Hydrangea cinerea Small — Ashy hydrangea

Introduction

Ashy hydrangea (Hydrangea cinerea Small) is a deciduous woody shrub of the east-central and southeastern United States and the gray-leaved member of the Hydrangea arborescens complex. It belongs to Subgenus Hydrangea, Section Hydrangea, and shares the general architecture of smooth hydrangea: opposite unlobed leaves, terminal dome-shaped inflorescences, completely or nearly completely inferior ovaries, and a persistent woody framework characteristic of Tree TSAR's woody tree, shrub, or vine growth-form category.

Its most reliable distinction is the indumentum of the mature leaf underside. H. cinerea has a grayish abaxial surface that is uniformly velutinous, pilose, or tomentose. The hairs are conspicuous across the surface but do not produce the dense white felt characteristic of H. radiata. In H. arborescens, by contrast, mature leaves are green beneath and glabrous or nearly so except sometimes along the main veins.

Character	H. arborescens	H. cinerea	H. radiata
Mature leaf underside	Green; glabrous or becoming nearly so, sometimes sparsely hairy along the main veins	Grayish; evenly velutinous, pilose, or tomentose, but not densely felted	White to grayish; densely tomentose and conspicuously felted
Relative distribution	Broadest-ranging member	More restricted and generally more southern/interior	Narrow southern Appalachian range
Intermediacy	Occasional intermediates with H. cinerea	Occasional intermediates with H. arborescens	Usually conspicuous morphologically
Historical alternative	H. arborescens ssp. arborescens	H. arborescens ssp. discolor	H. arborescens ssp. radiata
Tree TSAR treatment	Species	Species	Species

The diagnostic synthesis follows Flora of North America, which recognizes all three at species rank.

The history of H. cinerea illustrates the difficulty of translating a subtle but persistent biological distinction into nomenclature. Seringe described the plant as H. arborescens var. discolor in 1830, and McClintock later treated it as H. arborescens subsp. discolor. Small's H. cinerea, published in 1898, is the accepted species name. Pilatowski's subsequent study argued that the gray-leaved plants represent a distinct species on combined chemical, morphological, reproductive, and geographic evidence rather than merely one endpoint in a continuum of H. arborescens.

The separation is not absolute. Flora of North America notes occasional specimens intermediate between H. cinerea and H. arborescens, which is entirely compatible with closely related species retaining limited gene flow. For Tree TSAR, that permeability is a reason to maintain a review trigger, not a reason to erase a lineage that remains ordinarily diagnosable across much of its range.

Current Kew treatment places six names in synonymy, including H. arborescens var. discolor, H. arborescens subsp. discolor, H. arborescens var. deamii, and H. ashei. The synonymy records repeated historical recognition of the same gray-pubescent element at different ranks.

Horticultural and Agricultural Uses

Ashy hydrangea occupies only a small place in mainstream horticulture compared with H. arborescens. Wild-type plants occur principally in specialist collections, botanic gardens, and collections assembled to represent species diversity rather than mass-market ornamentals. This relative obscurity should not be confused with horticultural inadequacy; its restrained inflorescences and gray-backed foliage simply lack the exaggerated mophead form that made H. arborescens famous.

A notable exception is the sterile-flowered horticultural lineage historically called ‘Sterilis’. Sterile forms of H. cinerea have been cultivated since at least the early twentieth century, and the botanical names H. cinerea f. sterilis and var. sterilis are now treated as synonyms rather than accepted infraspecific taxa.

Cultivation also creates a taxonomic complication. The three members of the arborescens complex are sufficiently compatible for hybridization to occur in gardens, while the identity of older cultivated accessions is not always secure. Plants labeled simply “H. arborescens” may occasionally belong to H. cinerea or include mixed ancestry. Careful examination of mature leaf undersides therefore remains useful even in collections.

Conservation Concerns

NatureServe currently assigns Hydrangea cinerea a global rank of G5, indicating species-wide security, but its state-level ranks reveal a more uneven picture. The species is ranked S2 in North Carolina and S1 in South Carolina, while several other states either lack a current rank or record historical status. The contrast is a useful reminder that a globally secure species can still be locally uncommon or vulnerable near portions of its range.

Taxonomic resolution matters to that interpretation. If gray-leaved populations are routinely submerged under H. arborescens in inventories or cultivated collections, the true distribution and abundance of H. cinerea can be obscured. Future nuclear population-genomic work across contact zones would be especially valuable for distinguishing pure populations, introgressed populations, and morphological intermediates and for testing whether any additional geographic structure deserves recognition.

Additional Information

iNaturalist — Hydrangea cinerea (opens in a new tab): iNaturalist: Observation-based resource useful for photographs and locality leads; identifications within the arborescens complex should be checked against mature leaf indumentum.

Plants of the World Online — Hydrangea cinerea (opens in a new tab): Plants of the World Online: Accepted name, distribution, and current synonymy.

Flora of North America — Hydrangea cinerea (opens in a new tab): Flora of North America: Detailed morphological treatment and species-level interpretation of the H. arborescens complex.

Trees and Shrubs Online — Hydrangea cinerea (opens in a new tab): Trees and Shrubs Online: Taxonomic and horticultural synthesis, including the history of discolor, H. cinerea, and cultivated ‘Sterilis’.

NatureServe Explorer — Hydrangea cinerea (opens in a new tab): NatureServe Explorer: Current global and subnational conservation ranks.

References and Further Reading

Freeman CC (2016) Hydrangeaceae Dumortier. In: Flora of North America Editorial Committee (eds.) Flora of North America North of Mexico, vol. 12. Oxford University Press, New York & Oxford, pp. 462–491. https://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=10425 (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)

Pilatowski RE (1982) A taxonomic study of the Hydrangea arborescens complex. Castanea 47(1): 84–98. https://efloras.org/florataxon.aspx?flora_id=1&taxon_id=115977 (opens in a new tab)

Small JK (1898) Studies in the botany of the southern United States—XIII. Bulletin of the Torrey Botanical Club 25(3): 134–151. https://doi.org/10.2307/2477711 (opens in a new tab)

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