Differs from Hibiscadelphus wilderianus in its denser pubescence especially on leaves, petioles, peduncles, involucral bracts, and corolla; linear-subulate to lanceolate involucral bracts, with acute to acuminate apices; evenly 5-lobed calyx; wider, densely pubescent, externally purple, internally yellow corolla lobes; and ovate to sub-globose capsules, 2.5–3.5 × 2.2–3.2 cm with scattered long hairs on the endocarp.

USA. HAWAIIAN ISLANDS: West Maui, Lahaina District, Kaua`ula Valley, south slope, 841 m, 13 Feb 2014, Oppenheimer, Bustamente & Perlman H21404 (holotype: BISH; isotypes: MO, NY, PTBG, US).

Small trees 3–6 m tall, many branched, trunks to 30 cm dbh, bark smooth, light tan to gray, young branchlets densely white to tan pubescent with 8–12-rayed stellate trichomes 0.3–0.4 mm in diam., surface scurfy-waxy, glabrescent with age; petiole scars prominent, subcircular, 2.5–4 mm in diam. Leaves chartaceous, new growth densely stellate-pubescent, mature leaves with blades broadly-ovate to suborbicular or subreniform in outline, occasionally shallowly 3-lobed, 7.5–16(–18) cm long, (8)9.5–13.5(–18) cm wide, veins prominulous, primary veins 7–9 radiate from base, midvein with 3–4 pairs of secondary veins arising along midrib, light green to occasionally red tinged when fresh, higher order venation prominulous on both surfaces, margins irregularly broadly crenate, base cordate, with a wide to narrow but usually open sinus, apex acute to obtuse or rounded, green when fresh with scattered tan stellate pubescence on both surfaces, densely so along veins and adaxial surface, trichomes 0.2–0.4 mm in diam. with (2–)8–16 rays, abaxial surface with principal vein axils domatiate with dense tufts of tan to white trichomes 0.2–0.3 mm long; petioles 3.5–6cm long, green or sometimes red-tinged, pubescent with dense white to tan stellate trichomes as on branchlets; stipules lanceolate to subulate, 2–3.5 mm long, apex acute, green, sparsely to densely tan or white stellate pubescent, soon caducous. Flowers solitary, axillary, erect to spreading, pedicels 22–30 mm long, green or sometimes red-tinged, densely white to tan stellate pubescent as in petioles, involucral bracts 5–6 (–7), linear-subulate to lanceolate (rarely spathulate), acute to acuminate apically, connate only at base, 9–22 mm long, 1–2 mm wide at base, erect, appressed or spreading perpendicular to the floral axis in anthesis, green, densely tan or white stellate pubescent with trichomes 0.2–0.3 mm in diam. Calyx tubular-saccate, mostly 5-lobed, tube 22–30 mm long, 19–20 mm wide, the lobes triangular, acute to short acuminate 5–10 mm long, 7–8 mm wide, green, surface obscured by dense tan stellate pubescence as in bracts, in mature fruit splitting along one side but persistent. Corolla zygomorphic, adaxially curved, 5–6.5 cm long, lobed nearly to base, lobes coalescent, 6–6.5 cm long, 3.5–4 cm wide, obovate-spathulate, apex obtuse, tips and outer margins slightly reflexing with age, outer exposed portion purple, purple-green or purple-yellow, inner concealed portion yellow, conspicuously veined, densely covered with gray or tan stellate trichomes especially along veins, internally yellow or purple-tinged distally, purple toward base, corolla usually becoming purplish with age, staminal column and apex of the style exserted for 1.5–2.5 cm; staminal column 8–8.5 cm long, antheriferous in distal 3.5 cm, maroon-purple, antheriferous in distal 3.5 cm, stamens c. 100, anthers reniform-curved, 0.8–1.5 mm long, purple, filaments 6–12 mm long, purple, pollen grains purple turning golden yellow after anther dehiscence; style 8.5–9 cm long, style branches 3–5 mm long, villose, stigmas rounded, c. 1 mm long, yellow, ovary dome-shaped, 8 mm long and wide. Fruit a woody capsule, globose-cuboid to -ovoid, 5-locular, 5-valved, 2.5–3.5 (–4) cm long, 2.2–3.3 cm in diameter, surface yellowish brown, rough densely covered with dense tan stellate hair clusters, appearing tuberculate, mericarps 10, mesocarp well developed, reticulate, endocarp chartaceous, loose, with scattered long hairs, testa brown. Seeds 1–2 per mericarp, reniform, 8–10 mm long, 6–8 mm wide including the dense, lanate yellowish-tan hairs 0.4–1 mm long.

Known only from west Maui, Hawaiian Islands at 20.87°N, 156.62°W (Fig. 3).

