Hydrangea arguta (Gaudich.) Y.De Smet & C.Granados — Kanawao hydrangea

Introduction

Kanawao hydrangea, Hydrangea arguta, is a woody shrub or small tree endemic to the Hawaiian Islands and the sole accepted species of Section Broussaisia in Tree TSAR. It is one of the most morphologically distinctive members of broad Hydrangea: plants are evergreen, often 2–5 m tall, the flowers are functionally unisexual on separate plants, and the fruits mature as fleshy red to maroon berries rather than the dry capsules familiar from many temperate hydrangeas. Leaves may be opposite or arranged in whorls of three, sometimes with both arrangements occurring on a single plant. The species inhabits moist to very wet native forest across Kauaʻi, Oʻahu, Molokaʻi, Lānaʻi, Maui, and Hawaiʻi. (Fosberg 1939) Plants of Hawai‘i (opens in a new tab)

For much of its taxonomic history the species was placed in the endemic Hawaiian genus Broussaisia. That treatment was understandable: its dioecy, united styles, berry-like fruits, and insular distribution make it look profoundly unlike an ordinary hydrangea. Molecular analyses instead showed that Broussaisia is nested within the broader evolutionary radiation of Hydrangea. De Smet et al. (2015) therefore transferred the species to Hydrangea, while retaining Broussaisia as a section. The more densely sampled plastome analysis of Yang et al. (2024) recovered Section Broussaisia as sister to the remainder of Subgenus Dichroa, making the Hawaiian lineage one of the deepest branches within that subgenus. Berry-like indehiscent fruits combined with united styles are particularly characteristic of the section.

The species has also accumulated an instructive history of overdivision. Broussaisia pellucida was once distinguished from B. arguta, and Fosberg (1939) subsequently recognized varieties and forms based on characters including style length, anther development, pubescence, and leaf arrangement. His own broad specimen survey, however, showed extensive overlap and continuous variation; most strikingly, opposite and ternate leaves can occur on the same individual. Tree TSAR therefore follows the modern treatment in accepting a single variable species without subordinate taxa. Broussaisia pellucida, B. arguta var. pellucida, and the named forms are retained as synonyms and as part of the history of interpreting variation within the species. Bishop Museum (opens in a new tab)

Horticultural and Agricultural Uses

Kanawao is principally significant as a Hawaiian native forest plant rather than as a conventional ornamental crop. Its glossy foliage, colored flowers, fleshy fruits, and unusual architecture give it considerable potential for native-plant horticulture and ecological restoration in suitable Hawaiian environments. Because the species is functionally dioecious, seed production depends upon compatible male and female plants occurring within pollination distance; Bishop Museum cultivation guidance consequently recommends collecting fruits where multiple plants grow together when viable seed is desired. Plants of Hawai‘i (opens in a new tab)

Hawaiian sources also record traditional cultural associations with the fruit, including an old belief connecting its consumption with fecundity and comparisons between abundant fruiting and the increase of chiefly families. These records are culturally and historically significant, but they should not be interpreted as evidence for a modern medicinal effect. Plants of Hawai‘i (opens in a new tab)

Conservation Concerns

Kanawao remains distributed across six of the major Hawaiian Islands and can be locally common in wet native forest (Fosberg 1939). NatureServe lists it under the name Broussaisia arguta among the species associated with Hawai‘i wet cliff and ridge-crest shrubland and gives it a global rank of G3 (Vulnerable) (NatureServe n.d.).

Its breeding system also deserves attention in restoration and ex situ work. A dioecious population can remain numerically present while losing effective sexual reproduction if one sex becomes uncommon or populations become too fragmented for pollination. Provenance and sex representation are therefore meaningful considerations when establishing conservation collections. Population-genomic sampling across the archipelago would also be valuable for determining whether the historically recognized pellucida morph or individual island populations contain cryptic evolutionary structure.

Additional Information

References and Further Reading

De Smet Y, Granados Mendoza C, Wanke S, Goetghebeur P, Samain M-S (2015) Molecular phylogenetics and new (infra)generic classification to alleviate polyphyly in tribe Hydrangeeae (Cornales: Hydrangeaceae). Taxon 64(4): 741–753. https://doi.org/10.12705/644.6 (opens in a new tab)

Fosberg FR (1939) Taxonomy of the Hawaiian genus Broussaisia (Saxifragaceae). Occasional Papers of Bernice P. Bishop Museum 15(4): 49–60. https://hbs.bishopmuseum.org/pubs-online/pdf/op15-4.pdf (opens in a new tab)

NatureServe (n.d.) Hawai‘i wet cliff and ridge crest shrubland. NatureServe Explorer. https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.770638/Hawaii_Wet_Cliff_and_Ridge_Crest_Shrubland (opens in a new tab) (accessed 9 October 2026).

Yang X, Xue T, Gao T, Liang Y, Smets EF, Gadagkar SR, Yu S (2024) An updated infrageneric classification based on phylogenomics and character evolution in Hydrangea (Hydrangeaceae). Taxon 73(2): 503–518. https://doi.org/10.1002/tax.13158 (opens in a new tab)