Classification of the genus Acacia (in the wider sense) has been subject to considerable debate. It is generally agreed that there are valid reasons for breaking it up into several distinct genera, but there has been disagreement over the way this should be done. As of 2017, it is widely (but not completely) accepted that the section that includes the majority of the Australian species (including this one) should retain the name Acacia, whilst other sections of the genus should be transferred to the genera Acaciella, Mariosousa, Senegalia and Vachellia[].

Common Name: Koa Acacia

General Information

Koa acacia is an evergreen tree that usually grows about 20 metres tall, though it sometimes reaches up to 35 metres. Some populations are smaller and more shrub-like[].
This species (as of 2006) provides one of the most expensive timbers in the world, used mainly for furniture, veneer and crafts[]. The only Acacia that is native and endemic to Hawaii, plantations have been established there in order to provide native vegetative cover on degraded sites[].
Acacia koa is currently known to be the dominant canopy endemic tree occurring on six of the Hawaiian islands. The species occurs in mesic forest in a wide range of habitats and elevations. The plant is classified as 'Least Concern' in the IUCN Red List of Threatened Species(2013)[].

Known Hazards

Especially in times of drought, many Acacia species can concentrate high levels of the toxin Hydrogen cyanide in their foliage, making them dangerous for herbivores to eat.

Botanical References

Range

Pacific Islands - Hawaii.

Habitat

Both pure and mixed forest stands, usually with native ohia (Metrosideros polymorpha). Also a co-dominant in several other major forest types including Koa/Mamane (Sophora chrysophylla) Montane Dry Forest and Koa/Ohia/A'e (Sapindus soponaria) Forest[].

Properties

Conservation Status	Least Concern
Other Uses Rating
Habit	Evergreen Tree
Height	20.00 m
Growth Rate	Fast
Pollinators	Bees
Self-fertile	Yes
Cultivation Status	Cultivated, Wild

Cultivation Details

Trees grow naturally in the tropics, usually at an elevation between 180 - 6,000 metres[]. They grow in areas where the mean annual rainfall is in the range 1,900 - 5,100mm[].
It grows well in moderately to well-drained, medium to very strongly acid soils on both flatland and steep slopes[, ]
Trees occurring in dense, wet native forest stands typically retain a straight, narrow form[]. In the open, trees develop more spreading, branching crowns and shorter, broader trunks[]. The tree has one main tap root and an otherwise shallow, spreading root system[].
Observations suggest Acacia koa can flower almost any time of year, depending upon local weather conditions. Trees appear to be self-fertilizing[]. Pods reach maturity at 4-6 months, depending on location and weather conditions[]. Seed production begins when trees are 5 years old[]. A. koa bears seed often and abundantly[].
A fast-growing tree[]. On favourable sites, planted seedlings grow to 9 metres in 5 years[].
This species has a symbiotic relationship with certain soil bacteria, these bacteria form nodules on the roots and fix atmospheric nitrogen. Some of this nitrogen is utilized by the growing plant but some can also be used by other plants growing nearby[]. A. koa is nodulated by the slow-growing Bradyrhizobium spp. Common in tropical soils. It nodulates heavily in a variety of soils, suggesting it is effective with a wide variety of Bradyrhizobia strains[].

Edible Uses

None known

Medicinal

The bark of all Acacia species contains greater or lesser quantities of tannins and are astringent. Astringents are often used medicinally - taken internally, for example. they are used in the treatment of diarrhoea and dysentery, and can also be helpful in cases of internal bleeding. Applied externally, often as a wash, they are used to treat wounds and other skin problems, haemorrhoids, perspiring feet, some eye problems, as a mouth wash etc[, ].
Many Acacia trees also yield greater or lesser quantities of a gum from the trunk and stems. This is sometimes taken internally in the treatment of diarrhoea and haemorrhoids[].

Agroforestry Uses:

Most plantations in Hawaii have been established to provide vegetative cover on sites degraded by decades of intense grazing[].

Other Uses

The wood is hard. It is used by local people to make spears, paddles etc[].
The heartwood is highly valued for its unique grain, varied colour and workability. It seasons well without serious warping or splitting. Curly-grained wood, the result of both stress and genetics, is preferred over straight-grained wood[]. Wood colour ranges from a subtle yellow to a striking dark red-purple. The specific gravity of wood averages 0.40, but with curly-grained wood can be as high as 0.65. It is the premier Hawaiian timber for furniture, cabinetry, interior work and woodcrafts[].

Propagation

The seed of most, if not all, members of this genus has a hard seedcoat and may benefit from scarification before sowing to speed up germination. This can usually be done by pouring a small amount of nearly boiling water on the seeds (being careful not to cook them!) and then soaking them for 12 - 24 hours in warm water. By this time they should have imbibed moisture and swollen - if they have not, then carefully make a nick in the seedcoat (being careful not to damage the embryo) and soak for a further 12 hours before sowing. Once treated, seeds are sown in nursery beds. 1 week after germination, seedlings are transplanted into nursery tubes or bags. Seedlings are ready for transplanting into the field when they are approximately 20cm tall, (after 3-4 months in the nursery). Establishment by direct seeding or encouragement of natural regeneration is recommended as heart rot occurs during transplanting. One study recommends air-layering as the best vegetative propagation technique.

The seeds are durable and easy to store. They germinate after many years of storage if kept in a cool, dry place. The most effective method for improving seed germination is mechanical scarification. However, hot water soaking works well and is a more practical method, seed should be soaked in boiled water for 24 hours. Seeds are seldom dispersed far from the tree and remain viable in the soil for up to 25 years.

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