Koa (Acacia koa) is a legume tree native to Hawaii. It is found on all the big islands and grows from near sea-level to the tops of the mountains, although it prefers the moist sites between 3000 to 6000 feet. It has been reported that trees form areas of heavy rain produce straight grain, while those at higher elevations produce more figured wood. Koa is the best known wood from the Islands.
Koa belongs to the genus Acacia. This genus is composed of nearly 800 species, 21 of which are native to the United States, with the rest native to the tropics and subtropics. The word koa is a native name meaning warrior or soldier.
Koa trees reach a height of 100 feet with a diameter of 4 feet. Koa grows rapidly and has been planted as a soil conservation measure as well as in plantations. Much of the koa sold today is harvested from plantations. Obviously, there are no concerns about scarcity; it is not an endangered tropical trees.
Historically, koa was used in Hawaii for dugout canoes. It also was the royal wood of Native Hawaiians and was therefore used for everything in contact with the royal family, including steps. Other historical uses for the wood included royal coffins and spear handles. The bark is quite astringent and has been used both medicinally and for tanning leather.
Current uses for koa include cabinetry, high quality furniture, interior trim, ukuleles (the wood is quite resonant) and other musical instruments, oars, paddles, poi bowls, turnery and carvings. It is certainly a useful wood and has an appearance much like teak. Lumber prices are moderate.
Processing suggestions and characteristics
Density. Koa has a density of 37 pounds per cubic foot at 6 percent MC. This means that a board foot will weigh about 3 pounds. This density is similar to many other U.S. hardwoods and a little below hard maple.
Drying. Koa dries easily, without splitting or cupping. It is similar to hard maple, except that color issues are not a concern with koa.
Shrinkage in drying is about 6 percent.
Gluing and Machining. Koa glues well without any special requirements.
Koa is brittle, especially if over-dried, and has variations in density. Also, some pieces have swirly grain. These characteristics mean that from time to time koa will be difficult to machine to achieve a premium surface unless care is taken to use sharp tools and sandpaper. The wood surface will tend to burnish if tools and sandpapers are not exceptionally sharp. However, with good tools and fresh sandpaper, the final surface is excellent. Koa also carves well and polishes to a high, lustrous finish.
Stability. The wood is fairly stable, requiring a 5 percent MC change to result in a 1 percent size change, across the grain. There is not much stability difference between quartersawn and flatsawn lumber.
Strength. Koa is fairly strong, with an ultimate strength (MOR) of 13,300 psi. The bendability (MOE) is 1.57 million psi. The hardness is 1110 pounds. For comparison, teak values are 14,600 psi, 1.55 psi and 1000 pounds. Soft (red) maple is 13,400 psi, 1.64 million psi, and 950 pounds.
Color and Grain. Koa has very little yellowish white sapwood in the tree, so most lumber will be 100 percent heartwood. The heartwood color can be variable from tree to tree and site to site; color ranges form light brown to dark brown, with golden or red tinges, or darker streaks. The wood’s surface texture is fine. Often the grain is swirly giving an attractive figure to the flatsawn lumber. The wood takes a high polish.
It is fluorescent under UV light.
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