Acacia koa (see Etymology for current name discrepancy)
Fabaceae or Pea family [Nature Clearly]
Synonyms: Acacia hawaiiensis, Acacia heterophylla, Acacia heterophylla var. latifolia, Acacia kauaiensis, Acacia koa var. hawaiiensis, Acacia koa var. lanaiensis, Acacia koa var. latifolia, Acacia koa var. waianaeensis, Acacia koa var. weimeae, Racosperma kauaiense, Racosperma koa, Rhacosperma koa
Koa
Natural Range & Environment
Acacia koa is endemic to all the main islands of Hawai‘i except Ni‘ihau and Kaho‘olawe; however, only Hawai‘i Island, Maui, O‘ahu, and Kaua‘i have densities great enough to be mapped as koa forests [Baker et al. 2009]. Koa is most common in mesic environments but can also be found growing in drier and wetter regions. And, while koa is most common in areas with organic-rich soils (partly of their own making), it can also be seen growing in rocky (lava) soils and infertile clays. Because of this moisture and soil adaptability, koa has a wide elevational range from 60 to 2,060 m (197 to 6,759 ft) [Wagner et al. 1990]. However, most large populations grow between 3,000 and 6,000 feet (914 and 1829 m) [Baker et al. 2009].
The yellowish white flower heads of koa last several days before withering and turning brown (photograph courtesy of Forest & Kim Starr CC BY 3.0). Pollinated flowers develop into pods, each containing a few to several shiny flattened black seeds (photograph courtesy of J.B. Friday ©). Koa seedlings begin life with bipinnately compound leaves and then transition to sickle-shaped phyllodes, which are flattened petioles (leaf stalks) (photograph of Hawai‘i Island koa courtesy of Gerry Carr CC BY-NC 4.0). There is variation in the shape of the phyllodes of individual koa, with those on Hawai‘i Island often being wider than those on koa seen on O‘ahu and Kaua‘i (photograph of O‘ahu koa phyllodes courtesy of J.B. Friday ©). The roots of koa form a symbiotic relationship with nitrogen-fixing bacteria called Bradyrhizobium. The bacteria multiply inside pinkish nodules along the roots (photograph courtesy of J.B. Friday ©).
Koa, while fast-growing, takes decades, perhaps even centuries, to become as large as this tree (photograph courtesy of Gerry Carr CC BY-NC 4.0). More realistic examples of cultivated koa include this young tree in ‘Ulupalakua, Maui (photograph courtesy of Forest & Kim Starr CC BY 3.0) and a slightly older tree in Kula, Maui (photograph courtesy of Forest & Kim Starr CC BY 3.0).
Description
No other native Hawaiian plant has been researched and written about more than koa; countless articles and entire books (e.g., Baker et al. [2009]) have been written about the species. Therefore, any description here will be incomplete but, hopefully, will contain enough information to convince you to grow this tree and grow it successfully. One excellent, comprehensive, and recommended-reading is an online article by Elevitch, Wilkinson & Friday [2006].
Acacia koa is thought to be the second most common tree in Hawai‘i, with ‘ōhi‘a lehua (Metrosideros polymorpha) being the first [Little & Skolmen 1989]. It is also among the largest and can grow to 35 m (115 ft) in height [Wagner et al. 1990], with very large trees having a trunk nearly 3 m (10 ft) in diameter (see photograph above). While koa is a fast-growing tree, very large trees are decades, perhaps even centuries, old. A koa's form is significantly influenced by the environment it is growing in, with the tallest trees with the straightest trunks found in moist, forested valleys and gullies, while short and often wide trees with gnarled trunks are common in dry, open, windswept sites.
The bark of koa trees is gray and initially thin and smooth, becoming thick, rough, and scaly as the tree grows larger. The valuable wood is moderately hard but easily worked, with the heartwood varying in color from golden yellow to reddish-brown, dark brown, and purplish red. This is surrounded by a pale, light-colored sapwood that can vary in thickness depending upon the growth rate of the tree. An individual tree's wood color often varies with the grain, this being one of the features that makes the wood so attractive and valuable.
