Native Plants of Hawai‘i - Sesbania tomentosa

Sesbania tomentosa

Fabaceae or Pea family [Five-minute families]

Synonyms: Sesbania arborea, Sesbania hawaiiensis, Sesbania hobdyi, Sesbania molokaiensis, Sesbania tomentosa f. arborea, Sesbania tomentosa var. molokaiensis

ʻOhai

Pū ʻohai

Oʻahu riverhemp

Natural Range & Environment

Historically much more common, Sesbania tomentosa was endemic to all eight of the main Hawaiian Islands as well as Nīhoa and Necker (Mokumanamana) in the Northwestern Hawaiian Islands. Currently, it is extinct on Niʻihau (and, possibly, Lāna‘i) and restricted to relict populations on the other main Hawaiian Islands; it is still a rather common plant on Nīhoa and Necker. S. tomentosa is federally listed as endangered throughout its range. According to the last USFWS 5-year review (2021), there are "hundreds of plants on the Northwestern Hawaiian Islands of Nihoa and Mokumanamana, with fewer than 700 wild individuals of Sesbania tomentosa documented on Kaua‘i, O‘ahu, Moloka‘i, Maui, and Hawai‘i" [USFWS;ECOS]. Fortunately, there are active restoration efforts on most of the islands where S. tomentosa grows, and, in some cases like on O‘ahu, the number of planted S. tomentosa greatly exceeds the number of remaining wild plants.

Sesbania tomentosa grows in dry regions on sandy beaches and dunes, lava, and pond margins (on Kaua‘i) from sea level to 830 m (2,723 ft) [Wagner et al. 1990].

Populations of ‘ohai often differ in leaf, flower, and fruit characteristics, as well as size and stature. Here are three of several flower-color variants (top to bottom): ʻApua Point on Hawai‘i Island, Maui, Nīhoa. Ripe seed pods are dry, long, and thin. A cultivated prostrate Maui form in a container. A cultivated silvery low-shrub O‘ahu form in the ground. A wild upright ‘ohai in the coastal sands of Kaua‘i. A wild tall ‘ohai on a dry lava field on Maui. (Photographs of seed pods and upright Kaua‘i plant courtesy of Kenneth R. Wood [NTBG] CC BY-NC 4.0; Photograph of silvery O‘ahu plant courtesy of thatplantguy CC BY-NC 4.0; Photograph of tall Maui plant courtesy of Forest & Kim Starr CC BY 2.0; Header photograph of O‘ahu plant courtesy of Shelomi Doyle CC BY-NC 4.0.)

Description

Sesbania tomentosa varies significantly in stature, flower color, and leaf pubescence (hairiness). This variation is almost entirely seen between geographically separate populations with little noticeable differences among plants within a single population. For example, the wild ʻohai at Ka‘ena Point, O‘ahu, are all silvery sub-shrubs with peach-colored flowers (see header photograph). These plants differ dramatically in stature from the extremely prostrate ʻohai at Ka Lae (South Point) on Hawai‘i Island or the tree-like ʻohai growing at Kanaio on Maui. Some botanists, noting these population differences, have proposed formally dividing Sesbania tomentosa into two or more species, subspecies, or varieties; however, to date (2025), it is still classified as a single species without formal divisions.

Our field observations of ʻohai at Ka‘ena Point, O‘ahu, confirm that wild plants typically live longer than five years, perhaps even more than ten. We suspect, but have not (yet) confirmed, that wild ʻohai on the other Hawaiian Islands likely have a similar lifespan. This is in marked contrast to our experiences with several forms of ʻohai from different islands cultivated by us on Oʻahu. In most cases, these cultivated plants have lived only a few years. We suspect that either cultivation somehow accelerates the life cycle of Sesbania tomentosa, or a disease or pest within the soil (probably root-knot nematodes) is responsible for this shortened lifespan.

