Hydrangea caucana Engl. — Gland-tooth sprawling hydrangea

Introduction

Gland-tooth sprawling hydrangea is a functionally dioecious woody root-climber of the Colombian Andes, known from Antioquia, Nariño, and Valle del Cauca. It inhabits mountain cloud forest at approximately 750–1,365 m elevation and has been collected in flower or fruit primarily between November and March. Its precise stature remains surprisingly uncertain: the modern revisers did not encounter it alive, and the labels of the available historical collections did not record plant height, although their morphological reconstruction suggests a liana probably not exceeding roughly 15–20 m (Samain et al. 2021).

The species is striking when examined in detail. Its free-growing shoots and flowering axes are clothed in persistent reddish-brown stellate hairs, while its relatively small, flat leaves are ovate to lanceolate-elliptic, acuminate, and edged with small gland-tipped teeth. Numerous small acarodomatia—cavities associated with leaf-vein axils that can house mites—are present beneath the leaves, although they can be partly obscured by the reddish pubescence. The inflorescences bear enlarged marginal flowers, while the smaller reproductive flowers can have bright red to purple petals. That combination of red-brown indumentum, glandular-dentate foliage, and reddish-purple floral coloration produces a coherent phenotype quite unlike true H. peruviana (Samain et al. 2021).

That distinction is important because H. caucana spent decades submerged in the enormous twentieth-century concept of H. peruviana. Type-based re-examination showed that true H. peruviana has slightly spoon-shaped, elliptic to obovate leaves with serrate or only weakly dentate margins and dense reddish pubescence, whereas H. caucana has flatter, more narrowly shaped leaves with conspicuously glandular teeth. Their geography is also non-overlapping in the modern treatment: H. caucana is Colombian, while true H. peruviana is restricted to Ecuador (Samain et al. 2021).

Unlike most of the better-sampled species in this complex, H. caucana has not yet been placed molecularly because suitable fresh material was unavailable for the Cornidia studies. Its recognition therefore rests primarily on type-anchored morphology, geography, and ecology rather than on phylogenetic data. Tree TSAR retains it as a species on that evidence while treating fresh Colombian collections and nuclear genomic sampling as important tests of its position.

This is also a useful reminder of what the 2021 revision actually accomplished. The authors were not merely elevating old names because they existed. They compared types against newly collected material across the entire peruviana–oerstedii aggregate and concluded that features previously dismissed as individual variation instead occur in repeatable, geographically structured combinations. Hydrangea caucana is one of the clearest examples of a recognizable species that disappeared taxonomically through twentieth-century over-lumping before field-informed revision restored it (Samain et al. 2021).

Horticultural and Agricultural Uses

There is no documented horticultural tradition for H. caucana, and no authenticated cultivated material is known to have played a meaningful role in horticulture. This is therefore not a species for which generic Cornidia cultivation advice should be substituted for actual experience.

Its living-collection value would nevertheless be considerable. The persistent reddish indumentum, gland-toothed leaves, colored flowers, and comparatively compact stature suggested by preserved material could make it horticulturally distinctive, but even more importantly, living plants would allow the mature architecture, reproductive phenology, and sexual dimorphism of the species to be documented for the first time in modern study.

Any future introductions should retain precise wild provenance. Material from Antioquia, Nariño, and Valle del Cauca could ultimately prove to contain meaningful geographic structure, and collections made merely under the species name would lose information essential to both conservation and systematics.

Conservation Concerns

Samain et al. (2021) applied IUCN criteria and assessed H. caucana as Endangered, with an estimated area of occupancy of about 32 km², fewer than five locations, and continuing reduction caused by habitat destruction and fragmentation. This was a specialist assessment rather than a formal IUCN Red List listing. Particularly concerning is the age of the record: the most recent collection available to the revisers was from 1992.

The immediate need is therefore not simply to count herbarium records but to relocate living populations. Fresh survey work could establish whether all historically known populations persist, provide material for the first molecular placement of the species, document both functional sexes, and create provenance-based ex situ collections.

Current global backbones still illustrate why this matters. Plants of the World Online presently treats H. caucana as a synonym of H. peruviana var. peruviana, whereas the later type-anchored specialist revision recognizes it as a morphologically and geographically distinct species (Royal Botanic Gardens, Kew 2026).

Additional Information

Open-access 2021 taxonomic revision (opens in a new tab): — includes the amended description, type discussion, distribution map, and conservation assessment.

Trees and Shrubs Online — Section Cornidia treatment (opens in a new tab): — concise modern comparison of the peruviana–oerstedii species complex.

iNaturalist observations for Hydrangea caucana (opens in a new tab): — community-science records should be interpreted cautiously because older broad H. peruviana identifications remain widespread.

References and Further Reading

Granados Mendoza C, Martínez Salas EM, Goetghebeur P, Wanke S, Samain M-S (2021) Molecular phylogeny, character evolution, and biogeography of Hydrangea section Cornidia, Hydrangeaceae. Frontiers in Plant Science 12: 661522. https://doi.org/10.3389/fpls.2021.661522 (opens in a new tab)

Samain M-S, Granados Mendoza C, Martínez Salas EM (2021) On Hydrangea peruviana, an endangered species from Ecuador, and Hydrangea oerstedii, very common in Costa Rica and Panama, and seven threatened Central and South American hydrangeas, which have been confounded with these. PhytoKeys 171: 91–153. https://doi.org/10.3897/phytokeys.171.56351 (opens in a new tab)

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