Hydrangea panamensis
Panamanian sprawling hydrangea
On this page
Introduction
Panamanian sprawling hydrangea is a functionally dioecious root-climbing liana of lowland to lower-montane tropical rainforest in Costa Rica and Panama. It grows from roughly 200 to 1,000 m elevation—lower than any of the other species treated in the 2021 peruviana–oerstedii revision—and is often associated with streams at the lower end of that range. Mature plants can reach about 20 m but characteristically remain among the lower branches of their host trees rather than climbing through and above the canopy (Samain et al. 2021).
Its relatively small foliage gives the species a distinctive character. The leaves are obovate to slightly elliptic, usually only about 5–12 cm long, with a rounded apex terminating in a minute point. Their margins are only slightly glandular-dentate and slightly revolute. Most diagnostically, the lower leaf surface bears very few acarodomatia—usually none, one, or two—rather than the numerous vein-axil cavities characteristic of true H. peruviana. Brownish to reddish stellate hairs cover young petioles, leaves, and flowering axes before becoming sparser with age (Samain et al. 2021; Sutton 2025).
The enlarged marginal flowers are normally pale, but color is not absolutely fixed. Field observations were predominantly white, while herbarium labels have recorded red flowers or reddish-pink coloration around the fertile flowers. This intraspecific variability is worth preserving in the narrative because it cautions against using floral color alone to identify members of the complex (Samain et al. 2021).
Where H. panamensis becomes especially significant is the molecular evidence. It was one of only three morphologically circumscribed Cornidia species recovered with strong monophyletic support in the 2021 nuclear analysis. Its sampled accessions form a 100% bootstrap-supported lineage, itself sister to the sampled true H. peruviana within equally strongly supported Clade K (Granados Mendoza et al. 2021).
That topology is a powerful validation of the taxonomic revision. The species most closely related to H. peruviana is not a slight geographic variant of it: H. panamensis is both morphologically coherent and genetically strongly delimited, with different leaf shape, acarodomatia, ecology, altitude, and geography. The evidence therefore simultaneously supports their close relationship and their recognition as separate species.
Tree TSAR accordingly accepts H. panamensis at species rank despite major backbones still synonymizing the name under broad H. peruviana.
Horticultural and Agricultural Uses
No substantial horticultural tradition is documented for H. panamensis. Its smaller foliage, lower-canopy habit, evergreen growth, and occasional reddish-pink floral coloration could make it an interesting specialist climber, but it comes from warm, humid rainforest and is unlikely to possess the temperate hardiness of the northern and southern Cornidia.
Its relatively contained vertical architecture is notable. A plant that remains in the lower canopy could ultimately be easier to manage and observe than 40–60 m Cornidia whose flowering crowns disappear above a large host tree.
For scientific collections, its particular value is as the best-supported living comparison with H. peruviana. Authenticated accessions from multiple Costa Rican and Panamanian populations would make it possible to examine variation within the strongly supported molecular lineage and compare acarodomatia, reproductive characters, and architecture directly with its Ecuadorian sister species.
Conservation Concerns
Despite its geographic range across two countries, H. panamensis is much less common than H. oerstedii. The 2021 revision found only 17 collections and assessed it as Endangered, with an estimated area of occupancy of about 60 km² and continuing reductions associated with rainforest loss and fragmentation (Samain et al. 2021).
Because the species often follows low-elevation watercourses, riparian habitat preservation may be especially important. Lowland tropical forests also face different development pressures from the higher cloud forests occupied by much of Cornidia.
Its strong molecular coherence makes conservation units particularly tangible: loss of H. panamensis would mean the loss of one of the very few Cornidia species already demonstrated as a strongly supported nuclear lineage.
Additional Information
- Open-access 2021 Hydrangea panamensis treatment (opens in a new tab): — complete description, field photographs, distribution, ecology, and conservation assessment.
- 2021 Cornidia molecular phylogeny (opens in a new tab): — documents the unusually strong monophyly of H. panamensis and its sister relationship with H. peruviana.
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)