Oahu Riverhemp

Perennial, Shrubs, Stems woody below, or from woody crown or caudex, Taproot present, Nodules present, Stems or branches arching, spreading or decumbent, Stems prostrate, trailing, or mat forming, Stems less than 1 m tall, Stems 1-2 m tall, Stems greater than 2 m tall, Stems solid, Stems or young twigs glabrous or sparsely glabrate, Stems hairs pilose or spreading, Stems silvery, canescent, tomentose, cobwebby, or wooly, Leaves alternate, Leaves petiolate, Stipules inconspicuous, absent, or caducous, Stipules setiform, subulate or acicular, Stipules deciduous, Stipules free, Leaves compound, Leaves even pinnate, Leaf or leaflet margins entire, Leaflets opposite, Stipels present at base of leaflets, Leaflets 10-many, Leaves hairy on one or both surfaces, Flowers i n axillary clusters or few-floweredracemes, 2-6 flowers, Inflorescences racemes, Inflorescence axillary, Bracteoles present, Flowers zygomorphic, Calyx 5-lobed, Calyx glabrous, Petals separate, Corolla papilionaceous, Petals clawed, Petals reddish brown, maroon, Petals orange or yellow, Petals bicolored or with red, purple or yellow streaks or spots, Banner petal suborbicular, broadly rounded, Wing petals narrow, oblanceolate to oblong, Wing petals auriculate, Wing tips obtuse or rounded, Keel petals auriculate, spurred, or gibbous, Keel abruptly curved, or spirally coiled, Keel tips obtuse or rounded, not beaked, Stamens 9-10, Stamens diadelphous, 9 united, 1 free, Filaments glabrous, Style terete, Fruit a legume, Fruit stipitate, Fruit unilocular, Fruit tardily or weakly dehiscent, Fruit elongate, straight, Fruit or valves persistent on stem, Fruit exserted from calyx, Fruit compressed between seeds, Fruit beaked, Fruit glabrous or glabrate, Fruit 11-many seeded, Seeds ov oid to rounded in outline, Seed surface smooth, Seeds olive, brown, or black.

Sesbania tomentosa, commonly known as Oahu riverhemp and ʻōhai, is an endangered species of plant in the pea family, Fabaceae, that is endemic to the main Hawaiian Islands as well as Nihoa and Necker Island. It inhabits low shrublands and, rarely, dry forests, at elevations from sea level to 2,500 ft (760 m). Associated native plant species include akiʻaki (Sporobolus virginicus), ilima (Sida fallax), naupaka kahakai (Scaevola taccada), and pili (Heteropogon contortus). Off-road vehicles, wildfires, grazing, and alien species competition have destroyed their habitat on the main islands, but they are still quite common on Nihoa and Necker. At least 2000 specimens grow on Nihoa, while there are far less on Necker.

ʻŌhai is highly polymorphic, exhibiting broad variations in color and shape. Plants that grow on Nihoa have reddish-orange flowers and young leaflets that are relatively hairless. Necker plants have salmon to orange colored-flowers, and leaflets that are very hairy. A form that grows as a standing tree exists on Molokaʻi. ʻŌhai grows as a prostrate shrub with semi-glaucous hairless leaves on the southernmost tip of the island of Hawaiʻi, Ka Lae.