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 disagreement in the way this should be done. Some authorities have wanted to transfer this species to the genus Racosperma, but the latest decision (in 2011 and still not fully accepted) is that it remains in Acacia[
Acacia hawaiiensis (Rock.) O.Deg. & I.Deg.
Acacia kauaiensis Hillebr.
Racosperma koa (A.Gray) Pedley.
Common Name: Koa Acacia
Koa acacia is an evergreen tree that grows about 20 metres tall.
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[
Pacific Islands - Hawaii.
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[
|Other Uses Rating||
|Cultivation Status||Cultivated, Wild
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[
Prefers a moderately to well drained, medium to very strongly acid soil[
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 A. 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[
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[
Most plantations in Hawaii have been established to provide vegetative cover on sites degraded by decades of intense grazing[
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[
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.
If you have any useful information about this plant, please leave a comment. Comments have to be approved before they are shown here.