Hydrangea steyermarkii Standl. — Tacaná sprawling hydrangea

Introduction

Tacaná sprawling hydrangea is a rare functionally dioecious root-climbing hydrangea whose true identity was obscured for much of the twentieth century by an extraordinarily broad misapplication of its name. In its modern, type-anchored sense, Hydrangea steyermarkii is restricted to the Volcán Tacaná region along the Mexico–Guatemala border. Plants elsewhere in Mexico and Central America that were routinely filed under this name proved to represent several quite different species, including H. albostellata, H. breedlovei, H. nahaensis, H. sousae, and H. tapalapensis (Samain et al. 2014, 2019). Phytotaxa (opens in a new tab)

True H. steyermarkii remains a woody root-climber rather than a shrub or epiphyte. It is distinguished within the Mexican radiation by relatively elongated internodes, lanceolate to obovate leaves whose non-revolute margins are crenate and armed with conspicuous glandular teeth, and white flowers lacking enlarged marginal flowers. Young foliage can be densely pale-pubescent beneath. Unlike H. sousae and H. tapalapensis, it does not characteristically project shrub-like flowering growth above the crown of its host tree (Samain et al. 2014, 2019).

The history of the name demonstrates how severely herbarium-based taxonomy can distort a lineage of enormous climbing plants. A herbarium sheet captures a handful of leaves and flowering branches, but not whether an individual remains below the canopy, overtops its support, produces long runners, or changes its architecture after reaching the light. Samain and colleagues found that those whole-plant characters, combined with type comparison and detailed leaf morphology, revealed several species hiding beneath what had been treated as one variable H. steyermarkii (Samain et al. 2019).

Nuclear data place true H. steyermarkii within Mexican Clade I, in the shallowly resolved group containing H. tapalapensis and H. sousae. The present evidence therefore supports a young cluster of morphologically and geographically differentiated lineages rather than a set of ancient, deeply separated branches. Broader genomic sampling from Tacaná remains desirable, but nothing in the limited nuclear tree requires a return to the old broad steyermarkii concept (Granados Mendoza et al. 2021).

Horticultural and Agricultural Uses

No reliably documented horticultural tradition exists for true H. steyermarkii. This qualification is important because plants in horticultural or botanical collections carrying that name may have been identified under the historically broad concept.

Trees and Shrubs Online, for example, discusses a Costa Rican collection labelled aff. steyermarkii but notes that its provenance and morphology make H. albostellata a more plausible identity. Any cultivated accession claimed as H. steyermarkii should therefore be re-evaluated against the narrow Tacaná concept before it is used for horticultural, conservation, or breeding conclusions (Sutton 2025).

Conservation Concerns

The taxonomic correction dramatically changes the conservation picture. A supposed widespread Central American species became a highly localized volcanic-mountain endemic once misidentified material was removed. Samain et al. (2019) applied IUCN criteria and assessed the narrow species as Endangered, with an area of occupancy only slightly above 32 km², fragmented and declining populations, and threats from habitat destruction, fires, and increasing human pressure. They also noted the potential risk from volcanic activity.

The Guatemalan side of the range is particularly poorly known: the 2019 revision reported no recent Guatemalan collections, with the last cited specimens dating to Steyermark's collections in the 1940s. Renewed field survey there is therefore a major priority. Confirmation of surviving populations could materially change estimates of distribution, genetic structure, and conservation need.

A further complication is that major taxonomic backbones have not fully caught up with the specialist revision. Plants of the World Online currently displays H. steyermarkii across Mexico and several Central American countries, a range that reflects the historically broad usage and conflicts with the type-anchored Tacaná treatment adopted here (Royal Botanic Gardens, Kew 2026).

Additional Information

Samain et al. 2014 — revision of Hydrangea steyermarkii (opens in a new tab): The 2014 Phytotaxa paper is essential for understanding why the name was restricted to Tacaná.
Trees and Shrubs Online — Section Cornidia (opens in a new tab): Trees and Shrubs Online follows the narrow specialist concept and explicitly warns against identifications from elsewhere in Central America.
Plants of the World Online — Hydrangea steyermarkii (opens in a new tab): Plants of the World Online is useful for nomenclatural interoperability but currently retains a much broader range than the specialist circumscription.

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, Hernández Najarro F, Martínez Salas EM (2014) First record of the critically endangered Hydrangea steyermarkii Standl. (Hydrangeaceae) in Mexico, and description of a new widespread Hydrangea species of Mesoamerica. Phytotaxa 162(4): 181–197. https://doi.org/10.11646/phytotaxa.162.4.1 (opens in a new tab)

Samain M-S, Hernández Najarro F, Martínez Salas EM (2019) The climbing hydrangeas (Hydrangeaceae) of Mexico, including description of six (critically) endangered new species. Acta Botanica Mexicana 126: e1463. https://doi.org/10.21829/abm126.2019.1463 (opens in a new tab)