Hydrangea diplostemona (Donn.Sm.) Standl. — Shrubby sprawling hydrangea

Introduction

Shrubby sprawling hydrangea is a functionally dioecious climbing hydrangea most securely understood from Costa Rica. It bears relatively glabrous to glabrescent leaves with entire or only very sparsely toothed margins and simple, umbel-like inflorescences lacking enlarged marginal flowers. Its reddish to pinkish-orange flowers are particularly distinctive among Cornidia, many of which flower in shades of white, cream, or greenish white (Sutton 2025).

The name has an unusually clear nomenclatural foundation. The species was first described in 1916 as Gilibertia diplostemona Donn.Sm.; Standley transferred it to Hydrangea in 1928. Gilibertia diplostemona is therefore a secure homotypic synonym referring to the same nomenclatural type (Royal Botanic Gardens, Kew 2026).

Its biological circumscription, however, is considerably less secure than its name. Modern Plants of the World Online assigns H. diplostemona a range from Costa Rica through Panama, Colombia, and Peru to Bolivia. Older Costa Rican floristic treatments likewise applied a very broad concept. Plants of the World Online The Costa Rican populations and type-anchored concept are the firmest core of the species; whether every South American collection historically assigned to H. diplostemona belongs to that same biological lineage has not been demonstrated. Plants of the World Online (opens in a new tab)

The 2021 phylogeny makes that caution especially important. A Costa Rican H. diplostemona accession was strongly paired with one Mexican H. albostellata accession even though H. albostellata itself appeared in several widely separated parts of the tree. Granados Mendoza et al. explicitly suggested genetic exchange between Central American H. albostellata and H. diplostemona as a plausible explanation (Granados Mendoza et al. 2021).

This is not good evidence for synonymizing the species. Morphologically, H. diplostemona retains a recognizable Costa Rican phenotype, notably its relatively glabrous foliage, absence of enlarged marginal flowers, and reddish flowers. Instead, the result makes it part of the section's growing reticulation story: neighboring Cornidia may remain identifiable as organisms while exchanging genes sufficiently to disrupt a simple bifurcating gene tree.

Older synonymy also requires reinterpretation. The 2007 Manual de Plantas de Costa Rica placed H. inornata and H. preslii in the synonymy of H. diplostemona (Morales 2007). Subsequent specialist work on South American Cornidia has shown that such broad synonymizations often combined several distinct lineages. Those two names should therefore not simply be carried forward as secure synonyms of the narrow Costa Rican species without renewed type-anchored study. Only Gilibertia diplostemona is retained here as secure formal synonymy.

Horticultural and Agricultural Uses

Hydrangea diplostemona has attracted occasional horticultural attention but remains very uncommon. Trees and Shrubs Online explicitly describes it as worthy of cultivation, an understandable judgment given its unusual pinkish-orange flowers, evergreen climbing habit, and relatively clean foliage (Sutton 2025).

The Costa Rican flora records it from very wet forest, rain forest, and oak forest, as well as some disturbed forest situations, from roughly 700 to 2,100 m. This suggests somewhat greater ecological breadth than might be inferred from a single cloud-forest locality, although the mild and moist tropical montane climate remains very different from that of most temperate gardens (Morales 2007).

Any introduction should retain collection locality. Until South American records have been critically reassessed, cultivated plants from Costa Rica are much more useful as representatives of H. diplostemona sensu stricto than an accession of uncertain origin bearing the same name.

Conservation Concerns

A modern species-specific IUCN assessment is not established in the sources reviewed here. The older Costa Rican flora documents the species in several types of wet forest and even some disturbed habitats, which indicates that it is not necessarily confined to pristine primary cloud forest in Costa Rica (Morales 2007).

At the same time, the extremely broad Costa Rica-to-Bolivia range shown by major databases cannot safely be converted into an assumption of low conservation concern. If a substantial portion of those South American records belongs to other species, the effective range of true H. diplostemona could be far smaller.

The highest conservation priority is consequently linked to taxonomy: establish the distribution of the Costa Rican/type lineage, sample neighboring Panamanian populations, and determine where—or whether—the same lineage continues into South America. Genomic testing should also examine its interaction with H. albostellata, since introgression could create important local evolutionary structure even if both species remain taxonomically distinct.

Additional Information

Plants of the World Online — Hydrangea diplostemona (opens in a new tab): provides current nomenclature, including the secure basionym Gilibertia diplostemona, but its broad Costa Rica-to-Bolivia distribution should be interpreted against the more recent specialist framework.

Trees and Shrubs Online — Hydrangea species of Section Cornidia (opens in a new tab): provides a concise modern morphological and horticultural summary.

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)

McClintock E (1957) A monograph of the genus Hydrangea. Proceedings of the California Academy of Sciences, Fourth Series 29(5): 147–256. https://www.biodiversitylibrary.org/part/51707 (opens in a new tab) https://doi.org/10.5281/zenodo.15905786 (opens in a new tab)

Morales JF (2007) Hydrangeaceae. In: Hammel BE, Grayum MH, Herrera C, Zamora N (eds.) Manual de plantas de Costa Rica, vol. VI: Dicotiledóneas (Haloragaceae–Phytolaccaceae). Monographs in Systematic Botany from the Missouri Botanical Garden 111. Missouri Botanical Garden Press, St. Louis, pp. 18–21. https://www.researchgate.net/publication/273945180_Morales_J_F_2007_Hydrangeaceae_In_B_E_Hammel_M_Grayum_C_Herrera_N_Zamora_eds_Manual_of_Plants_of_Costa_Rica_vol_VI_Monogr_Syst_Bot_Missouri_Bot_Gard_111_18-21 (opens in a new tab)