Hydrangea arborescens L. — Smooth hydrangea

Introduction

Smooth hydrangea (Hydrangea arborescens L.) is a deciduous woody shrub of eastern North America and the most widespread member of the closely allied H. arborescens complex in Subgenus Hydrangea, Section Hydrangea. It inhabits deciduous woods, wooded slopes, shaded bluffs, ledges, and stream banks across a broad range of moisture conditions and elevations. Mature plants commonly form colonies through underground spread, while the aboveground framework remains fully woody, placing the species securely within Tree TSAR's woody tree, shrub, or vine growth-form category. Flora of North America recognizes H. arborescens, H. cinerea, and H. radiata as separate species rather than subspecies of a single variable H. arborescens.

The three species are unmistakably close relatives. McClintock (1957) treated them as subspecies, but Pilatowski (1982) argued for species rank based on combined chemical, morphological, reproductive, and geographic discontinuities. Flora of North America follows that interpretation, while importantly acknowledging occasional specimens intermediate between H. arborescens and H. cinerea. The resulting picture is not one of absolute reproductive isolation, but of closely neighboring evolutionary lineages that ordinarily remain morphologically recognizable despite some permeability at their boundaries.

The most useful distinction among the three is found on the underside of a mature leaf:

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 above follows the modern Flora of North America treatment.

In H. arborescens, the abaxial surface is characteristically green and glabrous or nearly so at maturity, sometimes retaining sparse hairs along the principal veins. This contrasts with the evenly gray-pubescent H. cinerea and especially the densely silver-white underside of H. radiata. The distinction is much more useful than gross leaf shape, which overlaps extensively among members of the complex.

The species has accumulated an unusually extensive nomenclatural history. Current Kew treatment places thirty names in synonymy, ranging from Hydrangea cordata Pursh and numerous varieties and forms of H. arborescens to several illegitimate or invalidly published names. These names reflect more than two centuries of attempts to partition the considerable morphological variation present across its broad range; they do not presently justify recognition of formal infraspecific taxa.

Horticultural and Agricultural Uses

Smooth hydrangea has become one of the most important North American species in ornamental Hydrangea. Its horticultural success rests partly on a useful biological feature: flowers are produced on current-season shoots, allowing plants to flower even after severe dormant-season pruning. The species is hardy, adaptable to woodland garden conditions, capable of spreading into broad colonies, and comparatively easy to incorporate into breeding programs.

The cultivated history is dominated by transformations of the wild lacecap-type inflorescence into densely sterile-flowered mopheads. ‘Grandiflora’, originally a wild discovery in Ohio, became an important early selection. It was eventually eclipsed by ‘Annabelle’, collected near Anna, Illinois, in 1910 and brought into broad horticultural circulation during the 1960s. ‘Annabelle’ became the benchmark against which later smooth hydrangeas were judged, although its extremely large inflorescences commonly overwhelm its relatively weak stems.

Modern breeding has consequently emphasized stronger stems, compact growth, remontant flowering, and flower colors beyond the traditional white. North Carolina State University breeding used ‘Annabelle’ and several pink-flowered selections to develop successive pink and red-pink smooth hydrangeas, while induced polyploidy was used to generate low-fertility triploid selections. This breeding history illustrates how dramatically horticulture can reshape a wild lineage while leaving its underlying species identity intact.

Cultivated plants attributed to the arborescens complex nevertheless require some taxonomic caution. H. cinerea and H. radiata can be crossed with H. arborescens, and the ancestry of some older garden material is not fully documented. Cultivar identity should therefore remain conceptually separate from the biological limits of the three wild species.

Smooth hydrangea also has a documented history of Indigenous use. Flora of North America records the use of bark and sometimes leaves by Cherokee and Delaware people in preparations for several purposes, as well as Cherokee use of young woody material. These records are important ethnobotanical history rather than evidence for modern medical efficacy.

Conservation Concerns

No evidence reviewed here suggests an immediate species-wide conservation crisis for H. arborescens. It occupies by far the largest range of the three members of its complex, and in parts of the southern Appalachians and Piedmont it can be locally abundant. Cultivated abundance, however, should not substitute for preservation of wild geographic diversity.

The more consequential unresolved issue is evolutionary rather than purely demographic. Range-wide nuclear population genomics could determine how much gene flow occurs between H. arborescens and H. cinerea, whether apparent intermediates represent contemporary hybridization or older introgression, and whether geographically structured lineages occur within the currently broad H. arborescens concept. Such work could alter conservation priorities even if species rank itself remains unchanged.

Additional Information

iNaturalist — Hydrangea arborescens (opens in a new tab): iNaturalist: Field observations and photographs of smooth hydrangea; cultivated observations should be separated from wild records when interpreting distribution.

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

Flora of North America — Hydrangea arborescens (opens in a new tab): Flora of North America: Detailed morphology, habitat, distribution, and treatment of the H. arborescens complex.

Trees and Shrubs Online — Hydrangea arborescens (opens in a new tab): Trees and Shrubs Online: Detailed systematic and horticultural treatment, including the history of the major cultivars and modern breeding.

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)

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