Hydrangea hydrangeoides Ulleungdo Form — Ulleung hydrangea vine

Introduction

The Ulleungdo Form represents the geographically isolated Hydrangea hydrangeoides population of Ulleungdo, a volcanic island east of the Korean Peninsula. Julian Shaw described this population formally as Schizophragma hydrangeoides var. ullungdoense in 2012, later transferring it to Hydrangea as var. ullungdoensis. Kew continues to accept that variety. Tree TSAR instead recognizes the population as the editorial Ulleungdo Form, preserving its distinctive geography and horticultural identity while treating the currently demonstrated biological differentiation as insufficient for formal varietal rank.

Shaw's variety was based on BSWJ 8522, collected on Ulleungdo and subsequently cultivated at Crûg Farm. Unlike Jeju var. taquetii, Ulleungdo plants lack the conspicuous prolonged dwarf juvenile phase. Their principal proposed distinction is unusually luxuriant growth and relatively large leaves, reported at approximately 9.5–17 × 8–14 cm. These dimensions are impressive in cultivated plants but overlap the upper end of the range reported for ordinary Japanese H. hydrangeoides. BSWJ 8505 represents another documented Ulleungdo collection (Shaw 2012; Sutton 2025).

The available molecular evidence further weakens the argument for formal rank. Sutton's synthesis of Joung et al. (2010) reports that Ulleungdo material clustered with Japanese H. hydrangeoides rather than with the Jeju population recognized as var. taquetii. The study used RAPD markers and rbcL variation rather than modern population genomic methods, so it cannot prove that the Ulleungdo population lacks genetic distinctness. It does, however, mean that the only clearly demonstrated axis of strong differentiation is geography, while the proposed leaf-size difference overlaps ordinary Japanese variation.

Tree TSAR therefore regards the former variety as meaningful but over-ranked. The Ulleungdo population is not erased into anonymous synonymy because it is a discrete island population with a historical name, recognizable horticultural material, and obvious biogeographic interest. The Form page allows those facts to be preserved while accurately communicating that its evolutionary independence has not been demonstrated.

This treatment is especially appropriate because biological significance does not require formal taxonomic rank. Ulleungdo has a distinctive island flora, and an isolated population of a widespread species can be valuable for understanding colonization, founder effects, and local adaptation even when it remains genetically nested within the parent species. Nuclear population genomics could eventually demonstrate enough structured divergence to restore varietal or higher rank; equally, it could confirm that the island population is essentially a robust local expression of the Japanese lineage.

Horticultural and Agricultural Uses

The Ulleungdo population has a substantial specialist horticultural identity. Crûg Farm has grown documented wild-origin material, including BSWJ 8522 and BSWJ 8505, and cultivated plants can develop into impressive wall specimens with exceptionally lush foliage. This horticultural visibility is one reason Tree TSAR retains a dedicated Form page rather than reducing the former varietal names to an unexplained synonym.

The Royal Horticultural Society also maintains a horticultural entry for Hydrangea hydrangeoides “from Ulleungdo, Korea,” recording earlier labels including Schizophragma hydrangeoides ‘Korean’ and “Korean form.” These are horticultural aliases, not botanical synonyms, but they demonstrate that Ulleungdo material has a persistent identity in cultivation independent of the formal varietal name.

For Tree TSAR, the Form provides a place to connect those cultivated plants with their wild geographical origin while avoiding the stronger implication of a botanical variety. Cultivated accessions with reliable collection numbers should be particularly valuable for future genomic comparison with Korean and Japanese populations.

Conservation Concerns

The Ulleungdo Form has no separate formal conservation assessment. Available ecological evidence does not suggest that H. hydrangeoides is exceptionally scarce on the island. A long-term study of broadleaved forest understory dynamics on Ulleungdo included Schizophragma hydrangeoides among the major woody understory species and identified it as the dominant species at the particular survey site. This should not be extrapolated to every part of the island, but it demonstrates that at least some Ulleungdo populations can be locally abundant (Jung et al. 2021).

The conservation importance of the Form therefore lies less in demonstrated rarity than in preserving a geographically isolated component of species diversity. Its island provenance should be maintained in seed banks, living collections, and herbarium records rather than homogenized under generic Japanese H. hydrangeoides.

A modern population-genomic study comparing multiple Ulleungdo populations with Hokkaido, Honshu, western Japanese, and Jeju material is the principal review trigger. Discovery of sustained genomic differentiation concordant with morphology could justify restoration of formal rank.

Additional Information

Plants of the World Online — Hydrangea hydrangeoides var. ullungdoensis (opens in a new tab): Plants of the World Online — current external treatment accepting the Ulleungdo population as var. ullungdoensis.

RHS — Hydrangea hydrangeoides from Ulleungdo, Korea (opens in a new tab): Royal Horticultural Society — horticultural record for wild-origin Ulleungdo material and its historical garden aliases.

Trees and Shrubs Online — Hydrangea hydrangeoides (opens in a new tab): Trees and Shrubs Online — diagnosis, molecular context, and cultivated collections.

References and Further Reading

Joung YH, Suh JK, Lee NS, Eum SM, Choi I-Y, Roh MS (2010) Identification of Hydrangeaceae accessions of wild origin from Jeju, Korea, using molecular markers. Plant Genetic Resources 8(3): 235–241. https://doi.org/10.1017/S1479262110000286 (opens in a new tab)

Jung S, Sim HS, Kim JS, Bae KH, Cho Y-C (2021) Processes driving understory community dynamics in Ulleungdo Island broadleaved forest, South Korea. Ecological Research 36(4): 686–700. https://doi.org/10.1111/1440-1703.12231 (opens in a new tab)

Royal Botanic Gardens, Kew (2026) Hydrangea hydrangeoides var. ullungdoensis (J.M.H.Shaw) J.M.H.Shaw. Plants of the World Online. https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:77296095-1 (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)