Microscopy: Technical Information Sheet


Koa Acacia koa A. Gray Leguminosae

The genus Acacia is composed of 600 to 800 species, 21 of which are native to the United States, with the rest native to the tropics and subtropics. The word acacia is the classical Greek name of a thorny tree of Egypt, thought to be of this genus, form the Greek word for thorn. The word koa is a native name meaning warrior or soldier.

Other Common Names: Black Koa, Curly Koa, Figured Koa, Hawaiian Mahogany, Koaia, Koa-Ka, Koalaunui, Round-log Koa, Square-log Koa

Distribution: Hawaiian Islands

The Tree: Koa trees reach a height of 100 feet with a diameter of 4 feet. It grows a most elevations on the islands, but grows best in areas of heavy rainfall form 3,000 to 6,000 feet. It is the most conspicuous tree growing between the low, open dry forest and the wet ohia forest. Koa trees have gray bark, spreading branches and flat, curved petioles (phyllodes) which act as leaves. Koa is readily propagated, grows rapidly and has been planted as a soil conservation measure. The bark is astringent and has been used medicinally and for tanning leather.

 General Wood Characteristics: Koa sapwood is narrow and a yellowish white, while the heartwood ranges form light brown to dark brown, with golden or red tinges, or darker streaks. The color is supposed to be influenced by growing conditions. It has no distinctive odor or taste, although it imparts a flavor when used for food service. It is fluorescent under UV light. It has been reported that trees form areas of heavy rain produce straight grain, while those at higher elevations produce more figured wood.


Moisture content Specific gravity lb/ft3 kg/m3
Green 0.53 33 529
12% 0.55 34 545
11.2% 0.46 29 465
6% 0.59 37 593
Ovendry 0.49 31 497

a Reference (5).

Mechanical Properties

Property Greena Dryb
MOE 1.52 lbf/in2 10.48 GPa 1.57x106lbf/in2 10.82 GPa
MOR 9.00x103lbf/in2 62.05 MPa 13.3x103lbf/in2 91.70 MPa
C|| 3.90x103lbf/in2 26.89 MPa 7.32x103lbf/in2 50.47 MPa
C_|_ NA NA 1.36x103lbf/in2 9.38 MPa
WML 12.9 in-lbf/in3 88.94 kJ/m3 9.13 in-lbf/in3 62.95kJ/m3
Hardness 870 lbf 3870 N 1110 lbf 4938 N
Shear|| NA NA 1.78x103lbf/in2 12.27 MPa

a Reference (11).
a Reference (5).

Drying and shrinkagea

  Percentage of shrinkage
(green to final moisture content)
Type of shrinkage 0%MC 6%MC 20%MC
Tangential 6.19 NA NA
Radial 5.47 NA NA
Volumetric 12.39 NA NA

a Koa dries well, without splitting or cupping, including veneers. Reference (5).

 Kiln drying schedulea

Condition 4/4, 5/4, 6/4 stock 8/4 stock 10/4 stock 12/4 stock 16/4 stock
Standard T6-D4 T3-D3 NA NA NA

a References (1&10).

Working Properties: Koa is brittle and has variations in density, making it difficult to work. It is difficult to plane by machine or by hand. It glues well, but tends to burn when machined with routers or drum sanders. It carves well and polishes to a high finish.

Durability: Resistant to insects and fungi.

Preservation: No information available at this time.


Historical: Dugout canoes, early surfboards, royal coffins, spear handles, ukuleles, general construction, ships knees. The royal wood of Native Hawaiians, used for everything in contact with the royal family, including steps.

Current: Cabinetry, furniture, interior trim, boats, boat planking, veneers, ukuleles, organs, pianos & other musical instruments, radio cabinets, oars, paddles, picture frames, handles, gun stocks, crutches, wheels, posts, fencing, bridges, railway cars, poi bowls, turnery & carvings.

Toxicity: No information available at this time.

Additional Reading & References Cited (in parentheses):

1. Boone, R.S., C.J. Kozlik, P.J. Bois & E.M. Wengert. 1988. Dry kiln schedules for commercial woods - temperate and tropical. USDA Forest Service, FPL General Technical Report FPL-GTR-57.

2. Carlquist, S. 1980. Hawaii. A natural history. Geology, climate, native flora and fauna above the shoreline. SB Printers, Inc., Honolulu, Hawaii. 468 pp.

3. Carlson, N.K. and L.W. Bryan. 1959. Hawaiian timber for the coming generations. A Report on the Honaunau Forest, South Kona, Hawaii. Its present condition and its potential. Bishop Estate, Land Development and Control Section.

4. Elias, T.S. 1980. The complete trees of North America, field guide and natural history. Van Nostrand Reinhold Co., New York, 948 pp.

5. Gerry, E. 1955. Information leaflet, foreign woods. Koa or Koa-ka. USDA Forest Service, FPL, Report No. 2023.

6. Little, Jr., E.L.1979. Checklist of United States trees (native and naturalized). USDA Forest Service, Ag. Handbook No. 541, USGPO, Washington, DC.

7. Markwardt, L.J. and T.R.C. Wilson. 1935. Strength and related properties of woods grown in the United States. USDA Forest Service, Tech. Bull. No. 479. USGPO, Washington, DC.

8. Panshin, A.J. and C. de Zeeuw. 1980. Textbook of Wood Technology, 4th Ed., McGraw-Hill Book Co., New York, 722 pp.

9. Record, S.J. and R.W. Hess. 1943. Timbers of the new world. Yale University Press, New Haven, 640 pp.

10. Simpson, W.T. 1991. Dry kiln operator's manual. USDA Forest Service, FPL Ag. Handbook 188.

11. Skolmen, R.G. 1974. Some woods of Hawaii... properties and uses of 16 commercial species. USDA Forest Service General Technical Report PSW-8.

12. Summitt, R. and A. Sliker. 1980. CRC handbook of materials science. Volume 4, wood. CRC Press, Inc., Boca Raton, FL. 459 pp.

13. Whitesell, C.D. 1964. Silvical characteristics of Koa (Acacia koa Gray). USDA Forest Service Research Paper PSW-16.


4/4 nominal 1-inch (standard 25.4-mm) thickness lbf pound-force
5/4 nominal 1-¼-inch (standard 32-mm) thickness m meter
6/4 nominal 1-½-inch (standard 38-mm) thickness MC moisture content
8/4 nominal 2-inch (standard 51-mm) thickness MOE modulus of elasticity
10/4 nominal 2-½-inch (standard 64-mm) thickness MOR modulus of rupture
12/4 nominal 3-inch (standard 76-mm) thickness Mpa megapascal (106 Pa)
16/4 nominal 4-inch (standard 102-mm) thickness N newton
C|| compression parallel to grain, maximum crushing strength NA information not available
C_|_ compression perpendicular to grain, stress at proportional limit Pa pascal
Dry 12 percent moisture content Shear|| shear parallel to grain, maximum shearing strength
Gpa gigapascal (109 Pa) SG specific gravity
Hardness side hardness WML work to maximum load
kJ kilojoule (103 J)