Hydrangea caerulea
Blue herb hydrangea
Synonyms
- Deinanthe caerulea
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Introduction
The blue herb hydrangea, Hydrangea caerulea, is a rhizomatous herbaceous perennial restricted to western Hubei in central China. Together with the Japanese H. bifida, it forms section Deinanthe, the wholly herbaceous branch of subgenus Cardiandra. Its seasonal aerial stems arise from persistent underground rhizomes rather than from the suffrutescent framework characteristic of section Cardiandra. POWO accepts the species in broad Hydrangea and treats Deinanthe caerulea Stapf as its homotypic synonym (Royal Botanic Gardens, Kew 2026).
The species inhabits moist montane valleys and forests, historically reported particularly from western Hubei. Although it shares the remarkable bifid-leaf morphology of H. bifida, it is not a recently isolated Chinese counterpart of the Japanese plant. Genomic evidence indicates that the two Deinanthe lineages diverged approximately 7–9 million years ago in the late Miocene, after which they survived as widely separated East Asian climate relicts (Sakaguchi et al. 2021).
Their later demographic histories were quite different. The central-Chinese lineage retained much less genetic diversity than the Japanese populations sampled by Sakaguchi et al. (2021), and demographic reconstruction indicated a pronounced Holocene decline associated with isolation in montane refugia. The two species therefore tell parallel but distinctly different stories of survival through East Asia's climatic history.
Horticultural and Agricultural Uses
Blue herb hydrangea is cultivated as a specialized shade perennial for its broad bifid foliage and blue to bluish flowers. Its natural habitat suggests cool, sheltered, reliably moist conditions rather than the exposed situations tolerated by some familiar woody hydrangeas.
The distinction between clonal and sexual propagation has particular conservation importance here. Division is appropriate for maintaining a desirable garden clone, but it produces copies of the same genotype. Seed-grown collections can preserve recombination among parents and are more useful for representing wild genetic diversity, provided that the source populations are documented.
Conservation Concerns
The restricted distribution of H. caerulea is compounded by evidence of low genetic diversity and limited exchange among surviving populations. Sakaguchi et al. (2021) reported markedly lower diversity in the central-Chinese lineage than in H. bifida and reconstructed a strong decline during the Holocene.
More detailed field and experimental work has reinforced that concern. Xu et al. (2025) found low genetic diversity and low interpopulation gene flow, together with reproductive constraints including pollen limitation, substantial seed abortion, and poor survival during natural regeneration. The authors identified anthropogenic disturbance, habitat loss and fragmentation, geographic isolation, and reproductive bottlenecks as interacting drivers of endangerment.
These findings make conservation of several wild populations especially important. Protecting habitat without maintaining connectivity and genetic representation could leave isolated populations vulnerable even where adult plants persist. Ex situ conservation should likewise avoid overrepresenting a few easily propagated clones at the expense of population-level diversity.
Additional Information
References and Further Reading
De Smet Y, Granados Mendoza C, Wanke S, Goetghebeur P, Samain M-S (2015) Molecular phylogenetics and new (infra)generic classification to alleviate polyphyly in tribe Hydrangeeae (Cornales: Hydrangeaceae). Taxon 64(4): 741–753. https://doi.org/10.12705/644.6 (opens in a new tab)
Sakaguchi S, Asaoka Y, Takahashi D, Isagi Y, Imai R, Nagano AJ, Qiu Y-X, Li P, Lu R, Setoguchi H (2021) Inferring historical survivals of climate relicts: The effects of climate changes, geography, and population-specific factors on herbaceous hydrangeas. Heredity 126(4): 615–629. https://doi.org/10.1038/s41437-020-00396-4 (opens in a new tab)
Wei Z, Bartholomew B (2001) Cardiandra, Deinanthe, Hydrangea, Platycrater, Schizophragma. In: Wu ZY, Raven PH (eds.) Flora of China, vol. 8. Science Press, Beijing & Missouri Botanical Garden Press, St. Louis, pp. 406–422. https://www.iplant.cn/foc/pdf/Saxifragaceae.pdf (opens in a new tab)
Xu J, Li T-T, Xiao P-N, Cheng C-H, Liu T, Wang Z-X (2025) Study on the endangering and driving factors of Deinanthe caerulea, a rare and endemic plant in China. Acta Ecologica Sinica 45(9): 4517–4534. https://doi.org/10.20103/j.stxb.202311032390 (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)