The roots of koa form a symbiotic relationship with bacteria called Bradyrhizobium. These bacteria multiply inside pinkish nodules along the roots and convert atmospheric nitrogen into fixed nitrogen compounds like ammonia that it shares with the plant. In return, the koa provides the bacteria with energy-rich chemicals like sugars. Often the area surrounding a large koa or within a cluster of koa will have a distinctive ammonia-like scent.
Acacia koa has two distinct leaf types. As a seedling or young plant, koa has true bipinnately compound leaves with many leaflets. As the plant matures, the compound leaves are replaced by sickle-shaped phyllodes, which are flattened petioles (leaf stalks). The grayish or silvery green phyllodes can vary in shape, with koa on Hawai‘i Island often having wider phyllodes than trees seen on O‘ahu and Kaua‘i (see photographs above). This leaf variation is largely responsible for some early taxonomists dividing the species into formal varieties (that are no longer recognized). Root suckers and stems growing from a damaged trunk or branch will curiously and temporarily revert back to producing juvenile compound leaves. The most common hypothesis to explain the transition from true compound leaves to phyllodes is that it is an adaptation to reduce water loss [Pasquet-Kok et al. 2010].
Koa produces clusters of ball-like flowerheads about ⅓ inch (8.5 mm) in diameter made up of many tightly clustered, tiny, pale yellow to white, perfect (bisexual) florets with their stamens and style extending beyond the florets' corollas. Each flowerhead blooms for several days before browning and withering. While flowering period can vary depending upon geography and climate, it most commonly peaks in middle to late winter (i.e., January to March), with ripe fruits most common in late summer and fall (but often persisting on trees into winter).
Pollinated flowers develop into long, flat, initially green pods, each containing a few to several seeds. The pods turn brown and dry when ripe and reluctantly (but not always) split open to reveal the black, flat, oval to elliptic seeds inside (see photograph above). (See Special Features and Information below for how to distinguish Acacia koa pods from those of Acacia koaia.) Koa seeds have a durable, waterproof seedcoat that permits them to remain dormant for many years in the soil such that an opening or clearing in a forest often produces a dense field of koa seedlings covering the ground.
Growth Requirements
General
No other native Hawaiian plant has been researched and written about more than koa; countless articles and entire books (e.g., Baker et al. [2009]) have been published about the species. Therefore, any guidelines and recommendations here will be incomplete but, hopefully, will contain enough information to help you grow this tree successfully. One excellent, comprehensive, and recommended reading is an online article by Elevitch, Wilkinson & Friday [2006].
If not for the recently introduced and deadly fungal disease koa wilt (Fusarium oxysporum f. sp. koae; first identified in 1980) and the introduced black twig borer (Xylosandrus compactus), koa would be a relatively easy native tree to grow and maintain. Yes, koa is vulnerable to numerous other native and non-native pests and diseases (see Diseases and Pests below). However, these are all minor or manageable. Koa wilt is not, and infected trees typically die within a year after the infection becomes visible (i.e., rapid leaf loss). Therefore, if you want to keep a koa tree in a landscape longer than a few to several years, the trick is to avoid infection by the fungus. The most successful way of doing this is to restrict your koa plantings to upland landscapes above 2,000 ft (610 m). Perhaps it’s the relative isolation of these upland areas (compared to lowland landscapes) that keeps koa alive. However, it’s more likely it’s the cooler soil temperatures that make a koa wilt infection less likely or reduces the fungus’ growth rate after infection. It’s also possible that at these temperatures a koa’s physiology is better able to defend against or combat this microbe. Unfortunately, I haven't found any research that tests these or other hypotheses to explain why koa wilt is less of a problem at higher elevations.
