Hydrangea hydrangeoides var. yakushimensis
Yakushima hydrangea vine
Synonyms
- Schizophragma hydrangeoides var. yakushimense
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Introduction
The Yakushima hydrangea vine, Hydrangea hydrangeoides var. yakushimensis, is a striking but still incompletely understood expression of the species from Yakushima, the exceptionally wet mountainous island south of Kyushu. Kew accepts it formally as a variety endemic to Yakushima. Tree TSAR also retains it at varietal rank, but provisionally, because its combination of morphology and island geography is persuasive enough to warrant recognition while the available sampling remains too limited to regard the boundary as settled.
Julian Shaw described the plant as Schizophragma hydrangeoides var. yakushimense in 2012 from the Crûg Farm collection BSWJ 6119. He later transferred it into Hydrangea as var. yakushimensis. Kew recognizes the earlier Schizophragma name as its single homotypic synonym.
Its most remarkable feature is developmental. Young plants can persist for a prolonged period as extremely dwarf terrestrial shoots spreading across the forest floor. When they eventually enter the climbing adult phase, the lateral shoots carry leaves smaller than those of typical H. hydrangeoides, reported at about 8–9.5 × 7.5–8 cm. Shaw also described differences in marginal teeth and sterile sepals. The type collection grew over mossy boulders in Yakushima's wet forest environment (Shaw 2012; Sutton 2025).
The principal weakness of the taxon is not the absence of distinctive morphology but the limited evidence that this morphology characterizes a coherent Yakushima population. Sutton specifically notes that it remains unclear whether all H. hydrangeoides on Yakushima correspond to the named variety or whether the type collection represents only one local expression. No population-genetic analysis of the entity was located.
Tree TSAR nevertheless retains the variety because two meaningful evidence axes currently coincide: an isolated island distribution and a distinctive developmental/morphological syndrome. That exceeds the evidentiary basis of the Ulleungdo Form, where the main morphological distinction overlaps typical Japanese plants and available molecular evidence points toward the Japanese population. The difference between the treatments is therefore deliberate rather than inconsistent.
Var. yakushimensis remains more weakly supported than var. taquetii. Its page should consequently make its provisional status visible. Broader collections from Yakushima could demonstrate that the dwarf juvenile phase and reduced adult foliage are island-wide characteristics, in which case the variety would become substantially stronger. Alternatively, discovery of extensive ordinary H. hydrangeoides intermixed with plants of the named morphology could favor reduction to a Tree TSAR Form.
Horticultural and Agricultural Uses
The type collection BSWJ 6119 has been propagated and distributed in European horticulture, providing unusually direct access to the material on which the varietal concept was founded. Its prolonged dwarf juvenile phase is horticulturally intriguing and could result in plants that establish differently from ordinary H. hydrangeoides before eventually adopting the familiar climbing architecture.
This cultivation history also makes careful labelling particularly important. Plants propagated from BSWJ 6119 document one wild Yakushima genotype or population; they should not be treated as proof that every wild plant on Yakushima belongs to the same variety. Retention of the collection number allows cultivated material to contribute meaningfully to later morphological and genomic work.
The compact juvenile habit and relatively small adult leaves could prove desirable horticultural traits if shown to persist consistently in cultivation, but the taxonomic narrative should remain separate from cultivar selection. A naturally distinctive population can be horticulturally valuable without every horticultural selection becoming a taxonomic entity.
Conservation Concerns
No formal conservation assessment was located for var. yakushimensis. Its known distribution is confined to Yakushima, but current evidence is insufficient to establish either the abundance of plants matching the varietal diagnosis or their distribution across the island. Consequently, neither rarity nor security should be inferred merely from the taxon's island-endemic treatment.
Yakushima's geographic isolation makes the population intrinsically valuable for understanding the diversification of H. hydrangeoides. Conservation collections should represent multiple independently collected wild plants rather than relying only on the cultivated type accession BSWJ 6119.
Field work should determine how consistently the dwarf juvenile architecture and smaller adult foliage occur across Yakushima, whether they correlate with particular elevations or habitats, and whether morphologically typical H. hydrangeoides occurs on the same island. Nuclear population genomics comparing Yakushima with Kyushu and other Japanese populations would provide the decisive test of formal varietal recognition.
Additional Information
- Plants of the World Online — Hydrangea hydrangeoides var. yakushimensis (opens in a new tab): Plants of the World Online — accepted name, Yakushima distribution, and homotypic synonym.
- Trees and Shrubs Online — Hydrangea hydrangeoides (opens in a new tab): Trees and Shrubs Online — original collection, morphology, habitat, cultivation, and explicit uncertainty over the breadth of the varietal concept.
References and Further Reading
Liu W, Zhu X-Y (2011) Leaf epidermal characters and taxonomic revision of Schizophragma and Pileostegia (Hydrangeaceae). Botanical Journal of the Linnean Society 165(3): 285–314. https://doi.org/10.1111/j.1095-8339.2010.01101.x (opens in a new tab)
Royal Botanic Gardens, Kew (2026) Hydrangea hydrangeoides var. yakushimensis () . Plants of the World Online. https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:77296096-1 (opens in a new tab)
Shaw JMH (2012) Climbing hydrangeas and their relatives. The Plantsman, New Series 11(4): 226–231. https://www.rhs.org.uk/about-us/pdfs/publications/the-plant-review/general-pdfs/index-for-2012.pdf (opens in a new tab)
Sutton J (2025) Hydrangea hydrangeoides. Trees and Shrubs Online. International Dendrology Society. https://www.treesandshrubsonline.org/articles/hydrangea/hydrangea-hydrangeoides/ (opens in a new tab)