Hibiscadelphus stellatus occurs on very steep, rocky slopes between 800 and 900 m elevation. These sites have a windward aspect and are situated mid-slope between the upper rim of a deep valley and a perennial stream below. Soils at these sites are of typical volcanic, basalt origin, from the Wailuku Series of original shield building flows. The vegetation where Hibiscadelphus stellatus grows forms a mosaic of trees and shrublands with an open canopy, best characterized as Lowland Mesic Forest (Wagner et al. 1999). Rainfall averages from 12 to 1400 mm annually and the substrate is well-drained.

Associated tree species include: Alectryon macrococcus Radlk., Antidesma pulvinatum Hillebr., Coprosma foliosa A. Gray, Diospyros sandwicensis (A. DC) Fosberg, Dodonaea viscosa Jacq., Metrosideros polymorpha Gaudich. var. glaberrima (H. Lév.) H. St. John, Myoporum sandwicense A. Gray, Myrsine lanaiensis Hillebr., Myrsine lessertiana A. DC., Nestegis sandwicensis (A. Gray) O. Deg, I. Deg. & L.A.S. Johnson, Pisonia sandwicensis Hillebr., Pittosporum confertiflorum A. Gray, Pouteria sandwicensis (A. Gray) Baehni & O. Deg., Psychotria kaduana (Cham. & Schltdl.) Fosberg, Psydrax odorata (G. Forst.) A.C. Smith & S.P. Darwin, Santalum ellipticum Gaudich., Sophora chrysophylla (Salisb.) Seem., Streblus pendulinus (Endl.) F. Muell., Zanthoxylum dipetalum H. Mann, and Zanthoxylum hawaiiense Hillebr. Understory species include: Achyranthes splendens Mart. ex Moq., Bidens micrantha Gaudich., Charpentiera ovata Gaudich., Euphorbia multiformis Gaudich. ex Hook. & Arn., Osteomeles anthyllidifolia (Sm.) Lindl., Pipturus albidus (Hook. & Arn.) A. Gray, Pleomele auwahiensis H. St. John, Remya mauiensis Hillebr., Urera glabra (Hook. & Arn.) Wedd., and Wikstroemia oahuensis (A. Gray) Rock. Ferns are locally common in the understory and include: Asplenium nidus L., Doodia kunthiana Gaudich., Dryopteris sandwicensis (Hook. & Arn.) C. Chr., Lepisorus thunbergianus (Kaulf.) Ching, and Microlepia strigosa (Thunb.) C. Presl. Vines are represented by Alyxia stellata (J.R. Forst. & G. Forst.) Roem. & Schult., Ipomoea tuboides O. Deg. & Ooststr, Lipochaeta connata (Gaudich.) DC, Sicyos pachycarpus Hook. & Arn., and Smilax melastomifolia Sm. Grasses and sedges are sparse and include: Eragrostis variabilis (Gaudich.) Steud., Panicum nephelophilum Gaudich., Trisetum inaequale Whitney, Carex meyenii Nees, and Carex wahuensis C.A. Mey.

Hibiscadelphus stellatus has been observed with buds, flowers and immature and mature fruit capsules in February and April. Flowers open mid-day and produce abundant nectar.

Stellatus – Latin, star shaped, alluding to the stellate pubescence that characterizes the Malvaceae in general, including Hibiscadelphus. The name also refers to the “star-shaped” pattern formed by the five involucral bracts, which contrasts with the cruciform pattern formed by the four bracts in Hibiscadelphus crucibracteatus. Additionally, stellatus acknowledges the beautiful and stellar (outstanding) flowers of this species. The Hawaiian name hau kuahiwi has been applied to other species of the genus (Rock 1913). Hau (Hibiscus tiliaceus L.), a lowland tree; kuahiwi–lit. mountain or high hill (Pukui and Elbert 1986). Hawaiians recognized the similarities of the taxa while observing that Hibiscadelphus grows at higher elevations.

The conservation status of Hibiscadelphus is precarious at best. Three species (Hibiscadelphus crucibracteatus, Hibiscadelphus giffardianus, and Hibiscadelphus wilderianus) were each only known from a single naturally occurring tree (Hobdy 1984; Rock 1913). However, Hibiscadelphus giffardianus survives in cultivation and is planted within the type locality at Kipuka Puaulu in what is now Hawai`i Volcanoes National Park. Hillebrand provided no information on the abundance or scarcity of Hibiscadelphus bombycinus when he first collected it but the species is presumed extinct. Hibiscadelphus crucibracteatus is presumed extinct in the wild since the single known tree died a few years after its discovery from damage by introduced axis deer (Axis axis) despite it being fenced; there is no ex situ material although there were several attempts at propagation (R. Hobdy, pers. comm.). Hibiscadelphus woodii was known from four individuals, but evidently has recently gone extinct (Wood 2012). There are no plants in cultivation despite attempts to propagate it. Hibiscadelphus hualalaiensis is considered extinct in the wild as of 1992 but is in cultivation. Hibiscadelphus wilderianus is also presumed extinct. Although Rock mentioned that Wilder (who discovered the species with Rock, later returning and making several additional collections from the only known tree) had succeeded in raising a single seedling (Rock 1913) no surviving material is known. Hibiscadelphus distans is known from two wild populations of approximately 15–20 individuals total on Kaua`i, and over 100 ex situ collections at the McBryde and Limahuli gardens of the National Tropical Botanical Garden (NTBG). With 99 known plants, Hibiscadelphus stellatus has the largest known wild populations plus the only known naturally occurring seedlings of any species in the genus.