If your landscape is below 2,000 feet, it's still possible to include a koa tree. However, consider the tree to be a short-lived (i.e., three to ten years) addition to your landscape because of koa wilt. You can sometimes extend the tree's life beyond this range by (1) never overwatering, (2) only planting in a site with excellent drainage, and (3) never planting in a site you suspect is already harboring the fungus in the soil (e.g., a koa or other plant has died in the site within the last several years of a vascular disease). Once your lowland koa dies of wilt, you'll need to find another distant location in your landscape (e.g., the other side of the property) if you want to try again with another koa. State (e.g., Hawai‘i Agricultural Research Center) and federal (e.g., USDA Forest Service) agencies have been working for years now to discover and propagate naturally wilt-resistant strains of koa. However, to date (2026), based on my (Koebele) experiences with a few of these wilt-resistant koa (and my observations of other wilt-resistant plantings), they still haven't found a wilt-immune strain; the four wilt-resistant koa I planted all died in less than ten years.
Koa is a large tree, not well suited for small yards. Therefore, unless you have the space in your landscape, koa's smaller sister species, koai‘a (Acacia koaia), is likely a better choice.
Wild koa vary in morphology and grow in a wide range of environmental conditions. For example, on O‘ahu, you can see short broad trees growing on the dry southern slopes of the Wai‘anae Mountains, while midway up a Ko‘olau Mountain trail, you'll encounter koa giants. Therefore, inquire as to the origin of your koa (i.e., Where did its parent grow?) and try to purchase or collect seeds from a koa that best matches the climatic conditions of your landscape.
Koa grows best in full sunlight and will languish in the shade. And, while koa is moderately drought tolerant (for a few months) once its root system is fully established (i.e., a few months after planting), for the best growth and least stress, keep the soil moist — not wet — at all times. Drought-stressed koa are more prone to attacks by black twig borers and other pests. Koa tend to grow fastest in organic-dominant soil but are also okay with clay, lava/cinder, or even sandy soils provided the soil drains water well; koa will not survive in salty soils. Koa can withstand some wind, and a regular breeze can help reduce black twig borer attacks. However, violent or persistent winds will normally shape a tree into something you may find unattractive. Most koa require little to no pruning to obtain an aesthetic shape. Remove any dead branches or those that might congest the center of the crown, but always make sure the cuts are clean with minimum bark damage. Generally, it's safest to keep pruning to an absolute minimum because of the potential for infection or rot invading the cut. Koa has both a deep and shallow root system. Therefore, avoid damaging its surface roots (and its trunk) with lawnmowers, trimmers, or excessive foot traffic; any damage can invite infection and rot.
A young koa that doesn't already have a population of Bradyrhizobium bacteria growing within its roots usually acquires the symbiotic bacteria soon after being planted in a landscape. However, if you think your koa has failed to pick up these bacteria — symptoms include chlorotic leaves and slow growth — you can inoculate your koa by collecting some soil from beneath an established koa (or other nitrogen-fixing plant species), mixing the soil with some water, and then sprinkling the water around your koa. Many growers inoculate their koa seedlings while they are still in a container. However, if you do this, you need to get your koa into the ground soon after because the roots of the rapidly growing seedling will not respect the boundaries of the pot and quickly become permanently disfigured.
Young koa grow quickly, often 5 ft (1.5 m) or more per year, and can reach 33 ft (10 m) in height with a trunk a foot (30 cm) in diameter in about five years. Expect to see your first flowers after three to five years in the ground. Because koa are nitrogen-fixing trees, most grow well without additional fertilizers. However, if your koa grows slower than expected, try applying a fertilizer containing little to no nitrogen to see if you can improve its growth rate.
For a video summary of much of the information presented on this webpage along with a how-to on propagating koa from seed, check out this YouTube video by GreenGardenGuy1.
Pests and Diseases
Koa wilt, discussed above, is by far the most serious and deadly disease to afflict koa. Friday & Dudley (CTAHR) provide an excellent summary of the disease along with links to the agencies conducting ongoing research to identify and develop wilt-resistant trees.
