Uhiuhi (Mezoneuron kavaiense; formerly Caesalpinia kavaiensis) – The value of cultivation

    Back in the mid-1990s, it was impossible to get seeds of O‘ahu uhiuhi for cultivation. This was understandable. At the time, there were only about three dozen wild uhiuhi left on O‘ahu, all mercilessly being attacked by the alien black twig borer. State botanists were diligently and tirelessly collecting seeds from these last trees for long-term storage until some plan could be formulated to deal with this foe and reintroduce uhiuhi back into O‘ahu’s remaining dry forests. Seeds from Big Island uhiuhi were nearly as hard to come by. Howver, I was determined to grow and learn about this plant and was fortunate enough to acquire a few seeds from an uhiuhi already in cultivation. When one of the seedlings was large enough, I planted it out on the campus of Leeward Community College (LCC) where I was then an Instructor of Biology. The uhiuhi did amazingly well, growing into a vigorous fruiting tree that almost never was attacked by borers. 

    I left LCC in 1998, and five years later Frani Okamoto became the LCC Shadehouse & Native Plant Collection Manager. In 2004, Frani and I heard about four orphaned O‘ahu uhiuhi being kept in the State’s (DOFAW) Rare Plant Facility at Pahole. The story was, as best as we could determine, that parent plant documentation on the four uhiuhi was in question so the plants had not been used in a reforestation project but remained at the Pahole facility. Frani and I felt this was the perfect opportunity to get the O‘ahu uhiuhi into cultivation. We convinced DOFAW (and the O‘ahu Army Natural Resources Program because two of the uhiuhi belonged to them) that they should give the four plants to Frani who would plant them out on the LCC campus. 

    It was a sad but necessary day when we dug up and removed the eight-year-old Big Island uhiuhi from LCC; the O‘ahu uhiuhi had begun to flower and we could not risk producing hybrid seeds from the two island varieties. Like the Big Island uhiuhi before them, the O‘ahu plants grew vigorously at LCC. They also started to produce an enormous amount of seeds that Frani religiously collected and stored; by 2009, she had over 1,700 seeds. In 2008, Frani obtained endangered species commercial sale tags, and, for the very first time, O‘ahu uhiuhi were available for purchase by the public. In addition, Frani has given 2,550 seeds (2005-2010) back to the Rare Plant Facility at Pahole and Lyon Arboretum’s Seed Conservation Laboratory. DOFAW plans to propagate the seeds for test plantings throughout the Wai‘anae Mountains to locate viable uhiuhi recovery sites. This way, the much more valuable wild-plant seeds collected by botanists many years ago will not be wasted on failed attempts to find the uhiuhi a safe home on O‘ahu.

Habitat & Appearance: Today, uhiuhi inhabit a few of the dry and mesic forests on O‘ahu and Hawai‘i. It’s my understanding that uhiuhi populations once existed on Kaua‘i, Lāna‘i, and Maui but these are now gone. However, because botanists collected seeds from these now-extinct populations, cultivated plants can still be located. Young trees tend to be multi-branched while much older uhiuhi usually have one or two main trucks. Very old trees can be quite tall; Wagner et al. (1990) give a height of 3 - 10 meters (10 – 33 feet) but at least one tree I remember in Kona (Hawai‘i) was taller than this. The rough gray bark and compound leaves make uhiuhi an attractive tree but it is the numerous inflorescences of pink to red pea-like flowers and large flat seedpods that really make this tree stand out in a forest or garden. (Uhiuhi flowers grace the header photograph for the homepage of this website.) 

Uhiuhi in Hawaiian Culture: Uhiuhi wood is extremely dense and sinks in seawater. It is often very dark – even black – but I’ve also seen nearly white uhiuhi wood. Like kauila, Hawaiians used this dense wood for weapons, tools, fishhooks, hale beams, and kapa boards (Abbott 1992, Krauss 1993). Perhaps, the most interesting use of uhiuhi wood was for the runners of holua, sleds used for recreation on steep grassy slopes (Handy & Handy 1992). According to Chun (1994), a medicine to purify the blood was made from young uhiuhi leaves, leaf buds, and bark. Today, some lei-makers use uhiuhi flowers, so it’s likely this was also an ancient practice.

