Agroforestree Database

Local names:
 English (koa acacia,koa,Hawaiian mahogany), Hawaian (koa), Trade name (koa)

Acacia koa is a large, evergreen tree to 25 m tall, stem diameter to 150 cm at breast height. 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. A. koa has one main tap root and an otherwise shallow, spreading root system. Bark gray, rough, scaly and thick.

A. koa belongs to the thorn-less, phyllodinous group of the Acacia subgenus Heterophyllum. Young seedlings have bipinnate compound true leaves with 12-15 pairs of leaflets. Where forest light is sufficient, seedlings stop producing true leaves while they are less than 2 m tall. True leaves are retained longer by trees growing in dense shade. Phyllodes are sickle-shaped and often more than 2.5 cm wide in the middle and blunt pointed on each end.

Inflorescence is a pale yellow ball, 8.5 mm in diameter, 1-3 on a common stalk. Each inflorescence is composed of many bisexual flowers. Each flower has an indefinite number of stamens and a single elongated style.

Pods are slow to dehisce, 15 cm long and 2.5-4 cm wide. They contain 6-12 seeds that vary from dark brown to black.

The generic name ‘acacia’ comes from the Greek word ‘akis’, meaning point or barb.

Ecology

Occurring in both pure and mixed forest stands, A. koa is most commonly associated with the native ohia (Metrosideros polymorpha). It is 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.

Native range
United States of America

Tree management

On favorable sites, planted seedlings grow to 9 m in 5 years. Its wide branching form is the result of open growth. Dense stocking of seedlings, which mimics the competitive environment where superior trees grow, encourages straight and rapid height growth. Initial spacing of 1.2 x 1.2 m is currently recommended. Observation indicates that effective self-thinning results in an adequate number of potential crop trees by age 25.

Plantation establishment is most easily and successfully accomplished through the stimulation of natural regeneration where scattered A. koa cover is adequate. Pasture soils are scarified and competition from grasses reduced by the application of a contact herbicide. Gaps in the regeneration are filled with planted seedlings. Fertilizers are applied to give seedlings an initial boost. Plantation thinning prescriptions should be based on desired products and management capabilities. The most important factors to consider in picking A. koa crop trees is stem form and height. Seed production begins when trees are 5 years old. A. koa bears seed often and abundantly.

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.

Occurring in both pure and mixed forest stands, A. koa is most commonly associated with the native ohia (Metrosideros polymorpha). It is 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.

Propagation is most successful from seed. 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 20 cm 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.

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

Cattle, sheep and pigs browse A. koa foliage aggressively, especially its juvenile leaves.

Apiculture:  The tree is visited by the honey bee and may be a source of nectar.

Timber: A. koa heartwood is highly valued for its unique grain, varied color 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 color 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.

Nitrogen fixing: 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.