Koa wilt is not the only fungus that attacks koa. There are at least five different types of koa rust fungi, some native and some non-native, with the most common being the indigenous Atelocauda digitata [Nelson 2009]. This fungal disease is systemic but becomes active and visible in growing tissues such as new leaves, stems, flowers, and pods. While not fatal, the fungus deforms new growth, can destroy the tree's seed pods, and is unsightly. Unfortunately, there is no known "cure" for koa rust. However, early removal of afflicted plant parts can reduce the spread of fungal spores to new plants. Other types of fungi are responsible for the wood rot sometimes seen on older trees or on damaged or cut roots and branches; therefore, it's important to always minimize any damage to these plant parts (e.g., unnecessary pruning).
The most serious koa pest is the black twig borer (Xylosandrus compactus). Stressed (because of drought, disease, or other pests) trees are most often attacked by this wood-boring beetle. Trees placed in a poor environment such as a shady, windless site are also more prone to attack. Refer to the Pests & Diseases page for more information and treatments.
Koa is sometimes attacked by other common pests such as aphids, Chinese rose beetles, mealybugs, and whiteflies. See Pests & Diseases for more information and treatments. Seed predators such as seed weevils (Aracerus levipennis and Stator spp.) and the koa seedworm (Cryptophlebia illepida) are common on wild trees. To combat these on cultivated trees, use a systemic pesticide early before the pods begin to develop (and, maybe again, as the pods develop). Bagging the flowers soon after pollination can also help. Accidentally introduced in the 1960s, the tiny acacia psyllid (Acizzia uncatoides; syn: Psylla uncatoides) pierces phyllodes to extract the sap, causing curling, yellowing, and distortion of phyllodes, and stunting and forking of terminal shoots. Often, natural predators like ladybird beetles help control these psyllids. However, if the infestation becomes severe, sprays like neem or horticultural oil and insecticidal soap will kill this insect; avoid more deadly insecticides because they will kill natural predators.
Lastly, koa is also "attacked" by several native insects, where we recommend that no treatment (in order to preserve these insects) is the best treatment. These include the koa looper moth (Scotoythra paludicola), which is clearly the most destructive (and sometimes lethal) of this group of native pests. Thankfully, severe koa looper moth infestations are rare, and healthy trees normally recover from the defoliation this insect can cause; sickly trees sometimes don't. If you want to mitigate the damage, removing the caterpillars by hand is the safest option. Other native "pests" include the koa butterfly (Udara blackburnii), one of only two native (and endemic) Hawaiian butterflies, and the colorful endemic koa bug (Coleotichus blackburniae), sometimes referred to as the "stinkless sting bug" because of its close relationship with the more familiar and unpleasant non-native garden stink bug. Both insects can also be seen on ʻaʻaliʻi (Dodonaea viscosa).
Koa is vulnerable to many diseases and pests, some native and some non-native, with just a few notables pictured here. Therefore, always use caution when deciding to combat a disease or pest or ignore it and let "nature take its course." The most deadly koa disease is koa wilt (Fusarium oxysporum f. sp. koae), a vascular fungus (photograph of dead and dying young trees courtesy of Scot Nelson Public Domain). There are at least five different types of koa rust fungi, some native and some non-native (photograph of Atelocauda koae courtesy of J.B. Friday ©). Generally, they are non-lethal and can be minimized by prompt removal and disposal of infested parts. Larvae of the native koa looper moth (Scotorythra paludicola) occasionally defoliate entire trees; however, otherwise healthy trees often recover without assistance (photograph of koa looper moth caterpillar courtesy of Forest & Kim Starr CC BY 3.0 US). The koa butterfly (Udara blackburnii) is a minor pest on koa and ʻaʻaliʻi (Dodonaea viscosa) and is best ignored since it is one of only two native (and endemic) Hawaiian butterflies (photograph courtesy of J.B. Friday ©). The endemic koa bug (Coleotichus blackburniae), sometimes referred to as the "stinkless sting bug" because of its close relationship with the more familiar and unpleasant non-native garden stink bug, also feeds on koa and ʻaʻaliʻi. Juvenile koa bugs (photograph courtesy of Forest & Kim Starr CC BY 3.0 US) are red and black, while adults are green and red (photograph courtesy of Will Haines CC BY-NC 4.0).
