Hydrangea kawakamii Hayata — Taiwan roughleaf hydrangea

Introduction

Taiwan roughleaf hydrangea (Hydrangea kawakamii Hayata) is a deciduous woody shrub endemic to the mountains of Taiwan and one of the clearest examples of a lineage hidden by the historically broad H. aspera concept. Hayata described it as a species in 1908 from Taiwanese material, but later revisions reduced it to H. aspera. Current Plants of the World Online still follows that synonymized treatment.

Modern systematic evidence strongly favors restoration. De Smet's multilocus analyses, subsequent RADseq study, and reciprocal-monophyly tests all support H. kawakamii as an independent evolutionary lineage. The best-supported relationship places it as sister to the type-linked Hubei lineage of H. strigosa, rather than within H. aspera sensu stricto.

Character	H. aspera	H. aspera ssp. robusta	H. kawakamii	H. strigosa	H. villosa
Core geography	Mainland Asian complex	Eastern Himalaya–southern China–Indochina	Taiwan	Hubei type lineage	Central China
Leaf underside	Variable hairy	Usually robustly hairy	Long erect hairs; subtle papillae	Appressed strigose hairs + conspicuous white papillae	Villous; long brownish hairs on major veins
Seed coat	Striate	Striate	Striate with smaller transverse veins, giving a reticulate appearance	Striate	Striate
Cytology	2n = 34 or 36 reported	2n = 34 reported	2n = 36	2n = 34	2n = 34
Molecular status	Permeable complex	Subspecific differentiation	Strong species-level lineage	Strong Hubei lineage	Strong species-level lineage
Tree TSAR rank	Species	Subspecies	Species	Species	Species

The morphology is subtler than the genetic distinction. Young branchlets, petioles, and inflorescences bear dense yellow-brown pubescence, while the lower leaf surface has long erect hairs and small papillae. The seed coat provides a particularly useful microscopic distinction: thin transverse veins cross between the larger longitudinal striae and create a reticulate pattern lacking in H. aspera.

Cytology supplies another independent line of evidence. H. kawakamii has 2n = 36, whereas most investigated members of the aspera complex have 2n = 34. Mortreau et al. found sufficient differences in chromosome organization to distinguish a Taiwanese “Kawakamii group” from the mainland Villosa group even before the later molecular species-delimitation studies were available.

Chromosome number alone would not justify species rank, and these hydrangeas remain capable of hybridization. What makes the case strong is concordance among island geography, cytology, genomic differentiation, reciprocal monophyly, and subtle but reproducible morphology.

Taiwanese floristic sources currently place the name H. kawakamii in synonymy under H. aspera, preserving “Kawakami hydrangea” as an English vernacular name. The disagreement with Tree TSAR is therefore explicit: the same Taiwanese populations are being interpreted under different species concepts rather than representing an overlooked distribution record.

Horticultural and Agricultural Uses

Taiwan roughleaf hydrangea is an increasingly interesting collector plant. Living introductions demonstrate that the species is more variable than older botanical descriptions implied: plants can become large and spreading, sometimes tall, with broad velvety foliage, long petioles, and varying degrees of reddish or purple pigmentation in veins and petioles. Flowering in temperate cultivation ranges from midsummer into early autumn depending on clone.

The old cultivar or clone commonly called ‘Taiwan’ helped establish the horticultural concept, while more recent wild-origin introductions have broadened the known variation. Accurate provenance is especially valuable because cultivated plants provide a way to examine variation across Taiwanese populations rather than perpetuating one clone as representative of the entire species.

Its later-flowering tendencies can limit performance in short-season climates, but they also extend the flowering season of collections containing earlier members of Section Asperae.

Conservation Concerns

No formal IUCN assessment was located for H. kawakamii under its restored species concept. De Smet's fieldwork found large populations and no immediate major threats among the populations examined, but also emphasized the species' restricted Taiwanese distribution.

Its island endemism makes geographic representation important even in the absence of evidence for current decline. A conservation collection derived from one or two horticultural introductions cannot represent the full genetic variation of a lineage supported by both genomic and cytological evidence.

Recognition at species rank also changes conservation accounting. Under a broad H. aspera treatment, loss of Taiwanese populations could be obscured by the enormous mainland distribution assigned to the parent species. Under the lineage-based interpretation, the Taiwanese populations represent an irreplaceable endemic evolutionary unit.

Additional Information

TAI2 — Hydrangea aspera / kawakamii (opens in a new tab): Taiwan Plant Information Integration System: Taiwanese floristic treatment under H. aspera, with H. kawakamii in synonymy and morphological information.

Trees and Shrubs Online — Hydrangea kawakamii (opens in a new tab): Trees and Shrubs Online: Detailed horticultural and taxonomic discussion of H. kawakamii.

Plants of the World Online — Hydrangea kawakamii (opens in a new tab): Plants of the World Online: Current alternative treatment synonymizing H. kawakamii with H. aspera.

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)

Mortreau E, Siljak-Yakovlev S, Cerbah M, Brown SC, Bertrand H, Lambert C (2010) Cytogenetic characterization of Hydrangea involucrata Sieb. and H. aspera D. Don complex (Hydrangeaceae): Genetic, evolutional, and taxonomic implications. Tree Genetics & Genomes 6(1): 137–148. https://doi.org/10.1007/s11295-009-0235-8 (opens in a new tab)

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