Koai‘a (Acacia koaia) – Diversity matters!


    Evolution doesn't happen without genetic diversity. The reason we have so many different dog breeds today is because the ancestral dog thousands of years ago was a genetically diverse species that we were able to mold using artificial selection. The ancestors of many native Hawaiian plants had their genetic diversity severely reduced because for most only one or a few seeds were able to make it to the Islands via wind, water, or wings (i.e., birds). However, this limited diversity, increased over time by mutations, often expressed itself in the numerous open and favorable habitats in the Islands with an explosion of new species (e.g. Cyanea, Schiedea, Pritchardia). 

     Koai‘a hasn't yet diversified enough to be considered more than one species (with some taxonomists still considering it a subspecies of koa [Acacia koa] ). However, there is horticultural evidence that koai‘a has become a genetically diverse species both between and within the many populations found on Kaua‘i, O‘ahu, Moloka‘i, Lāna‘i, Maui, and Hawai‘i. This evidence is expressed in the sometimes dramatic differences in the health and lifespan of koai‘a in cultivation. Primary in these genetic differences is a tree's resistance or immunity to its number one killer, the fungal disease koa wilt (Fusarium oxysporum. f. sp. koae). Based on the observations made by me and other native plant botanists and horticulturists I have reached out to, and on formal research conducted on koa, your success with koai‘a will depend a lot on where your koai‘a came from (i.e., original seed source), where your garden or restoration site is geographically, and, unfortunately, blind luck. Below, I elaborate on these factors. 

Habitat & Appearance: A small tree endemic to open dry sites on Kauaʻi, Molokaʻi, Lānaʻi, Maui, and Hawaiʻi. Koaiʻa is also found on O‘ahu, but only in very small populations in the Koʻolau Range mountains. Seedlings and sucker growth on mature trees have feathery compound leaves. As the tree matures, these are replaced by sickle-shaped phyllodes which are flattened leaf stems. Small yellow spherical compound flowers develop into thin pods that turn brown and dry when ripe, each containing several black flattened seeds.

    Compared to koa, mature koaiʻa: (1) are normally much smaller, (2) have straighter phyllodes, (3) have much denser wood, and (4) have seeds arranged longitudinally in the pod rather than vertically. 

    Both koaiʻa and koa have a symbiotic relationship with a nitrogen-fixing bacterium called Bradyrhizobium in their roots that help the tree grow and fertilize the surrounding soil. These bacteria produce an ammonia-like fragrance that emerges from the soil surrounding the tree. 

(Top to Bottom) A beautiful but now-dead young koai‘a at Leeward Community College. Koai‘a usually has less curved phyllodes and thinner pods than koa. Ripe pods with seeds arranged longitudinally. Seedling. (Header photograph courtesy of David Eickhoff ©.)

Koai‘a in Hawaiian Culture: Hawaiians used the hard koaiʻa wood for numerous tools and weapons such as spears, fish lures, paddles, and kapa beaters, while leaves were sometimes used to cover hale. A mixture of ground koaiʻa leaves and bark with ʻauʻaukoʻi (Senna occidentalis) and kikānia pipili (Desmodium sandwicense) stalks was used in a steam bath to treat skin diseases.

Collecting Seeds: Flowering and fruiting is generally more sporadic in koai‘a than koa, with wild trees normally blooming once a year during the winter or spring and bearing ripe pods in spring or summer; cultivated trees sometimes flower and bear fruit more often or not at all. One thing that is rather consistent is koai‘a need to be cross-pollinated to set fruit; in other words, a lone tree may flower regularly but will not produce seeds. (If you have seen otherwise, a self-fertile koai‘a, please contact me.)

    Collect the ripe pods when they are brown and dry. Inside you will find a few to several shiny flat black seeds. Unfortunately, like koa, koai‘a seeds are attacked and eaten by larvae of the koa seedworm (Cryptophlebia illepida), a moth, sometimes so much so that it may be difficult to collect more than a few viable seeds from dozens of ripe pods. It is possible to reduce this predation by bagging the immature green pods early to prevent the female moth from laying her eggs on the pods, or by setting out sticky traps to capture and kill the moths. However, these preventative measures require more that one visit to the fruiting trees. Alternatively, if you have the time, you can search the ground beneath a mature koai‘a for viable seeds. I have done this more than once during off-season visits and successfully come home with a few to a dozen viable seeds. Seeds are also destroyed by the koa haole seed weevil (Araecerus levipennis) and the bruchid beetle (Stator limbatus). If you intend to inoculate your koai‘a seedlings with their symbiotic nitrogen-fixing bacteria, collect 2-3 cups of soil from beneath the mother tree(s) before leaving the site.

    Back at home examine each seed carefully for small holes, and test your seeds in a cup of water; the viable seeds will sink. Before sowing or storing your seeds, handwash them in soapy water and sterile the outer surface by soaking them for 15 minutes in 10% bleach. (The washing and sterilization will remove any tiny weevil, beetle, and seedworm eggs, and may help remove or destroy harmful fungal spores like those that cause koa wilt.) Stored in the refrigerator, koai‘a seeds remain viable for decades.    