Uses
Ancient Hawaiian
Koa was an extremely valuable resource in ancient Hawai‘i, primarily because of its use in making the hulls of wa‘a (canoes) of all types. Malo [1898/1903], Degener [1973], Abbott [1992], and Krauss [1993] all provide detailed descriptions of how Hawaiians made canoes in ancient times (see The Making of Wa‘a below for a summary). However, koa wood was also used to make (1) spears [Malo 1898/1903; Abbott 1992], (2) hoe (paddles) [Malo 1898/1903; Degener 1973; Abbott 1992; Krauss 1993], (3) kahili shafts [Abbott 1992], (4) ‘umeke (bowls) and pā (platters), but never those used for preparing, storing, or serving food since the wood imparted an unpleasant taste to the food [Krauss 197-; 1993], and (5) alaia (short surfboards; koa replaced wiliwili (Erythrina sandwicensis) for long boards after European contact) [Malo 1898/1903; Buck 2024 reprint of 1957 book; Abbott 1992; Krauss 1993]. While Krauss [1971-] states that koa trunks and branches were used as posts for hale (houses), and branches were fashioned into a booth for rituals such as the dedication of a heiau, Abbott [1992] states that "there is no record that this species (Acacia koa), favored for canoe construction, was ever cut for house building." It's very likely that koa wood was used to make other weapons, tools, etc., in ancient times; however, I (Koebele) have limited the above list to those items with credible sources in the literature.
Koa bark was used to make a red dye, presumably for kapa, and as one ingredient in a drink to "clean the blood" [Krauss 1993; Ka‘aiakamanu & Akina 1922]. Krauss [2001] and Ka‘aiakamanu & Akina [1922] also state that young koa leaves, pounded and crushed, were spread to cover a sleeping pallet of mats, causing a person with fever (or someone long confined to bed) to perspire and then fall asleep. Additionally, koa ash, along with other plant ingredients, was used to treat a weak child or one with ‘ea (thrush) [Krauss 2001; Ka‘aiakamanu & Akina 1922]. Ka‘aiakamanu & Akina [1922] and Ka‘aiakamanu [2003] also repeatedly prescribe koa bark or koa leaves as an ingredient in treatments for other illnesses and injuries. According to McDonald & Weissich [2003], koa leaves (phyllodes) were/are used in lei-making.
The Making of Wa‘a
An ancient canoe was a valued item lasting generations. Therefore, there was a great deal of ritual associated with the selection and felling of the tree and its transformation into a functioning canoe. Koa was the preferred wood for canoes. However, Abbott [1992] did discover one canoe at Bishop Museum made of milo (Thespesia populnea); all the others were made of koa. Additionally, on the rare occasion when a large log from the American West Coast would drift ashore, an ali‘i would claim it, and if it was sound, would have it fashioned into a canoe.
Canoe construction began with a kahuna kālai waʻa, a master canoe carver and priest, who knew both the skills involved in making a canoe and the prayers, offerings, etc., necessary, being employed, typically by an ali‘i. After a suitable koa tree was discovered, he, along with a group of skilled woodworkers, would bring offerings to Kū (primarily known as the god of war but also of fishing, farming, and canoe-making) to the base of the tree. In most accounts, the offerings included a pig, fish, nui (coconut), and ‘awa. After prayers, the men would sleep near the tree, and in the morning, along with more prayers, the food offerings were cooked in an imu (underground oven) and eaten.
The kahuna kālai waʻa would then mark near the base of the tree where the men should begin cutting. Using stone adzes, the felling was hard work but could be achieved in a single day if there were enough men continuously working. Once the tree was down, the kahuna kālai waʻa would offer more prayers and show the men exactly where to make the second cut to sever the crown of the tree from the lower trunk.
