Scientists think the primary pollinators of Hibiscadelphus hualalaiensis' curved flowers were native Hawaiian honeyeaters and honeycreepers (photograph courtesy of Eric White CC BY-NC-SA 3.0). Within the round fuzzy fruits of hau kuahiwi (photograph courtesy of Karl Magnacca CC BY-NC 4.0) are several angular fuzzy seeds. H. hualalaiensis has heart-shaped leaves, sometimes with red veins (photograph courtesy of  Karl Magnacca CC BY-NC 4.0). On young plants, the margins of the leaves often curiously curve downward (photograph courtesy of Forest & Kim Starr CC BY 3.0 US). Two mature, but maybe not quite full size, plantings: one in a restoration site on Hawai‘i Island (photograph courtesy of  Karl Magnacca CC BY-NC 4.0) and the other in a botanical garden (photograph courtesy of Eric White CC BY-NC-SA 3.0). Header photograph courtesy of Kenneth R. Wood (NTBG) CC BY-NC 4.0.

Growth Requirements

General

    While I (Koebele) have been repeatedly able to keep Hibiscadelphus distans (one of the other hau kuahiwi) alive and healthy for more than five (or even ten) years both in the ground and in pots, I have yet to have the same success with Hibiscadephus hualalaiensis. Generally, all my plants slowly declined in vigor and died whether I kept them in pots or planted them in the ground. Over the years, I've formulated a few hypotheses to explain these failures, but unfortunately, have been unable to confirm any of them — I leave those experiments to you and others with more conductive conditions. 

    My first hypothesis is that Hibiscadephus hualalaiensis simply demands a cooler habitat than what I could provide. I only had access to nurseries or landscapes that were at or below 500 ft (152 m) in elevation. Further, these were all on the leeward side of O‘ahu, which is much warmer and drier than O‘ahu's windward side. Successful restoration plantings on Hawai‘i Island have all been at cooler upland sites like Puʻu Waʻawaʻa at around 3,000 ft (914 m). My second hypothesis is that H. hualalaiensis requires a cinder-dominant substrate to survive longterm. Unfortunately, all my plantings on O‘ahu were in clay-dominant soil. My nursery plants were kept in cinder-rich (about 33% with sphagnum peat moss, vermiculite, and perlite) and generally survived longer than my plantings, but it never occurred to me to try and keep them in (or near) 100% cinders. My third hypothesis is that some unknown soil pathogen or pest in the highly-contaminated soil of lowland O‘ahu killed my H. hualalaiensis. Again, I failed at the time to conduct a proper autopsy of my dead plants or send a sample away for analysis (e.g. University of Hawai‘i at Mānoa Agricultural Diagnostic Service Center).

    All the above is a long way of stating that we need more horticulture research conducted on this species to truly bring it into large scale cultivation. Until then, my recommendation is: if your landscape is in upland Maui or Hawai‘i Island, please give this rare, beautiful, and unique species a try. However, if your landscape is in the lowlands of any of our Islands, then Hibiscadelphus distans is a safer bet. The following is some additional advice based on my past horticultural and field experiences with H. hualalaiensis.

    As an in-the-ground accent or container plant, Hibiscadelphus hualalaiensis has a fascinating story concerning its history of scientific discovery and relationship with Hawai‘i's native birds. It does best in a site with full sunlight for all or most of the day, but can also survive in a (very) bright shaded space. Once established in the ground (3 to 6 months), avoid overwatering to prevent root rot problems. Both in-the-ground and container plants should have soil or media, respectively, that drains excess water easily. If your site has poor drainage, improve it by mounding up the soil and planting your hau kuahiwi on top of the mound. Hau kuahiwi is somewhat drought tolerant. 

    Hibiscadelphus hualalaiensis grows at a rate comparable to its sister species, Hibiscadelphus distans. Unfortunately, none of my H. hualalaiensis ever flowered. I never fertilized any of my in-the-ground hau kuahiwi. However, for my potted plants, I usually applied a balanced granular controlled-release fertilizer every six months or so. Hand-prune your hau kuahiwi only if necessary, and do not remove too much woody material at any one time. 

Pests and Diseases

    The leaves of hau kuahiwi are often eaten by Chinese rose beetles, or occasionally infested by whiteflies. Protected by ants, aphids, mealybugs, spider mites, and thrips can also become a problem; they normally attack the stem-tips. Others have reported their hau kuahiwi being attacked by black twig borers. Refer to the Pests & Diseases page for ways to deal with each of these pests. 

Special Features and Information

General

    Hau kuahiwi belong to the Mallow family (Malvaceae), which contains about 4,225 species within 243 genera. Family members include herbs, shrubs, and trees found worldwide, but most commonly in the tropics. Some of the more economically important species include: cotton (Gossypium spp.), cacao (Theobroma cacao), linden (Tilia spp.), durian (Durio spp.), Hibiscus, and okra (Abelmoschus esculentus) [Britannica;Malvaceae]. 

    Hibiscadelphus is an endemic Hawaiian genus of eight endangered or extinct species. It is a complex genus and is considered to be one of the rarest groups of plants in the world. Here is a species summary based on [Baker 1980] and [WikipediA;Hibiscadelphus]:

• Hibiscadelphus bombycinus — Presumed extinct. Known from a single collection before 1868 from Kawaihae, Hawaiʻi.

