Hydrangea marunoi
Kimotsuki herb hydrangea
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Introduction
The Kimotsuki herb hydrangea, Hydrangea marunoi, is a narrowly endemic member of section Cardiandra known from the Kimotsuki Mountains of the Ōsumi Peninsula in southern Kyushu. Described in 2022, it demonstrates how substantial evolutionary diversity can remain concealed among superficially similar Japanese Cardiandra (Tagane et al. 2022).
Its marginal flowers commonly have three enlarged petaloid calyx lobes, making it immediately reminiscent of H. alternifolia. Detailed morphology separates the two: H. marunoi has more stamens, as well as longer styles and seeds. The distinction is reinforced by MIG-seq data, which recovered H. marunoi as a genetically coherent lineage more closely allied to H. amamiohsimensis and H. moellendorffii than to H. alternifolia (Tagane et al. 2022).
That relationship is also ecologically suggestive. Hydrangea marunoi occurs mainly on wet, semi-shaded rocks beside streams, resembling the humid streamside environments occupied by H. amamiohsimensis on Amami-Ōshima and H. moellendorffii on Iriomote. Its apparently familiar three-sepaled flowers therefore sit on a lineage whose genetic and ecological affinities lie to the south rather than with the most similar-looking Japanese species.
The protologue described H. marunoi as a “rhizomatous perennial herb,” but this terminology does not establish a fundamentally different growth architecture from the rest of section Cardiandra. Tree TSAR therefore retains it as a perennial subshrub from a woody base. Its persistent architecture should be reevaluated only if direct anatomical or developmental evidence demonstrates a genuinely herbaceous perennial structure.
Horticultural and Agricultural Uses
Because H. marunoi was described only recently and has a very limited wild range, it has little established horticultural history. Its immediate value is more botanical and conservation-oriented than commercial.
Its natural occurrence beside humid, shaded streams provides a starting point for specialist cultivation, but detailed horticultural prescriptions would presently go beyond the evidence. Provenance-documented plants in legitimate ex situ collections could be particularly valuable because they would permit observation of mature architecture while preserving material from a geographically restricted lineage.
Conservation Concerns
When the species was described, Tagane et al. (2022) provisionally evaluated it as Vulnerable under IUCN criterion D. They reported fewer than 1,000 individuals, an estimated extent of occurrence of about 162 km², and an area of occupancy of about 40 km², while noting that some populations occurred inside protected areas.
That preliminary assessment should no longer be presented as its most current Japanese status. The Ministry of the Environment's 2025 Fifth Red List lists Hydrangea marunoi as Endangered (EN). Ministry of the Environment, Japan (opens in a new tab) The change strengthens the conservation emphasis warranted by its narrow Kimotsuki distribution.
Protection of multiple natural populations and their streamside habitats is especially important. Ex situ propagation can provide insurance and research material, but extensive collection from such a localized species could itself become harmful if not carefully controlled.
Additional Information
References and Further Reading
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)
Tagane S, Fujii S, Hirota SK, Naiki A, Yahara T (2022) Hydrangea marunoi (Hydrangeaceae), a new species from Osumi Peninsula, southern Japan. PhytoKeys 211: 33–44. https://doi.org/10.3897/phytokeys.211.89452 (opens in a new tab)
Yang X, Xue T, Gao T, Liang Y, Smets EF, Gadagkar SR, Yu S (2024) An updated infrageneric classification based on phylogenomics and character evolution in Hydrangea (Hydrangeaceae). Taxon 73(2): 503–518. https://doi.org/10.1002/tax.13158 (opens in a new tab)