Depending on its origin, Sesbania tomentosa can be anywhere from a creeping shrub only inches above the ground to a small tree more than ten feet (3 m) tall. The pinnately compound leaves of ʻohai can be anywhere from densely covered in silky hairs to hairless, which alters their apparent color; silvery for pubescent forms, dark green for hairless forms. Interestingly, ʻohai leaves fold up at night. According to Rick Barbosa [per. comm.], on sunny days, the leaf clusters at the plant's stem tip have a fragrance resembling nectarines.

Sesbania tomentosa has perfect (bisexual) pea-like flowers about one inch (2.5 cm) long that vary somewhat in shape but dramatically in color. Depending on the original source (i.e., wild population) of the plant, flowers can be light to dark red, subtle to bright orange, apricot to yellow. On O‘ahu, ʻohai blooms sporadically throughout the year, but most often the peak period is during the winter and spring after heavy rains. We suspect the blooming of ʻohai on other Islands is also rain-dependent. Cultivated plants bloom sporadically year-round. Pollinated flowers develop into long thin green pods that dry and turn tan-colored when ripe. Within each pod are several squarish- to oval-shaped olive green to brown seeds about 5 mm (⅕ in) long.

Growth Requirements

General

Regrettably, we cannot recommend planting your new ʻohai in the ground — unless you happen to own some oceanfront property. This is because nearly all the Sebania tomentosa, both coastal or upland forms, we have ever planted inland (i.e., away from the ocean) have died prematurely (i.e., in less than five years). We attribute these deaths to an attack on their roots by either root-knot nematodes (confirmed in some cases) or a Fusarium fungus (presumed in other cases). Fortunately, both coastal and upland ʻohai can be grown in a container where they are better protected from root pests and diseases.

Start with a new or sterilized (see below on how to sterilize) container. If you are growing a coastal ʻohai, fill the container with coralline sand and crushed coral. You can buy these legally from most garden shops or home-improvement stores; taking sand or coral from the shoreline is illegal in Hawai‘i. If you are growing an inland form of ʻohai, fill your container with 100% black cinder. Avoid placing the container directly on the ground, but instead, place it on a raised platform (e.g., cinder blocks, bricks) to make it more difficult for soil pests to migrate into the container. Transplant your ʻohai into the container and water thoroughly.

Sesbania tomentosa does best in full sunlight for all or most of the day. Therefore, place your container with plant in an appropriate site. Somewhat surprisingly, you can also grow ʻohai inside, either right next to a sunny window or under LED lights. An indoor ʻohai will not grow as quickly or be as silvery as those grown outside in direct sunlight, but they do have the advantage of being farther away from harmful soil (and other) pests.

After the initial transfer and watering, set up a watering schedule that provides your ʻohai with the near-minimum amount of water required to keep it growing. In other words, just a bit more water than needed to prevent leaf wilting and excessive leaf loss. (Remember, ʻohai folds up its leaves at night, so don't get fooled.) Overwatered plants can develop a black sooty mold on their trunk, branches, and even leaves. Too much water can also lead to stem and root rot.

‘Ohai in containers grow at a slow to moderate pace, depending in part on the size of the container, but begin flowering even when small (e.g., one foot [30 cm]). If your plant is growing too slowly or seems stunted (i.e., pot-bound), try transferring it to a larger container, giving it a little more water, or giving it just a bit of fertilizer (e.g., an organic or inorganic water-soluble fertilizer at ⅓ to ½ the recommended strength). However, remain cautious because too much water and/or fertilizer can result in fewer flowers, abnormal growth, and pest or disease problems.

All ‘ohai are heat and drought-tolerant. Coastal forms are tolerant of salt spray, but we don't know if inland forms also share this trait. Coastal forms actually seem to benefit from an occasional (e.g., once a month) media drench or foliar spraying with seawater, perhaps because the seawater provides nutrients or helps deter pests.

Eventually, even under the best care, your ‘ohai will die; consider five to ten years a good lifespan for this species. What do you do then? Well, first, pull up your dead ‘ohai and closely examine its roots. If you cannot see any evidence of root-knot nematode damage (see the Pests & Diseases page) or other root abnormalities, then, you are good to go and can transfer a new ‘ohai into the old container and media. However, if you see any evidence of root-knot nematodes (or other suspicious root damage), you need to start again from scratch. (If your newly-planted ‘ohai dies prematurely, you will know you missed something and should start from scratch.)