It was then time to wait for Lea (the wife of Kū), in the form of a bird, to appear and determine if the log was sound and whether further work in its transformation should continue. If a bird landed on the log and repeatedly pecked at it, the bird being the endemic insectivorous ‘elepaio (Chasiempis spp.), or made a loud caw, the bird being an ʻalalā (Corvus spp.), Hawai‘i's endemic crow, this was considered a bad omen and the log would be abandoned (see **** below). However, if the ‘elepaio landed and did not peck at the log, this was considered a good omen, and the men continued work, roughly hewing the log and reducing its weight in preparation for its transport. (Unlike some other indigenous cultures, Hawaiians did not use fire to burn out the inside of the canoe.) After this work was completed, non-skilled workers, including women and children, were summoned to help haul the heavy unfinished canoe down from the highlands to a hālau wa‘a near the ocean. One or more ropes made of hau were attached to the maku‘u (a protrusion on the rear of the canoe), and with additional prayers and feasting, the large group carefully hauled the canoe downslope.
Upon arrival at the hālau wa‘a, the wood of the canoe was allowed to cure (if this had not already been done earlier in the uplands). Then, the long and skilled labor of finishing the canoe began. In addition to further shaping and hollowing out with assorted types of adzes, the canoe was also sanded smooth with puna and pōhaku ‘ānai (coral and stone rubbers). Other elements of the canoe, made of various other types of wood, were then added. Typically, the pale (gunwales) were fashioned from the yellow wood of ‘ahakea (Bobea spp.) and then lashed to the sides of the canoe. The stern and bow end pieces were usually made of ulu (breadfruit), while seats and internal struts/spreaders were often made of ‘ōhi‘a lehua (Metrosideros spp.). If it was a single-hulled canoe (a kaukahi), an ‘ama made of light wiliwili (Erythrina sandwicensis) wood was attached to the canoe by two long arched ‘iako made of hau. If the canoe had a sail, it was made of hala (Pandanus tectorius) leaves.
The outer hull of the canoe was painted, typically black, with the paint made of assorted plant ingredients (several paint "recipes" have been recorded) and then coated with kukui oil; normally the golden pale were left unpainted. Surprisingly, this coating was not permanent, and working canoes required regular recoating. When all the work was completed, the canoe was finally ready for its first voyage/ocean-testing. This was preceded and followed with considerable rituals involving prayers, offerings, and feasting.
**** Today, ʻelepaio are known as three separate species, one each on Kauaʻi, Oʻahu, and Hawaiʻi Island, while the endangered ʻalalā is known only from Hawaiʻi Island, although there were two other Corvus species found on Oʻahu and Molokaʻi at the time of early Polynesian settlers. Currently, there is no fossil evidence of ʻelepaio ever existing on any Hawaiian island other than the three listed above. Obviously, this creates a dilemma as to the identity of the bird element in regard to canoe building on the islands of Maui and Lāna‘i.
Modern
Koa wood is still very much prized and always in high demand, making it one of the most expensive woods in the world [Baker et al. 2009]. The wood is used to make both traditional items such as ‘umeke (bowls) and hoe (paddles) and also modern items such as fine furniture and small items such as jewel boxes, bracelets, and pens. Additionally, because koa is a tonewood, it is frequently used to make musical instruments such as ʻukulele and acoustic guitars (see photograph below).
People from ancient through historic to modern times have always valued koa (photograph of koa canoe at Hōnaunau National Historical Park courtesy of ph Public Domain; photograph of koa staircase at Iolani Palace courtesy of Daryl Mitchell CC BY-SA 2.0; photograph of Taylor Swift with koa acoustic guitar courtesy of Dwight McCann CC BY-SA 2.5.)
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. Many species host nitrogen-fixing bacteria within root nodules [WikipediA; Fabaceae]. In Hawai‘i, there are about 25 native species and about 150 introduced and naturalized species [Flora of the Hawaiian Islands].
While there are several introduced and naturalized species of Acacia in Hawai‘i, there are only two native (endemic) species, Acacia koa and Acacia koaia [Flora of the Hawaiian Islands]. The two species can usually be distinguished from one another by (1) the orientation of the seeds within their pods (A. koa seeds are positioned side-to-side [transversely] in a line within the pod, while A. koaia seeds are positioned end-to-end [vertically] in a line within the pod) and (2) wood density, with A. koaia wood being significantly harder and denser than A. koa wood. Less reliable is the shape of the tree's phyllodes, with A. koaia often having narrower and straighter phyllodes than those of A. koa. However, possible hybrids between the two species have been observed [Koebele], as well as trees (on Kaua‘i) that display morphological features intermediate between the two species [Wagner et al. 1990].
