Hydrangea aspera ssp. robusta (Hook.f. & Thomson) E.M.McClint. — Tall hydrangea

Introduction

Tall hydrangea (Hydrangea aspera ssp. robusta (Hook.f. & Thomson) E.M.McClint.) is the large, robustly built geographic expression of H. aspera extending from the eastern Himalaya through southern China and into Indochina. Hooker and Thomson originally described it as the species H. robusta in 1857, and current Plants of the World Online continues to accept it at species rank. Tree TSAR instead follows the integrative interpretation of De Smet, recognizing the morphological and altitudinal distinction while treating it as a subspecies of H. aspera.

Typical plants are imposing shrubs and occasionally small trees, with stout, often conspicuously four-angled young stems, thick petioles that may be quite long, and comparatively large, broadly ovate to elliptic leaves. The lower leaf surface varies from densely gray-white woolly to more sparsely clothed in stiff brownish hairs. Fertile flowers are often purple or otherwise pigmented rather than white.

Character	H. aspera	H. aspera ssp. robusta	H. kawakamii	H. strigosa	H. villosa
Core geography	Himalaya and mainland Asian complex	Eastern Himalaya through southern China and Indochina	Taiwan	Type-linked Hubei lineage	Central China; exact range needs refinement
Leaf underside	Variable appressed to more erect hairs	Often densely hairy; leaves commonly broad and substantial	Long erect hairs; small inconspicuous papillae	Short stiff appressed hairs plus conspicuous white papillae	Long erect/villous hairs, especially brownish on major veins
Habit	Variable shrub/small tree	Usually especially stout and tall	Large spreading shrub	Shrub	Usually compact to moderate shrub
Cytology	2n = 34 or 36 reported	2n = 34 reported in studied material	2n = 36	2n = 34	2n = 34
Molecular evidence	Genetically permeable complex	Insufficient independence for species rank	Strong lineage	Hubei lineage strong	Strong lineage
Tree TSAR rank	Species	Subspecies	Species	Species	Species

The subspecies is easily confused with H. longipes, because both can have strikingly elongated petioles. They are not the same taxonomic problem. H. longipes is generally thinner-leaved, has slender petioles, and usually white fertile flowers; ssp. robusta typically has more substantial leaves, thicker petioles and shoots, denser pubescence, and pigmented fertile flowers. Modern floristic treatments often recognize those morphologies as separate species-level taxa under H. robusta and H. longipes.

Its relationship with H. aspera sensu stricto is much less clear. Both traditional multilocus data and higher-resolution RADseq markers fail to demonstrate a consistently independent robusta lineage where populations occur near one another. Numerous intermediates link the morphology, and the genetic signal is compatible with substantial gene flow. De Smet found only limited morphological differentiation and some phylogenetic support for nominal robusta, insufficient to outweigh the broader evidence of shared ancestry and genomic permeability.

Subspecies rank therefore records something real without exaggerating it. The robust plants occupy a recognizable morphological and altitudinal portion of the variation, but current evidence does not demonstrate species-level independence.

The principal names associated with this concept include H. robusta, H. longialata, H. maximowiczii, H. rosthornii, H. rotundifolia, and H. robusta var. griffithii. Kew presently places these beneath accepted H. robusta rather than beneath H. aspera.

Horticultural and Agricultural Uses

Tall hydrangea can become one of the most imposing members of the aspera complex in a sheltered woodland garden. Stout stems, broad leaves, long petioles, and large flower heads produce a heavier architectural effect than the finer-textured H. longipes or H. villosa. Well-grown plants may reach several metres and need considerably more space than familiar compact garden hydrangeas.

Horticultural identification has historically been unreliable. Plants distributed as H. robusta, broad H. aspera ssp. robusta, or unrelated material have sometimes been mixed under the same names. Identification should therefore rely on morphology and provenance rather than nursery labels alone.

Because Tree TSAR treats robusta as a subspecies, cultivated material showing its characteristic robust morphology can still be discussed meaningfully without implying complete evolutionary isolation. This is precisely one of the purposes of subspecific rank.

Conservation Concerns

No formal IUCN assessment specific to Tree TSAR's H. aspera ssp. robusta was located. POWO's currently accepted species H. robusta has a broad reported distribution from eastern Nepal through southern China and Indochina, and Kew's machine-learning prediction does not suggest immediate species-wide extinction risk, but that prediction is not a formal Red List assessment.

The more pressing scientific issue is taxonomic resolution. Sampling from the type regions of H. aspera and H. robusta remains incomplete, and those populations are particularly important because a change in their relationships could alter the most appropriate rank. Future nuclear population genomics should sample the Himalayan extremes as well as Chinese regions of sympatry and morphological intermediacy.

Additional Information

Plants of the World Online — Hydrangea robusta (opens in a new tab): Plants of the World Online: Alternative species-rank treatment as Hydrangea robusta, including distribution and synonymy.

Trees and Shrubs Online — Hydrangea robusta (opens in a new tab): Trees and Shrubs Online: Morphology, cultivation, and the distinction between H. robusta and H. longipes.

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