Growing from Seed: After cleaning and sterilizing your seeds, scar each (I use a nail-clipper), and sow them using Method Two; use dibble tubes if you intend to plant out your koai‘a soon, treepots (i.e., extra-tall square pots) if your planting date is more distant. 

    Scarified koai‘a seeds sprout quickly, in 1-2 weeks, and grow fast! Within 3-6 months you should have a 12-18 tall seedling ready to plant into your garden. You can further enhance seedling growth with light fertilization (e.g., liquid Miracle-Gro®). I normally do not inoculate my seedlings with their symbiotic Bradyrhizobium, but rather wait for them to pick up these nitrogen-fixing bacteria after I plant them out. However, if you intend to use your koai‘a as part of a restoration project, you should give them every advantage and inoculate them a couple of weeks after they sprout. The easiest ways to do this is to add a pinch of the soil you collected beneath the mother tree to each pot, or mix the soil with some water and pour a bit of the slurry into each pot. In about a month you should know if your inoculation was successful because inoculated koai‘a grow about twice as fast as non-inoculated non-fertilized seedlings.

    Many, maybe most, native Hawaiian plants behave in dibble tubes. What I mean by this is that once their roots reach the bottom of the tube, both root and shoot growth slow dramatically, and the plant enters a kind of stasis. Koai‘a are a dramatic exception. Seedlings will continue to grow regardless of the container they are planted in. (So much so that I have seen on multiple occasions koai‘a [and koa] seedlings split their dibble tube along its entire length.) It is therefore very important to plant out your koai‘a before it outgrows its pot and becomes a problem (e.g., difficult to water, abnormal root growth).

    Koai‘a seedlings are occasionally infested with sap-sucking pests like mealybugs and scale insects; cottony-cushion scale has been my most common problem. They may also become infected by the koa wilt fungus within the nursery. These wilt afflicted plants yellow, drop their leaves, and soon die. First isolate the sick seedlings from their healthy siblings. Then spray them with horticultural oil or insecticidal soap because mites and other pests like psyllids can cause similar symptoms. If the seedlings do not improve, they should be discarded. 

Growing from Cuttings: I have never tried to grow koai‘a from cuttings. However, it may be possible since HARC scientists have successfully rooted cuttings of wilt-resistant koa.

Growth in the Garden: Because of its cultural significance and economic value there have been countless articles, webpages, and even books written about koa. And, since koai‘a and koa share a lot of the same horticultural characteristics, particularly when it comes to propagation, pests, and diseases, it is impossible here to give a complete account of all that is known or theorized about growing koai‘a. Therefore, for the most part, I will share here only my own experiences and advice, and those of people I personally know.

    Let me start with a depressing likelihood — your koai‘a will probably die from koa wilt within ten years of planting. Unfortunately, there is currently no effective treatment for this disease, and the possibility of your tree contracting koa wilt becomes increasingly likely the hotter and or more urban your garden is. This is because the wilt fungus is more likely already in the soil of urban landscapes, and is more virulent at higher temperatures. However, the good news is: 

    Plant your koai‘a seedling in a sunny, dry site in your garden that has good drainage. While wild and cultivated trees can go for months without watering, water your koai‘a regularly (how often will depend on the natural rainfall at your site) to prevent stress that could leave your tree more susceptible to koa wilt infection and pests like the black twigborer. Conversely, do not overwater, because this (i.e., waterlogged soil) can also promote infection. (Sounds a bit like the fairy tale, Goldilocks and the Three Bears?) Your tree should grow quickly, about three feet taller each year, until it starts flowering in its third-plus year in the ground. Then, most of the trees I have grown or seen dramatically reduce their upward growth, now being 10-15 feet tall. If you want to harvest seeds from your flowering koai‘a, remember to plant at least two trees in your garden so they can cross-pollinate.

Diseases & Pests: As discussed above, the main killer of koai‘a is koa wilt. (Here is one of many websites describing and illustrating this disease.) I have never read or heard of an effective treatment or cure for this fungal disease including the use of fungicides. Rather, the only advice I can give to you if your koai‘a contracts this disease is to try again, preferably with seed from another source and at a different site in your garden. Some koai‘a do have a natural resistance to koa wilt, you just have to be lucky enough to chance upon seeds from one of these special trees. (It's the old adage, "If at first you don't succeed, try, try again.") Second on the killers list is the black twigborer which can be a serious problem at some sites (often wetter windless areas) or non-existent at others. Chinese rose beetles will sometimes chew koai‘a leaves, but are not life-threatening. Occasionally, ants will nest at a tree's base and farm scale insects and mealybugs. Refer to Enemies in Garden for ways to combat each of the above-mentioned pests.

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