‘Ohai (Sesbania tomentosa) – Keep it in a pot!

If you have spent any time hiking in Hawai‘i, you know how beautiful an ‘ōhi‘a lehua is in full bloom, or a towering koa with the sunlight flickering through its sickle-shaped leaves. Why then are the roadsides and parks of Honolulu devoid of these native Hawaiian trees? Likewise, the inflorescence of a Hawaiian Trematolobelia is breathtaking, and, yet, these amazingly plants are absent from our front yards and gardens. Why? Coffee table and textbooks would have you believe the greatest threats to Hawai‘i’s native flora are habitat destruction, introduced herbivores like cattle, goats and pigs, highly competitive alien plants, and, most recently, climate change. But which of these explains why we do not see a long row of ‘ahakea shading the cars on Kalakaua Avenue? The truth is we have made the soil in our cities, neighborhoods, and front yards lethal to these and many other native Hawaiian plants with the introduction, over decades, of deadly alien bacteria and fungi and injurious microscopic animals like root-knot nematodes. Unfortunately, the 2018 identification of two species of Ceratocystis, the fungi responsible for the Rapid ʻŌhiʻa Death now threatening to destroy entire native Hawaiian forests, may be the event that finally awakens us to this long-ignored invisible invasion.

Habitat & Appearance: Ohai is an endangered prostrate shrub to small tree found wild today on Necker, Nihoa, Kaua‘i, O‘ahu, Moloka‘i, Kaho‘olawe, Maui, and Hawai‘i. It is also historically recorded from Ni‘ihau and Lāna‘i. Federal, State, and private restoration efforts have returned ‘ohai to many of its formerly recorded sites. In particular, Greg Mansker and his associates have done an amazing job of protecting and restoring the ‘ohai on O‘ahu. Today, there are hundreds of ‘ohai on northern and eastern O‘ahu shorelines where once there were none. ‘Ohai is most often found in coastal areas on calcareous beaches, sand dunes, rocky ridges and slopes. However, there are also inland populations growing in deep red soil and on lava plains.

‘Ohai populations vary in stature, flower color, and pubescence. Coastal forms are generally prostrate or sprawling shrubs while inland forms are taller or even tree-like. The pea-like flowers can be yellow, salmon, red, or a mix of these colors. The pinnately compound leaves of some ‘ohai populations are densely covered with silky hairs making them appear gray, while other populations have nearly glabrous green leaves. The flowers mature into long thin tan fruits (pods) contain 6-27 olive to brown seeds.

(Top to Bottom) ‘Ohai seeds and pods. Seedling. ‘Ohai on O‘ahu. Prostrate ‘ohai on Kaho‘olawe. Treelike ‘ohai on Maui (Kaho‘olawe and Maui photographs courtesy of Forest & Kim Starr ©).

‘Ohai in Hawaiian Culture: In the past, Hawaiians strung ‘ohai flowers into lei. Today, such lei are extremely uncommon because of the plant’s rarity.

Collecting Seeds: On O‘ahu, ‘ohai flowering is most common during the winter to spring rains, with fruits (pods) ripening in the spring and summer. In cultivation, flowering and fruiting is sporadic throughout the year. Collect the pods when they are dry and tan. Inside, the mature seeds will be olive green to brown. Examine each seed carefully. Often seeds will have holes or other damage caused by insects, rodents, or fungi. (At Ka‘ena Point, O‘ahu, ripe pods were rare and seedlings nearly non-existent until the State erected a fence around the Point and eliminated the mice and rats within the enclosure.) To detect unseen holes or internal damage, place the seeds in water; damaged seeds often float because of the trapped air inside. Seeds remain viable in the refrigerator for at least ten years.

Growing from Seed: Propagating ‘ohai from seed is easy. Carefully scar the small seeds with a nail-clipper or sandpaper immediately prior to sowing. You can use any of my Methods to germinate the seeds. However, I prefer to use Method Three with a media mix of 2:1:1, washed fine coral rubble, washed coral sand, peat moss; this mixture closely matches the natural soil of coastal ‘ohai varieties while reducing microbial contamination. The first seeds should sprout in about two weeks and continue for another month. Your seedlings should grow at a moderate pace and be 6-12 inches tall in 3-6 months. At this point, you can transfer them to a larger pot or permanent container (see Growth in the Garden below). Seedlings are frequently attacked by mites and sap-sucking insects such as aphids and mealybugs. Refer to Enemies in the Garden for methods to combat these pests.