Collecting Seeds: In the wild, uhiuhi normally flower and fruit only once a year during the winter months. However, in the garden, this period is often lengthened or trees will flower and fruit more than once annually. Collect the mature fruits when they are large, brown (or tan) and dry. Often the mature seed(s) inside will rattle when you shake the fruit. Young mature seeds are anywhere from the size of a penny to that of a quarter and brownish-green, turning dark brown with age. The seeds remain viable for a long time – at least ten years if stored in the refrigerator!

Growing from Seed: Soak uhiuhi seeds overnight before scarring (with sandpaper or nailclippers) and sowing. It’s best to sow the seed flat on well-drained, clean media, covering it with just ¼ - ½ inch of media (or sphagnum moss). The seed quickly swells to 2-3 times its original size and, within a week, a stout white root has emerged and begins penetrating deep into the media. If you have used Method One, transfer the developing seedling into a dibble tube or deep container at this time. (Sometimes, uhiuhi seeds are too large for the standard narrow dibble tube, in which case, you should sow the seed in a wider but still deep container.) After about two weeks, the seed’s shoot should have emerged (from the same end as the root) and begin developing its first set of leaves. Using the considerable energy stores within the seed, seedlings grow very rapidly to about eight inches in height in a few weeks. Wait until the seedling’s stem has turned dark and hardened somewhat (the seedling is normally 10-12 inches tall at this stage) before planting it out in your garden. Or, if you’re worried about snails, slugs, rats, birds or other pests attacking the young plant, consider repotting the uhiuhi and letting it grow to about two feet before planting it out. Sucking insects such as aphids, whitefly, or scale insects frequently attack seedlings in the nursery. Use insecticidal soap or horticultural oil to eliminate these pests.

Growing from Cuttings: I have never tried to grow uhiuhi from cuttings. Others have successfully air-layered uhiuhi; they take about three months to root.

Growth in the Garden: For the first few months after planting out, uhiuhi appear to stop growing, at least above ground. I suspect the young plant is diverting all its energy into new root growth or waiting for beneficial microbes such as mycorrhizae fungi to become associated with its roots. However, I’ve never dug up a young uhiuhi to test this hypothesis. In any case, be patient and water but don’t overwater (i.e., just enough to avoid wilted leaves) the uhiuhi during this stasis. Following this slow-growth period, uhiuhi grow rapidly and can be over five feet tall in a year. At this young stage, it’s common to have multiple large branches develop and the young tree becomes top-heavy. This is a problem since often the branches will break or the entire tree will fall over because of strong winds. You can protect against this happening by doing one or more of the following: pruning the tree, supporting the tree with lines or braces, weighing down the tree’s roots by surrounding the trunk base with large rocks, or planting the uhiuhi among taller trees to protect it from strong winds. Uhiuhi begins flowering and setting fruit after one to three years in the garden; be smart and collect and store this early seed in case your young tree meets an unnatural end (see below). After reaching about ten feet in height, uhiuhi grow more slowly, possibly because energy is being diverted to flowers and fruits and increasing the girth of its trunk(s) and branches.

Diseases & Pests: Uhiuhi is deceptively difficult to keep alive. I have grown and maintained a few uhiuhi for more than five years, as have others (see story above). But many more young Mezoneuron have died under my care (or, more properly, have died because of my insufficient care.) Young uhiuhi are targets for several insect pests, sometimes with fatal consequences. Scale insects, particularly hibiscus snow scale, frequently attack small uhiuhi, and, if left untreated, can kill young trees. Repeated sprayings with horticultural oil, a systemic insecticide, or both can eliminate the scale. The black twig borer can deliver a fatal hit to uhiuhi less than two feet tall. Taller uhiuhi are also attacked but usually only side branches are killed; when the main truck is killed, new sprouts appear below the borer holes. Refer to Enemies in the Garden for ways to minimize black twig borer attacks. Other borers such as the false powderpost beetle can quickly kill both small and quite large uhiuhi if they are drought-stressed. (This beetle recently killed one of my six-foot-tall uhiuhi in less than a week!) Interestingly, less stressed uhiuhi (and lonomea) defend themselves by producing a sticky sap that quickly fills the borers’ holes and hardens.

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