Starting from scratch means either using a brand-new container and new media or sterilizing the old container and media. To sterile a relatively small container: (1) scrub it clean with soap and water, (2) submerge it in a bucket of 10% to 15% bleach for a day, (3) submerge it again, this time, in clean water for another day, and (4) let it air dry, preferably in the sun, for a few days. For a large container: (1) scrub it clean with soap and water, (2) wipe or spray it thoroughly with 10% to 15% bleach, (3) let it air dry for a day, (4) repeat steps 2 and 3 two or three more times, (5) rinse it thoroughly with water, and (6) let it air dry for a few days. If you used coralline sand and crushed coral for media, you can sterilize these as well by: (1) washing/rinsing the media in a bucket of water (without soap!), (2) submerging the sand and coral in a bucket of 10% to 15% bleach for a day, (3) submerging it again, this time, in clean water for another day, (4) repeat step 3 for two or three more times, and (5) let the sand and coral air dry, preferably in the sun, for a few days. If you used black cinders as medium, we recommend not trying to sterilize them, but instead discarding them and using new cinders.

Finally, if you choose to disregard our advice and plant your ‘ohai in the ground, there are some things you can do to discourage root-knot nematodes, Fusarium fungi, and other soil pests and pathogens. Go to our Pests & Diseases page for details. Additionally, be sure to collect seeds from your ‘ohai as soon as possible so you're able to grow a replacement plant.

Pests and Diseases

‘Ohai are sometimes attacked by aphids, black stink bugs (it's easiest to kill these by hand), black twig borers, mealybugs, scale insects, and spider mites. Media drenches or foliar sprays with seawater can help discourage these attacks. Leland Miyano [per. comm.] has told us that these seawater drenches can also prevent or control root-knot nematodes. Refer to the Pests & Diseases page for ways to deal with all the above-mentioned pests. Overwatered plants can develop a black sooty mold on their trunk, branches, and even leaves, as well as lead to stem and root rot.

Uses

Ancient Hawaiian

ʻOhai flowers were strung into lei [Krauss 1993].

Modern

ʻOhai are still used today in lei and strung like other native pea-like flowers, such as ʻāwikiwiki, māmane, nanea, nuku ʻiʻiwi, and wiliwili [McDonald & Weissich 2003].

Special Features and Information

General

Fabaceae contains numerous agriculturally important plants, and is the third-largest plant family in number of species, behind only the Orchidaceae and Asteraceae, with about 765 genera and nearly 20,000 known species. The family has an essentially worldwide distribution, with species being found everywhere except Antarctica and the high Arctic. Trees are most common in tropical regions, while herbaceous plants and shrubs are predominant outside the tropics [WikipediA;Fabaceae].

Hawaiian Name

Some sources spell the Hawaiian name of Sesbania tomentosa with the kahakō macron over the "o" as ʻōhai; others do not. We have chosen to follow the Pukui & Elbert [1986] spelling without the kahakō, ʻohai. Hawaiians on Niʻihau called this plant ʻohai o Papiahuli; the meaning is unknown [Wichman & St. John 1994].

In Hawai‘i, ʻohai is also the name for the non-native monkeypod or rain tree (Samanea saman) and the white monkeypod (Albizia lebbecki). The name is also used, in part, for other non-native plants such as: ʻohai keʻokeʻo for the white-flowered form of Sesbania grandiflora, ʻohai ʻulaʻula for the red-flowered Sesbania grandiflora var. coccinea, ʻohai ʻula for the royal poinciana (Delonix regia), and ʻohai aliʻi for the pride of Barbados (Caesalpinia pulcherrima) [Pukui & Elbert 1986].

Etymology

The genus name, Sesbania, is derived from sesban, the Arabic name for Sesbania sesban (syn: S. aegyptiaca) or Egyptian pea [Wagner et al. 1990]. The species name, tomentosa, means "thickly matted with hairs" [Gledhill 2008], though not all forms of ‘ohai are hairy.

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