture relations, or other climatic responses, although these traits have not yet been evaluated systematically in cultivation.
Because morphological intergradation occurs near the northern edge of its distribution, scientifically useful living collections should retain locality information. Provenance is more informative than the varietal name alone when comparing plants from different parts of southern Kyushu.
Conservation Concerns
Southern sour hydrangea is geographically restricted but not presently listed as nationally threatened in Japan. Regional assessments nevertheless identify conservation concerns. Miyazaki Prefecture's 2025 revision lists the taxon as Near Threatened, while Kagoshima has recognized it as important because of its restricted distributional characteristics.
Its conservation value lies partly in preserving the southernmost mainland component of the H. acuminata radiation. The variety is not simply a collection of unusually broad-leaved plants: genomic work demonstrates geographically structured ancestry even though some intergradation with ordinary acuminata persists. Loss of southern populations would therefore eliminate evolutionary diversity that cannot be replaced simply by conserving the widespread variety farther north.
Maintaining multiple populations across both Kagoshima and Miyazaki is particularly desirable because the lineage occupies a range of habitats from lower-elevation forest margins to high volcanic slopes. Such environmental breadth may conceal adaptive variation not apparent from morphology alone.
Additional Information
- Plants of the World Online — southern Hydrangea acuminata taxon (opens in a new tab): Nomenclature and southern-Kyushu distribution; POWO currently retains the 2022 subspecific rank.
- iNaturalist — Hydrangea acuminata australis observations (opens in a new tab): Photographic occurrence records; observations may be filed under older combinations.
- Miyazaki Prefecture Red List (opens in a new tab): Current regional conservation framework.
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)
Yamazaki T (2001) A new variety of Hydrangea serrata () Journal of Japanese Botany 76(3): 174–175. https://doi.org/10.51033/jjapbot.76_3_9510 (opens in a new tab)