Koa has been successfully grown outside Hawai‘i in places like California, New Zealand, and Taiwan [GreenGardenGuy1].
By increasing soil nitrogen (via nitrogen-fixation), koa facilitates understory plant succession, particularly in barren lava fields where it acts as a pioneer species. Additionally, the decomposition of its phyllodes contributes organic matter to the soil. Combined, soil carbon and nitrogen content increases, which in turn improves nutrient and water retention and aids in long-term soil development.
As dominant trees in Hawaiian native forests, koa provides habitat for many species of birds. The aptly named koa-finches, Rhodacanthis palmeri and R. flaviceps, are believed to have fed on unripe koa seed pods. Sadly, these brilliantly colored honeycreepers became extinct in the 1890s [Pratt 2005].
Scholars such as Abbott [1992] have suggested that the abundance of large straight koa trees on Hawai‘i Island and Maui gave the ali‘i (chiefs) on these two islands a military advantage over ali‘i on the other Hawaiian Islands.
Hawaiian Name
According to Pukui & Elbert [1986], Koa has several other meanings in addition to being the name for this tree. It can mean (1) "brave, bold, fearless, valiant; bravery, courage" or (2) "soldier, warrior, fighter; military, hero, martial." Koa is also the name for a wind at Mālei, Molokaʻi, and the name for a "land crocodile" or "chameleon." Other Hawaiian dictionaries, such as Parker [1922], list additional meanings for Koa, such as "dry; without moisture" and "sterile, barren; unfruitful, said of land" [Uluka; Koa]. However, it's unclear why these meanings are not also mentioned by Pukui & Elbert [1986] or others. Koa etymology traces back to the Proto-Polynesian word Toa, used for Casuarina equisetifolia in places outside Hawai‘i; the common name here is "ironwood" [te māra reo; The Language Garden].
Plants of Hawai‘i lists Koaha as a Hawaiian name for this species but provides no explanation or origin for the name, while numerous Hawaiian dictionaries do not identify Koaha as a plant name [Uluka; Koaha].
Etymology
Morden & Faccenda [2025] have proposed that the primary scientific name for this species be changed to Acacia heterophylla subsp. koa (with A. koa remaining a synonym) based on the genetic analysis of Le Roux et al. [2014]. According to Le Roux et al. [2014], the most parsimonious explanation for A. heterophylla, a species found on the island of Réunion in the Indian Ocean yet genetically, morphologically, and ecologically very similar to A. koa in Hawai‘i, is an "extreme long-distance dispersal event" (most likely via a migrating bird) from Hawai‘i to Réunion ≤ 1.4 million years ago. Therefore, Morden & Faccenda [2025] have proposed the above reclassification to better represent the species' evolutionary history and maintain monophyly (a single common ancestor and every single one of its descendants). Further, they have proposed that the primary scientific name for koai‘a be Acacia heterophylla subsp. koaia rather than A. koaia. While some institutions have adopted these name changes (e.g., Plants of Hawai‘i, Kew Plants of the World Online), others (to date: 2026) have not (e.g., Flora of the Hawaiian Islands). Therefore, until there is further consensus, we will retain the older species name Acacia koa.
Acacia is derived from akakia, the Greek name for a thorny Egyptian tree (usually assumed to be Vachellia nilotica), which is likely derived from akē, a Greek term meaning a "point, thorn" [etymonline; acacia], referring to the thorns of this plant. The species name koa is taken directly from the Hawaiian name for this tree. Heterophylla is a compound word from the Greek héteros, meaning "different," and phúllon, meaning "leaf," referring to plants that have different or multiple forms of leaves on a single plant [Wiktionary; heterophyllus].
BPK