Hydrangea macrosepala Hayata — Large-sepal hydrangea

Introduction

Large-sepal hydrangea, Hydrangea macrosepala, is a Taiwanese shrub whose recognition by Tree TSAR departs deliberately from several current taxonomic backbones. Hayata described the species in 1913 from Taiwanese material, and the original type remains preserved in the Taiwan Forestry Research Institute herbarium from Kokei, collected by Sadaichi Nagasawa in 1903.

Hayata's original description portrays a shrub with glabrous brownish-purple young branches and narrow, lanceolate, papery leaves, approximately 8 cm long and only a little over 2 cm wide, becoming remotely serrulate toward the apex. The species name refers to its conspicuous enlarged floral sepals.

Its taxonomic history has been exceptionally unstable. Modern Taiwanese and international treatments have variously submerged H. macrosepala beneath H. chinensis or H. angustipetala. Flora of China included it among the synonyms of a broad H. chinensis, while POWO currently places it under the Taiwanese species H. angustipetala. Taiwan's own historical databases likewise reflect repeated movement of the name among broader species concepts.

The molecular record, however, gives good reason not to erase macrosepala without further testing. De Smet et al. (2015) sampled an accession identified as H. angustipetala and separate Taiwanese accessions identified as H. angustipetala “f. macrosepala,” demonstrating that the two concepts were represented independently in the molecular dataset rather than being assumed from the outset to be identical. More importantly, Yang et al. (2024) explicitly recognized H. macrosepala among the species forming the molecular clade for their newly established Section Kawagoeanae. Their account places macrosepala with H. kawagoeana, H. luteovenosa, H. chinensis var. lobbii, H. chinensis var. yayeyamensis, and H. scandens var. grosseserrata, while residual Chinenses occupies a separate lineage.

Tree TSAR therefore accepts H. macrosepala provisionally at species rank rather than following POWO's synonymy beneath H. angustipetala. This is not a claim that the entire Taiwanese angustipetala–formosana–macrosepala–obovatifolia problem has been solved. On the contrary, it identifies the exact problem that still needs a type-anchored Taiwanese revision. The present evidence demonstrates that macrosepala is phylogenetically consequential and that treating all of those historical names as interchangeable without testing risks obscuring real lineage structure.

Its relationship with H. angustipetala is therefore one of the principal review triggers for the entire section. Dense nuclear sampling tied explicitly to the types or topotypic populations of angustipetala, macrosepala, formosana, and obovatifolia could either strengthen the species treatment adopted here or demonstrate that some names should again be united.

Horticultural and Agricultural Uses

Authentically documented H. macrosepala is poorly known horticulturally. Many Taiwanese hydrangeas in cultivation have been distributed under H. angustipetala, H. chinensis, or horticultural forma names, and the same historical uncertainty that complicates herbarium taxonomy also affects living collections. Trees and Shrubs Online notes that some large-sepaled Taiwanese collections currently placed under H. angustipetala are particularly ornamental, while also emphasizing how difficult it is to distinguish the historical Taiwanese segregates consistently.

Until the complex is revised, cultivation records should therefore be used cautiously. A garden plant labelled “H. macrosepala” is most scientifically valuable when accompanied by a wild collection number, locality, and voucher that can eventually be compared with the nomenclatural type.

The conspicuous enlarged sepals that gave the species its name may make authentic material attractive horticulturally, but breeding or cultivation claims cannot yet be separated reliably from the broader angustipetala complex.

Conservation Concerns

No formal conservation assessment for H. macrosepala under the narrow Tree TSAR species concept was located. This is partly a taxonomic problem: if conservation databases treat the name as a synonym of H. angustipetala or H. chinensis, occurrences and threat information are necessarily aggregated under those broader concepts.

Historical Taiwanese collections demonstrate that macrosepala is not merely a nomenclatural abstraction—the type and additional early specimens are preserved in Taiwanese herbarium and biodiversity databases. What remains unclear is the modern distribution, population size, ecological specialization, and degree of geographic overlap with H. angustipetala sensu stricto.

This makes H. macrosepala a good example of taxonomic uncertainty as a conservation problem. A potentially distinct endemic lineage cannot be assessed properly until its diagnostic boundaries are mapped onto living populations. The immediate conservation priority is therefore not assigning a speculative threat category, but conducting type-informed field and genomic work across northern and montane Taiwan.

Additional Information

TAIF — Hydrangea macrosepala type specimen (opens in a new tab): The Taiwan Forestry Research Institute preserves the historical type of H. macrosepala, collected at Kokei in 1903.

Plants of the World Online — Hydrangea macrosepala (opens in a new tab): POWO currently treats H. macrosepala as a synonym of H. angustipetala; this differs from the provisional Tree TSAR species treatment.

Yang et al. (2024) (opens in a new tab): The modern sectional evidence supporting reconsideration comes from Yang et al. (2024).

References and Further Reading

De Smet Y, Granados Mendoza C, Wanke S, Goetghebeur P, Samain M-S (2015) Molecular phylogenetics and new (infra)generic classification to alleviate polyphyly in tribe Hydrangeeae (Cornales: Hydrangeaceae). Taxon 64(4): 741–753. https://doi.org/10.12705/644.6 (opens in a new tab)

Hayata B (1913) Icones plantarum formosanarum nec non et contributiones ad floram formosanam, vol. 3. Bureau of Productive Industry, Government of Formosa, Taihoku. https://doi.org/10.5962/bhl.title.56862 (opens in a new tab)

Li HL (1963) Woody flora of Taiwan. Livingston Publishing Company, Narberth, Pennsylvania. https://ci.nii.ac.jp/ncid/BA1891249X?l=en (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)