Synonyms
- Hortensia × mizushimarum ()
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Introduction
Mizushima hydrangea, Hydrangea × mizushimarum H.Ohba, is a naturally occurring Japanese hybrid between Hydrangea hirta and H. serrata. The name was formally published by Hideaki Ohba in Flora of Japan in 2001, based on material collected by Masami and Urara Mizushima at Hanaishi–Jakko-no-taki, Nikkō, Tochigi Prefecture, on 11 August 1952. The holotype is preserved at the University of Tokyo herbarium.
Hybrid formula: Hydrangea hirta × Hydrangea serrata
Under the modern infrageneric classification, this is a cross between Section Hirtae and Section Macrophyllae. The parents are readily distinguishable as ordinary species. H. hirta is a small, densely branched Japanese shrub with hairy, nettle-like leaves and inflorescences composed entirely of fertile flowers. H. serrata is the familiar mountain hydrangea, usually with a lacecap inflorescence containing enlarged marginal flowers. Their naturally occurring hybrids consequently bridge not only two species but two deeply distinct sectional lineages.
POWO accepts both the nothospecies and its parental formula, and recognizes only one homotypic synonym, Hortensia × mizushimarum (H.Ohba) H.Ohba & S.Akiyama. Its accepted native range is central Honshu.
Compared with many better-known hydrangea hybrids, H. × mizushimarum is poorly represented in the horticultural and experimental literature. This is important in itself: the parental hypothesis derives principally from field taxonomy and morphological intermediacy rather than from a modern genomic admixture analysis. The parentage is highly plausible and has been consistently accepted by Japanese and international treatments, but genomic verification would provide an additional independent test.
The hybrid also demonstrates that the species boundary around H. serrata is permeable in more than one direction. Mountain hydrangea can exchange genes naturally with H. macrophylla within Section Macrophyllae, but H. × mizushimarum shows that reproductive compatibility also extends outward to H. hirta in another section. Natural hybridization therefore crosses both shallow and deeper phylogenetic boundaries within the Japanese hydrangeas.
Whether such crosses routinely backcross into either parent remains poorly documented. That question matters because a named nothospecies does not necessarily correspond to a single stable lineage: individual populations may consist of repeated first-generation hybrids, later-generation recombinants, backcrosses, or some combination of these.
Horticultural and Agricultural Uses
Mizushima hydrangea is uncommon in cultivation and is principally encountered among specialist collectors of Japanese hydrangeas. Japanese horticultural sources recognize it among the naturally occurring H. hirta hybrid combinations, but it has not become an important commercial ornamental.
Its breeding interest is greater than its commercial history suggests. The cross combines the fine, densely branched habit and all-fertile inflorescences of H. hirta with the larger lacecap architecture, flower-color diversity, and cold-adapted mountain ecology of H. serrata. If fertile later-generation plants occur, they could provide a natural bridge for transferring traits between Sections Hirtae and Macrophyllae.
For living collections, wild provenance remains particularly valuable because the name denotes a natural hybrid combination rather than a uniform cultivar.
Conservation Concerns
No independent conservation status is appropriate for the nothospecies on present evidence. Its persistence depends upon continued coexistence of H. hirta and H. serrata in suitable central-Honshu forest habitats.
Natural hybrid populations nevertheless have genuine conservation value. They document evolutionary processes that disappear when parent populations become geographically isolated or their habitats are fragmented. Preserving sites containing both parental species therefore conserves the possibility of continuing hybrid formation as well as the species themselves.
The type locality near Nikkō also has historical value because it anchors the name to a geographically and botanically documented natural hybrid population.
Additional Information
References and Further Reading
Ohba H (2001) Hydrangea. In: Iwatsuki K, Boufford DE, Ohba H (eds.) Flora of Japan, vol. IIb. Kodansha, Tokyo, pp. 84–94. https://www.kspub.co.jp/book/detail/1546059.html (opens in a new tab)
Ohba H, Akiyama S (2016) Generic segregation of some sections and subsections of the genus Hydrangea (Hydrangeaceae). Journal of Japanese Botany 91(6): 345–350. https://doi.org/10.51033/jjapbot.91_6_10703 (opens in a new tab)