Koa Acacia
Acacia koa

Family: Fabaceae

What it is like

Acacia koa is an evergreen Tree growing to 20 m (65ft) by 15 m (49ft) at a fast rate. See above for USDA hardiness. It is hardy to UK zone 10. The flowers are pollinated by Bees. The plant is self-fertile. It can fix Nitrogen. Suitable for: light (sandy), medium (loamy) and heavy (clay) soils and prefers well-drained soil. Suitable pH: mildly acid and neutral soils and can grow in very acid soils. It cannot grow in the shade. It prefers dry or moist soil.

Height (m): 20

Where it is found

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 (Sapindus soponaria) Forest.

Pacific Islands - Hawaii.

Conservation Status: Status: Least Concern

Countries/locations it is found in

United States

How it is used

Food

Rating: 0

Medicine

Rating: 2

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.

Dysentery: Used in treating dysentery - an infection of the intestines that causes diarrhoea containing blood or mucus.

Other

Rating: 3

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. The reddish wood is similar in strength and weight to Black Walnut (Juglans nigra), with a specific gravity of 0.55. Koa is also a tonewood, used in the construction of ukuleles, acoustic guitars, and Weissenborn-style Hawaiian steel guitars. B.C. Rich used koa on some of their electric guitars and still uses a koa-veneered topwood on certain models. Fender made limited edition koa wood models of the Telecaster and the Stratocaster in 2006.

Agroforestry Services: Crop shade: Plants providing crop shade especially trees.

Agroforestry Services: Nitrogen: Plants that contribute to nitrogen fixation include the legume family – Fabaceae.

Management: Standard: Plants grow to their standard height. Harvest fruit, seeds, or other products. Non-Destructive management systems.

Regional Crop: These crops have been domesticated and cultivated regionally but have not been adopted elsewhere and are typically not traded globally, Examples in this broad category include perennial cottons and many nuts and staple fruits.

Agroforestry Services: Crop shade: Plants providing crop shade especially trees.

Agroforestry Services: Nitrogen: Plants that contribute to nitrogen fixation include the legume family – Fabaceae.

Management: Standard: Plants grow to their standard height. Harvest fruit, seeds, or other products. Non-Destructive management systems.

Regional Crop: These crops have been domesticated and cultivated regionally but have not been adopted elsewhere and are typically not traded globally, Examples in this broad category include perennial cottons and many nuts and staple fruits.

Carbon Farming: Plants that can be a critical part of the solution to climate problems. The Carbon Farming Solution - Eric Toensmeier.

Nitrogen Fixer: Plants that fix nitrogen in the soil

How it is grown

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.

Propagating it: 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.

Best place to grow:

Habit: Tree

Hardiness: 10-12

Growth: Fast

Soil: Light (sandy), medium, heavy (clay)

Shade: No shade

Moisture: Dry, moist

Things to keep in mind

Its other names

Local names

Synonyms

Acacia hawaiiensis (Rock.) O.Deg. & I.Deg. Acacia kauaiensis Hillebr. Racosperma koa (A.Gray) Pedley.