• Hibiscadelphus crucibracteatus — Presumed extinct. A single tree was discovered in 1981 on the slopes of Puhielelu Ridge on Lānaʻi (altitude 2,460 ft). This tree died in 1985, and all efforts to save the species failed because none of the collected seeds germinated.

• Hibiscadelphus distans — See Hibiscadelphus distans for Natural Range & Environment and Description. 

• Hibiscadelphus giffardianus — Joseph Rock discovered a single tree in 1911 at Kīpuka Puaulu, today part of Hawaiʻi Volcanoes National Park. This plant died in 1930, but cuttings were collected and rooted. Since then, several hundred clones have been planted in the park, but no natural regeneration has been observed, and only a few trees produce viable seeds.

• Hibiscadelphus hualalaiensis — See Natural Range & Environment and Description above for status and description.

• Hibiscadelphus stellatus — A recently discovered and described species with about 100 wild plants in Kaua‘ula Valley in Western Maui. This species is also in cultivation at State nurseries.

• Hibiscadelphus wilderianus — Presumed extinct around 1912. Known from a single tree discovered at Auwahi on Maui.

• Hibiscadelphus woodii — Discovered in 1991 on Kauaʻi. Only four plants were found at that time. Unfortunately, three were crushed by a boulder and died between 1995 and 1998, and the last was seen dead in 2011. Collected pollen was found to be inviable, no fruit was ever observed, and all attempts at propagation, including cross-pollination with H. distans, have failed. However, one plant was discovered on a cliff in Kalalau Valley in 2019, and a month later, two more were found.

• Hibiscadelphus x puakuahiwi — An unnatural hybrid between H. giffardianus and H. hualalaiensis. In the 1960s, both species were planted in Hawaiʻi Volcanoes National Park, where only the former occurred naturally. The two species cross-pollinated and hybrid seeds were unknowingly collected and planted. Since these two species do not naturally exist together and threatened the genetic integrity of H. giffardianus, the hybrids and the planted H. hualalaiensis were destroyed after their discovery in 1973, although at least one hybrid remains as of 2011. Other hybrids likely exist elsewhere in cultivation.

  Hibiscadelphus flowers produce nectar that attracts pollinating birds such as our native Hawaiian honeyeaters and honeycreepers. Scientists have speculated that the decline and extinction of some of the native honeyeater and honeycreeper species may have contributed to the decline and extinction of Hibiscadelphus species [Pratt 2005].

Saving a Rare Hibiscadelphus From Extinction

    A 2002 National Parks article entitled A Tree's Tribulation by Jenell Talley relates how a plant teetering on survival can be saved from total extinction by the efforts of a few conscientious people.

   "Hibiscadelphus giffardianus, also called hau kuahiwi, is endemic to Hawaii Volcanoes National Park. The species was first described in 1911 by Austrian botanist Joseph Rock. Before the original tree died in 1930, cuttings were collected and at least one tree was propagated on land adjacent to what is now Hawaii Volcanoes. This tree died in 1940, but one cutting remained, keeping the species alive. The plant survived in cultivation and was replanted in the park in the 1950s. The species has been reduced to a single tree at least three times. Each tree is derived from the 1911 original."

She continues: "Steps are being taken to increase the tree population, although currently Belfield, Linda Pratt, a botanist working for the U.S. Geological Survey Pacific Islands Ecosystems Research Center, and Tim Tunison, chief of resources management at Hawaii Volcanoes, are the only scientists dedicated to the recovery effort. The tree has no specific recovery program, but Belfield is working on the park's Rare Plants Stabilization Project, which began last year. The project focuses on examining the status of rare and endangered plant species, including the H. giffardianus, in four ecological zones in the park. Seeds, fruit, and cuttings are collected and raised in the park's greenhouse, then planted in the park. More than 200 trees have been planted in the last few years as part of an experiment conducted by U.S. Geological Survey researchers to study damage caused by rats. Despite these efforts, more work remains before the trees can make it off the endangered list. 'It is unrealistic to imagine that this species will ever be delisted unless it goes extinct,' Pratt says, adding, that reintroducing the species to its natural habitat and having it be self-sustaining is the park's goal."

    For updates to the above account, please refer to the USFWS ECOS page and linked Reviews and Reports for Hibiscadelphus giffardianus.

Hawaiian Name

    Hau kuahiwi translates to "hau of the mountains," referring to its mountain (kuahiwi) habitat and how its leaves resemble those of hau (Talipariti titiaceum, syn: Hibiscus tiliaceus), a Polynesian-introduced plant most frequently found in the lowlands [Pukui & Elbert 1986].

Etymology

    Hibiscadelphus is from the Greek words hibiscum, meaning "marsh mallow," and delphi, meaning "brother" [Gledhill 2008]. Together, they translate as "the brother of the marsh mallow," suggesting their close affiliation to the genus Hibiscus. 

    The species name, hualalaiensisis, is a geographic Latinization of Hualālai, the active shield volcano on Hawaiʻi Island, where this species was first found Radlkofer & Rock [1911].