Hydrangea serratifolia (Hook. & Arn.) F.Phil. — Sawtooth sprawling hydrangea

Introduction

Sawtooth sprawling hydrangea is a massive evergreen root-climbing liana of the cool, wet forests of central and southern Chile and adjoining southwestern Argentina. It is the southernmost wild hydrangea and one of the most conspicuous woody climbers of the Chilean temperate rainforest. Plants attach to tree trunks with adventitious roots and can ascend far into the canopy; ecological studies have documented large individuals tens of metres above the forest floor and extraordinary local abundance (Jiménez-Castillo & Lusk 2009; Sutton 2025).

Molecular evidence gives H. serratifolia an unusually clear place within an otherwise difficult section. It was one of only three conventionally recognized Cornidia recovered with strong monophyletic support in the 2021 two-gene nuclear phylogeny, with its accessions forming a clade at 99% bootstrap support. That lineage is sister to the extraordinary H. seemannii–H. integrifolia lineage linking northwestern Mexico with East Asia (Granados Mendoza et al. 2021).

Its reproductive biology provides a useful contrast with those two relatives. Hydrangea serratifolia retains the functionally dioecious condition reconstructed as ancestral for Cornidia: individual plants function primarily as male or female. Its inflorescences are usually thyrse-like, with a conspicuous central axis, and enlarged marginal flowers are absent or very rare. Hydrangea seemannii and H. integrifolia, in contrast, have hermaphroditic flowers and more nearly umbel-like inflorescences (Granados Mendoza et al. 2021; Sutton 2025).

The familiar epithet serratifolia is somewhat deceptive. Mature leaves may be entire or only remotely toothed, and consistently serrated foliage should not be expected. This has contributed to horticultural confusion, particularly because cultivated H. integrifolia and H. seemannii can also produce occasional leaf teeth. The reproductive and inflorescence characters are much more reliable than leaf margins for distinguishing the three species (Sutton 2025).

One of the most remarkable aspects of H. serratifolia is how young plants find a tree to climb. Rodriguez-Quintero et al. (2022) demonstrated negative phototropism in searching shoots: instead of growing toward brighter light, juvenile shoots orient toward darker portions of the forest understory, directing them toward the shaded base of a potential support tree. Once contact is made, ascending shoots can begin the long transition from forest floor to canopy. This gives H. serratifolia one of the clearest experimentally studied examples of active support-finding behavior among the climbing hydrangeas. https://doi.org/10.1080/15592324.2022.2122244 (opens in a new tab)

Horticultural and Agricultural Uses

Hydrangea serratifolia has a longer history in European cultivation than most Cornidia. Seed collected by Harold Comber in Chile in 1926 established material that became widely distributed in British gardens, aided by the relative ease with which these woody climbers can be propagated vegetatively. Long-lived historic plants survive in major collections, and cultivated material at the Royal Botanic Garden Edinburgh includes both functional sexes (Sutton 2025).

It is among the hardier evergreen Cornidia, with Trees and Shrubs Online indicating approximately USDA Zones 7–9 and RHS H4. Mature trees and large walls are appropriate supports, and plants can thrive in either sun or shade, although greater light generally encourages flowering. Dry sites and exposure to cold, desiccating winds are less suitable for a species adapted to high-rainfall temperate forest (Sutton 2025).

Its functional dioecy has practical consequences for collections. A single cultivated individual may flower splendidly without producing normal seed if a compatible plant of the opposite sexual function is absent. Vegetative propagation therefore remains especially useful for preserving documented wild-provenance material, while sexually produced accessions are valuable for retaining genetic diversity.

Conservation Concerns

No formal IUCN Red List assessment was located for H. serratifolia, and Trees and Shrubs Online records it as Not Evaluated. Its broad north–south range through parts of Chile and adjoining Argentina, together with its capacity to be locally abundant, distinguishes it from many tiny-range tropical Cornidia endemics. In one Chilean temperate-rainforest study, 54% of the 515 sampled trees at least 10 cm in diameter supported one or more H. serratifolia individuals (Jiménez-Castillo & Lusk 2009).

Local abundance nevertheless should not be treated as evidence that intact habitat is dispensable. Hydrangea serratifolia is a canopy-dependent woody climber of high-rainfall forest whose life cycle requires access to suitable host trees and the shaded structural environment that juveniles use to locate them. Forest fragmentation, altered moisture regimes, and loss of mature host trees could therefore affect populations even where the species is not presently recognized as globally threatened. Conservation collections are most informative when provenance and sex are recorded, because genetically distinct populations and both reproductive functions are necessary to represent the species adequately.

Additional Information

Trees and Shrubs Online — Hydrangea serratifolia (opens in a new tab): detailed morphology, identification, horticultural history, ecology, and comparison with H. integrifolia and H. seemannii.

Negative phototropism in Hydrangea serratifolia — Rodriguez-Quintero et al. 2022 (opens in a new tab): experimental study of how juvenile shoots locate support trees.

Host infestation patterns — Jiménez-Castillo & Lusk 2009 (opens in a new tab): ecological study of climbing behavior and host-tree occupation in Chilean temperate rainforest.

iNaturalist — Hydrangea serratifolia taxonomic record (opens in a new tab): observation-oriented resource; note that its displayed infrageneric placement should not be used in preference to the modern Section Cornidia framework.

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)

Jiménez-Castillo M, Lusk CH (2009) Host infestation patterns of the massive liana Hydrangea serratifolia (Hydrangeaceae) in a Chilean temperate rainforest. Austral Ecology 34(7): 829–834. https://doi.org/10.1111/j.1442-9993.2009.01990.x (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)

Rodriguez-Quintero WD, Moreno-Chacón M, Carrasco-Urra F, Saldaña A (2022) From dark to darkness, negative phototropism influences the support-tree location of the massive woody climber Hydrangea serratifolia (Hydrangeaceae) in a Chilean temperate rainforest. Plant Signaling & Behavior 17(1): 2122244. https://doi.org/10.1080/15592324.2022.2122244 (opens in a new tab)

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