Hydrangea hypoglauca Rehder — Whiteback hydrangea

Introduction

Whiteback hydrangea (Hydrangea hypoglauca Rehder) is a deciduous shrub of mountainous central and southwestern China. Its gray-white lower leaf surfaces suggested both its scientific epithet and its Tree TSAR common name, but the most taxonomically important feature is finer: the lower surface is characteristically glandular-granular. Its capsules have a conspicuous conical apical projection, and its fusiform seeds are longitudinally striate and conspicuously winged at both ends. Those reproductive characters align remarkably well with the modern diagnosis of Subgenus Heteromallae.

That morphological evidence conflicts with the available molecular result. Yang et al. (2024) sampled one accession identified as H. hypoglauca and recovered its plastome not with Heteromallae, but within Section Chinenses of Subgenus Dichroa. Tree TSAR therefore places H. hypoglauca provisionally in Subgenus Heteromallae, rather than treating the plastid topology as automatically decisive.

Several explanations remain viable. The sequenced specimen could have been misidentified; the species could retain a plastome acquired through ancient introgression or chloroplast capture; hybrid ancestry could have produced discordant nuclear and organellar histories; or geographically differentiated populations currently assigned to H. hypoglauca could conceal more than one lineage. Re-examination of the sequenced voucher together with type-anchored nuclear sampling across the species' range would provide the decisive test.

Morphologically, H. hypoglauca is most readily contrasted with H. sungpanensis. Both have glandular lower leaf surfaces, but H. hypoglauca is glandular-granular and has slender or only slightly enlarged fruiting styles, whereas H. sungpanensis is glandular-papillate and develops conspicuously club-shaped styles.

Horticultural and Agricultural Uses

Reliable horticultural information attributable specifically to H. hypoglauca is scarce. Material of the wider heteromalla complex has historically circulated under inconsistent names, making old cultivated records difficult to interpret.

Its broad elevational occurrence and attractive pale lower leaf surfaces suggest ornamental potential, but the species should not be assigned cultivation characteristics merely by analogy with H. heteromalla. Documented wild-origin material would be especially valuable because living collections could help test both its morphology and its disputed phylogenetic placement.

Conservation Concerns

The Flora of China records H. hypoglauca across several provinces and an exceptionally broad reported elevational range. No clear evidence presently indicates that the species as currently circumscribed is narrowly endemic.

Its most important conservation uncertainty is taxonomic. If geographically distinct populations prove to possess different genomic histories, the current broad name may mask more localized evolutionary units. Until that question is tested, conservation interpretation should emphasize provenance and population identity rather than treating all records as interchangeable.

Additional Information

Flora of China (opens in a new tab): detailed morphology, range, habitat, and the characters distinguishing H. hypoglauca from H. sungpanensis.

Plants of the World Online (opens in a new tab): accepted species status, distribution, and four-name synonym roster.

Yang et al. (2024) (opens in a new tab): source of the anomalous plastome placement within Section Chinenses.

References and Further Reading

Rehder A (1911) Hydrangea. In: Sargent CS (ed.) Plantae Wilsonianae, vol. 1, part 1. The University Press, Cambridge, Massachusetts, pp. 25–41. https://archive.org/details/plantaewilsonian11sargrich (opens in a new tab)

Wei Z, Bartholomew B (2001) Hydrangea. In: Wu ZY, Raven PH (eds.) Flora of China, vol. 8. Science Press, Beijing & Missouri Botanical Garden Press, St. Louis, pp. 411–422. https://www.iplant.cn/foc/pdf/Hydrangea.pdf (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)