Seeds were collected from 12 individuals of Hibiscadelphus stellatus representing the three known subpopulations. The subpopulations were mapped with GPS and each individual plant numbered and tagged. Cuttings from three plants were also made although these failed to take root. Material is being propagated at the Olinda Rare Plant Facility on Maui, NTBG on Kaua`i and the Lyon Arboretum on O`ahu. The first seeds germinated in conventional propagation approximately 50 days after sowing and under three weeks in tissue culture. As of May 2013 four parent trees from two sites are represented ex situ, with seeds from four additional trees in the third site now in propagation at Olinda and Lyon.

Threats to the existence of Hibiscadelphus stellatus include habitat erosion, fire, weeds, drought, probably rats (Rattus rattus, Rattus exulans) (Baker and Allen 1978) and mice, (Mus domesticus), slugs such as Derocerus and Limax or other invertebrates such as seed weevils (Giffard 1920) and caterpillars (Lorence and Wagner 1995), and potentially feral goats (Capra hirca) and/or pigs (Sus scrofa). Small populations of feral goats and pigs are encroaching in surrounding areas, although the West Maui Mountains Watershed Partnership is constructing strategic fencing. In 2007 a large wild fire burned within 180 m of the plants; succession of its habitat presently includes non-native fire-adapted grasses that were absent before the fire. Erosion is a natural process but is exacerbated by invasion by weeds and ungulates and the destruction of vegetation by fire. Woody non-native plants are currently low in diversity and number, but are represented by known aggressive, habitat – modifying species such as Grevillea robusta A. Cunn. ex R. Br., Lantana camara L., Psidium guajava L., and Schinus terebinthifolius Raddi. Herbaceous understory weeds are similarly low in number of taxa but include serious habitat modifiers such as Adiantum hispidulum Sw., Ageratina adenophora (Spreng.) R.M. King & H. Rob., Ageratina riparia (Regel) R.M. King & H. Rob., Buddlea asiatica Lour., Erigeron karvinskianus DC., and Oplismenus hirtellus (L.) P. Beauv., all of which may hinder establishment of seedlings.

When evaluated using the IUCN Red List criteria (IUCN 2013) Hibiscadelphus stellatus falls into the Endangered (EN) category, a designation for taxa facing a very high risk for extinction in the wild. The species merits this designation by meeting the following criteria: B2(a)(biii, v) + D, where the area of occupancy (AOO) is less than 500km² (B2), with severely fragmented or number of locations <5 (a), and a continuing decline observed, estimated, inferred or projected in (biii) quality of habitat and (bv) number of mature individuals; and D: <250 mature individuals. Although there is some reproduction observed, there is not a sufficient population structure that will allow enough immature plants to replace mature individuals as they perish, therefore a decline is almost a certainty under current conditions. The AOO is 2.28 hectares (5.63 acres) much less than the threshold. The habitat is inferred to be in decline due to the effects of introduced taxa such as invasive plants and rats, as well as the effects of introduced rats and diseases on pollinators. Continued monitoring over the next five years will possibly lead to an updated assessment to CR. Furthermore we recommend that the U.S. Fish & Wildlife Service list this new species as Endangered under the Endangered Species Act of 1973 and that the Service prepare and fund a recovery plan.

USA. Hawaiian Islands: Maui: west Maui, Lahaina District, Kaua`ula Valley, south side slope, 807 m, 17 Apr 2012, Oppenheimer et al. H41214 (BISH), 841 m, 24 Apr 2013, Oppenheimer et al. H41337 (US), 13 Feb 2014, Oppenheimer et al. H21403 (BISH), H21406 (BISH), H21407 (BISH), 820 m, Perlman, et al. 23853 (PTBG); Kaua`ula valley, below Helu, 817 m, 17 Apr 2012, Perlman et al. 22834 (PTBG, 2 sheets), Kaua`ula valley, south slope below Helu summit, 817 m, 18 Apr 2012, Perlman et al. 22837 (PTBG).

This new species clearly belongs to Hibiscadelphus based on its flowers that have their corolla lobes coalescent into a curved, tubular zygomorphic structure. Hibiscadelphus stellatus differs from its congeners in the following combination of characters: moderate to dense stellate pubescence on all parts; involucral bracts 5 (–7) in number that are linear-subulate to lanceolate, 9–22 mm long, and acute to acuminate apically; 5-lobed calyx with tube 22–25 mm long and lobes 5–8 mm x 7–8 mm; externally purplish-colored corolla 5–6.5 cm long; and globose-cuboid to ovoid capsules with scattered hairs on the endocarp. The species of Hibiscadelphus can be separated by the following key.