Hydrangea stenophylla Merr. & Chun — Willowleaf hydrangea

Introduction

Willowleaf hydrangea (Hydrangea stenophylla) is a slender woody shrub of southern China that Tree TSAR recognizes as a distinct species in Hydrangea subg. Dichroa sect. Chinenses. Current Plants of the World Online treats the name as a synonym of H. pottingeri, but the combination of historical morphology and modern molecular evidence makes continued species recognition more defensible.

Plants are approximately 0.8–2 m tall. First-year shoots are purplish, while second-year stems are characteristically pale to white, glabrous, and non-exfoliating. The narrow lanceolate leaves are commonly 8–20 cm long but only 1–2.7 cm wide, with 7–10 pairs of secondary veins and slightly reflexed, sparsely toothed margins. The terminal corymbs are borne on conspicuous peduncles approximately 4–12 cm long. Marginal sterile flowers usually have three or four unequal enlarged sepals, while the fertile flowers have green-white petals, 8–10 subequal stamens, three or four styles, and a nearly half-superior ovary. The broadly ellipsoid capsules contain small wingless, reticulate seeds.

Its documented classical distribution is much narrower than that of H. pottingeri: northern and western Guangdong and southwestern Jiangxi, principally in forest and thickets at about 700–800 m. Flora of China retained it as a separate species, while later backbone treatments absorbed it into H. pottingeri.

Yang et al. (2024) changed the evidentiary balance. Their expanded plastid sampling treated H. stenophylla independently and recovered it within sect. Chinenses but not with the sampled core H. chinensis lineage corresponding to H. pottingeri. In their tree, H. stenophylla instead groups with the accession representing H. vinicolor. The relevant node is maximally supported under both Bayesian and maximum-likelihood analyses. Because the study is plastid-based, this does not exclude chloroplast capture, but it provides independent molecular evidence that the traditional stenophylla morphotype is not simply ordinary variation surrounding the sampled core pottingeri lineage.

Comparison with Hydrangea pottingeri

Character	**Hydrangea pottingeri**	**Hydrangea stenophylla**
Habit	Shrub ca. 0.5–4 m	Shrub ca. 0.8–2 m
Younger stems	Reddish to brown	First-year shoots purplish
Second-year stems	Brownish; bark commonly exfoliating	Usually gray-white to white; bark not peeling at this stage
Leaves	Lanceolate, narrowly elliptic or obovate; usually 5–12 × 1.5–4 cm	Narrowly lanceolate to lanceolate; usually 8–20 × 1–2.7(–4.5) cm
Lateral veins	Usually 4–7 pairs	Usually 7–10 pairs
Inflorescence	Terminal, essentially sessile	Pedunculate, peduncle ca. 4–12 cm
Fertile petals	Yellowish, ca. 3 mm	Green-white, ca. 3–4 mm
Capsule	Globose to fusiform/ovoid	Broadly ellipsoid
Classical range	Broad eastern and southern Chinese complex	Northern and western Guangdong, southwestern Jiangxi
Plastid evidence	Core chinensis/pottingeri lineage	Distinct lineage; not recovered with core pottingeri
Tree TSAR	Accepted	Accepted, restored from synonymy

Tree TSAR interprets the morphological and plastid evidence together. Neither alone would necessarily justify overturning broad modern synonymy: morphological differentiation could represent geographically structured variation, and a plastid genealogy can be displaced by introgression. Their agreement, however, supports treating H. stenophylla as a working species hypothesis pending nuclear genomic testing.

Recent cytological work provides additional descriptive context. Chen et al. (2025) reported H. stenophylla as diploid with 2n = 2x = 36 and approximately 2C = 2.92 pg. Chromosome number does not itself establish species limits, but it provides a modern biological data point for a taxon that had otherwise largely disappeared into synonymy.

The name H. stenophylla var. decorticata should not follow the restored species automatically. Current evidence places that historical variety within the H. coenobialis complex, and POWO treats it as a synonym of H. coenobialis.

Horticultural and Agricultural Uses

Authenticated H. stenophylla appears to be exceptionally uncommon in cultivation. Its long, willow-like foliage, pale mature stems, and distinctly stalked lacecap inflorescences could make it horticulturally attractive, particularly in specialist collections emphasizing wild Hydrangea diversity.

No established cultivar tradition or agricultural use was located. Given the taxon's long synonymization beneath H. pottingeri, living collections bearing related names should be identified from provenance and morphology rather than relabelled solely from nursery names.

Conservation Concerns

No formal global IUCN assessment was located for Hydrangea stenophylla. Its traditional documented range in northern and western Guangdong and southwestern Jiangxi is relatively restricted compared with the broad range attributed to H. pottingeri.

If Tree TSAR's restored species concept is correct, treating these populations as part of a widespread H. pottingeri could obscure their conservation significance. Population surveys should therefore establish the extent, abundance, and connectivity of populations throughout the historical range and collect nuclear genomic data alongside voucher-confirmed morphology.

Additional Information

Flora of China — Hydrangea stenophylla (opens in a new tab): — Detailed morphology, habitat, distribution, and the traditional species concept.

Plants of the World Online — Hydrangea stenophylla (opens in a new tab): — Current backbone treatment synonymizing the name under H. pottingeri.

World Flora Online — Hydrangea stenophylla (opens in a new tab): — Morphological and nomenclatural record preserving the species concept.

References and Further Reading

Chen S, Lu H, Liu H, Si M, Cai M, Zhang Q, Yang Y, Zhang D (2025) Karyotype analysis and genome-size evaluation of native Hydrangea taxa in China. HortScience 60(7): 1021–1025. https://doi.org/10.21273/HORTSCI18540-25 (opens in a new tab)

Merrill ED, Chun WY (1930) Contributions to our knowledge of the Kwangtung flora. Sunyatsenia 1(1): 49–84. https://www.biotaxa.org/Phytotaxa/article/view/phytotaxa.269.4.1 (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)

Wei ZF (1994) A revision of the genus Hydrangea in China. Guihaia 14(2): 101–121. https://www.guihaia-journal.com/gxzwen/ch/reader/create_pdf.aspx?falg=1&file_no=199402002&quarter_id=2&year_id=1994 (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)