Hydrangea strigosa Rehder — Late hydrangea

Introduction

Late hydrangea (Hydrangea strigosa Rehder) is a deciduous woody shrub of central China whose modern interpretation exposes one of the most important distinctions between a morphotype and an evolutionary lineage in Section Asperae. Rehder described the species in 1911 from Ernest Henry Wilson collections, with the type represented by Wilson 765 from western Hubei near Yichang. That type geography is crucial to understanding what the name should mean today.

Historically, botanists recognized H. strigosa primarily by its strigose indumentum: short, stiff, appressed white hairs covering branchlets, petioles, inflorescence branches, and leaf undersides. Because this character is readily visible on herbarium specimens, the name eventually became applied to plants over a broad region of China.

Molecular evidence demonstrates that those plants do not constitute one evolutionary lineage.

Character	H. aspera	H. aspera ssp. robusta	H. kawakamii	H. strigosa	H. villosa
Core geography	Mainland Asian complex	Eastern Himalaya–southern China–Indochina	Taiwan	Type-linked Hubei lineage	Central China
Indumentum	Variable	Variable, often dense	Erect hairs	Short, stiff, appressed strigose hairs	Long erect/villous hairs
Abaxial papillae	Absent as a diagnostic feature	Absent as a diagnostic feature	Small, inconspicuous	Conspicuous white papillae	Absent
Cytology	2n = 34/36 reported	2n = 34 reported	2n = 36	2n = 34	2n = 34
Molecular result	Permeable lineage complex	Not sufficiently separate	Distinct	Hubei lineage distinct; Sichuan morphotype not equivalent	Distinct
Tree TSAR rank	Species	Subspecies	Species	Species, narrowly circumscribed	Species

De Smet's analyses recovered two very different genetic histories among plants identified morphologically as H. strigosa. Hubei populations form a distinct lineage and are strongly supported by multilocus coalescent delimitation and RADseq data. They are sister to Taiwanese H. kawakamii. By contrast, Sichuan plants with the same general strigose appearance can cluster with H. aspera and ssp. robusta, especially where the morphotypes occur together.

The name must follow the Hubei lineage because the type originates there. Tree TSAR therefore uses H. strigosa for the type-linked Hubei evolutionary lineage, not for every plant possessing strigose hairs.

One morphological character helps bridge nomenclature and biology. True allopatric Hubei H. strigosa combines the stiff appressed hairs with conspicuous white papillae on the lower leaf surface, giving fresh leaves a whitish appearance. These papillae were consistently present in specimens belonging to the independently supported lineage and absent from the intermediate populations examined by De Smet.

The result is an unusually instructive taxonomic reversal. Strigose hairs—the character that caused botanists to identify geographically scattered plants as one species—are not themselves enough to identify the biological lineage. The less conspicuous papillae, type geography, and molecular ancestry are more informative.

Current POWO continues to use a geographically broad H. strigosa and accepts fourteen synonyms beneath it. Tree TSAR uses a narrower concept. Consequently, historical synonyms should be transferred into the narrowed species only when their type material or geographic provenance can be reconciled with the Hubei lineage; names inherited merely from a broad morphospecies treatment require individual audit.

Horticultural and Agricultural Uses

Plants circulated as H. strigosa have long been valued for rough-textured foliage and late-season flowers, but horticultural identification is especially problematic. Material from Sichuan that looks convincingly strigose can belong genetically with the H. aspera complex rather than the type-linked species. A garden label reading H. strigosa therefore says less about lineage identity than might normally be expected from a botanical name.

Historically identified plants tend to flower comparatively late and have sometimes proved less hardy than higher-elevation members of the complex, consistent with the lower-altitude ecology associated with the classic morphotype.

For collections attempting to represent true H. strigosa, provenance from Hubei and examination of the abaxial papillae are especially important. Garden material lacking those data is better treated cautiously than forced into the narrowed biological concept.

Conservation Concerns

No IUCN assessment specific to the narrowed Tree TSAR concept was located. During De Smet's field studies, sampled Hubei populations were large and did not show obvious major threats, leading the author to regard immediate conservation concern as limited.

Taxonomic precision nevertheless matters. A broad distribution assembled from every strigose specimen could substantially overestimate the geographic security of the type-linked lineage. Conservation mapping should therefore distinguish confirmed Hubei H. strigosa from morphologically similar populations elsewhere until their ancestry is tested.

The species also provides a broader conservation lesson: protecting “representative” hairy hydrangeas based only on gross morphology could fail to preserve the actual evolutionary lineages present within the complex.

Additional Information

Plants of the World Online — Hydrangea strigosa (opens in a new tab): Plants of the World Online: Current broad species treatment and extensive synonymy; Tree TSAR uses a narrower type-linked circumscription.

Trees and Shrubs Online — Hydrangea strigosa (opens in a new tab): Trees and Shrubs Online: Detailed morphological and horticultural account and discussion of the Hubei–Sichuan molecular split.

Chinese Flora — Hydrangea strigosa (opens in a new tab): Flora of China / Flora Reipublicae Popularis Sinicae: Traditional broad morphologically based treatment, useful for understanding the historical strigosa concept.

References and Further Reading

De Smet Y (2020) Through the fog: Evolutionary insights provide novel genus- and species-level boundaries in tribe Hydrangeeae and genus Hydrangea. PhD thesis, Ghent University, Faculty of Sciences, Ghent, Belgium. https://biblio.ugent.be/publication/8716125 (opens in a new tab)

De Smet Y, De Clerck O, Uemachi T, Granados Mendoza C, Wanke S, Goetghebeur P, Samain M-S (2017) Multilocus coalescent species delimitation to evaluate traditionally defined morphotypes in Hydrangea sect. Asperae (Hydrangeaceae). Molecular Phylogenetics and Evolution 114: 415–425. https://doi.org/10.1016/j.ympev.2017.05.021 (opens in a new tab)

McClintock E (1957) A monograph of the genus Hydrangea. Proceedings of the California Academy of Sciences, Fourth Series 29(5): 147–256. https://www.biodiversitylibrary.org/part/51707 (opens in a new tab) https://doi.org/10.5281/zenodo.15905786 (opens in a new tab)

Sutton J (2025) Hydrangea strigosa. Trees and Shrubs Online. International Dendrology Society. https://www.treesandshrubsonline.org/articles/hydrangea/hydrangea-strigosa/ (opens in a new tab)