The Flowering Plants of Hawaii
Part 22 Hydrangeaceae - Hydrophyllaceae
Hydrangeaceae, which is home to the very common decorative hydrangea [Hydrangea macrophylla subsp. macrophylla (hortensia)] are represented on the islands by two species, the endemic, monotypic genus Broussaisia, and the recently naturalized Philadelphus karwinskyanus (reported in the Addendum to the Manual).
Broussaisia arguta (see images) is readily found in wet forest on all of the main islands. Visitors who hike the Pihea Trail on Kaua`i would find Broussaisia along the wetter parts of the trail, especially near the muddy steps portion of the trail; along the Pëpë`öpae Trail on Moloka`i; and in the wet forest at higher elevations along the road from Hilo to Volcanoes National Park on the Big Island. Flowers come in a variety of colors ranging from greenish blue to pink, while the fruits are dark red to purple (see images). The flower head is large, reminiscent of commercial hydrangea (see image below). Hawaiian names for Broussaisia are kanawao and pü`ahanui.
Philadelphus karwinskyanus (see image) and other members of the genus are commonly called mock-orange. We learn from a recent personal communication (28 Feb. 2012) from Forest Starr that “Philadelphus can become locally rampant here when planted in cool mesic areas. We believe Koke`e on Kaua`i is the only place it's displacing native vegetation, spreading from one of the cabins. Lack of sexual reproduction seems to limit its spread.”
Relationships within the family have been studied by Douglas Soltis (then at Washington State University) and colleagues (1995) using rbcL gene sequences of representatives of all genera thought to comprise the Hydrangeaceae including Broussaisia. The general resemblance of Broussaisia to Hydrangea macrophylla was borne out by the DNA analysis where Broussaisia, H. macrophylla, H. arborescens, and a representative of the genus Dichroa, emerged as a group of closely related species. A relationship to Dichroa, a genus native to Asia and Malesia, had been suggested by earlier workers (Manual, p. 794).
Species classically considered to comprise Hydrophyllaceae, the waterleaf family, are currently considered by some authorities to belong properly within Boraginaceae (Mabberley p. 421). Authors of the Manual follow the earlier usage and so will we. The sole member of this family that grows on the Hawaiian Islands is Nama sandwicensis a diminutive species that occurs on sandy soil and on raised limestone reefs on all of the main islands, except Kaho`olawe, and on Laysan and Lisianski (of the northwestern islands). The featured plant (see images) is known in Hawaiian as ahina kahakai, literally, the beach hinahina. It is necessary to indicate that this particular hinahina is a beach dweller since several other plants are also known by this name. When talking about hinahina with local folks, it is necessary to indicate where the plant was seen. Even then it can be confusing since another beach dweller, Heliotropium anomalum var. argenteum, is also called hinahina (see under Boraginaceae above), but more specifically hinahina kü kahakai. Among several other usages for kü the word can mean to stand, stop, halt, anchor, or moor (Pukui and Elbert, p. 167), suggesting that this is a hinahina that is anchored to, or lives on, a beach.