Growing from Cuttings: I have never tried to root ‘ohai cuttings. I do not know anyone who has.

Growth in the Garden: Nearly all of my knowledge of ‘ohai comes from my experiences growing the coastal form endemic to O‘ahu (i.e., Ka‘ena Point). Twice, I have grown the treelike ‘ohai endemic to Moloka‘i. On both occasions, the plants grew vigorously at two leeward O‘ahu sites (100 and 500 ft elevation, dry, mostly clay soil) for 2-3 years, flowered and fruited often, and then died within 1-2 months. On the first occasion, I examined the dead plants’ roots and discovered extensive root-knot nematode damage. After the second death, I just presumed root-knot nematodes were the cause. Early on, I planted my O‘ahu coastal ‘ohai into inland gardens where the soil was primarily clay and rock. And, like the Moloka‘i treelike ‘ohai, they would grow well for 1-3 years and then suddenly die. I did not always check the roots of these dead ‘ohai, but when I did I always found root deformities consistent with root-knot nematodes.

Professional and amateur horticulturists have prescribed an assortment of prophylactic treatments for root-knot nematodes.

  • Kerin E. Lilleeng-Rosenberger recommends using a nematicide mixed into the soil at the time of outplanting, or soil drenches every few months with neem oil extracts. Unfortunately, none of the effective commercial nematicides are available to the average homeowner in Hawai‘i. There are two organic nematicides available: those based on geraniol (the oil of geraniums), and those based on Quillaja saponaria, the soap bark root.

  • Leland Miyano suggests regular soil drenches with seawater.

  • Nick Sakovich (aka: Garden Guy Hawaii) recommends applying organic matter to the soil which will “encourage the growth of beneficial organisms, fungi, bacteria and good nematodes. They, in turn, will ‘attack’ the plant parasitic nematodes and deplete their numbers.”

  • Kevin Espiritu (founder of Epic Gardening [https://www.epicgardening.com/root-knot-nematode/]) suggests adding neem seed meal, crab meal, or oyster shell flour to your soil. “All three are fertilizers, but they’re great soil builders in the war against root knot nematodes.”

  • Several gardeners suggest planting companion marigolds next to nematode-susceptible plants. Unfortunately, Nick Sakovich tells us “planting a few marigolds next to each susceptible plant will NOT eliminate the problem.” Refer to his blog (http://www.gardenguyhawaii.com/search?q=root-knot) for details why.

To be brutally honest, I have not tried any of the above treatments in my attempts to maintain ‘ohai – because I am just too lazy to maintain a regular soil maintenance schedule. Rather, I have acted on an observation I made long ago: ‘Ohai I keep in pots in the nursery live much longer than ‘ohai I plant in the ground. So, I no longer plant out ‘ohai unless they are part of a coastal restoration project. Following my advice does not mean you cannot have an ‘ohai in your garden. Rather, I suggest you do what a lot of locals do – include container plants in your garden. For ‘ohai, the ideal situation is a large container, slightly elevated off the ground (on bricks, blocks, wooden legs) to make it difficult for root-knot nematodes to enter the container, containing a 2:2:1 mix of washed coral rubble, coral sand, and fresh peat moss or coconut coir, positioned in the sunniest, driest spot in your garden.

Water your ‘ohai as infrequently as possible. Depending on the plant and container size, one deep watering once a month or once a week is much better than daily light waterings. If you are less lazy than me, give your ‘ohai an occasional leaf spray or soil drench with seawater, or add some neem extract or organic matter to the pot every so often. I recommend starting out these treatments at a once every 3-4 months schedule. Using this keep-it-in-a-pot technique, I have maintained coastal ‘ohai for over five years, long enough to enjoy their beauty and collect a lot of seeds. When your ‘ohai dies – yes, even wild ‘ohai only live about ten years based on my observations – dig it up and inspect the roots. If you see root-knot nematode damage, start your new ‘ohai with all new materials (i.e., a new or sterilized pot and new clean media).

Diseases & Pests: As detailed above, root-knot nematodes are the most serious threat to ‘ohai. However, seedlings and mature plants are also occasionally infested by spider mites, aphids, and mealybugs. Refer to Enemies in the Garden for ways to combat these pests. I have also seen black stinkbugs on ‘ohai. These, I simply killed by hand.

Ā ā Ē ē Ī ī Ō ō Ū ū