From Common Forest Trees of Hawaii

Acacia koa
Pea family (Fabaceae)

Native species ()

Koa, the largest native tree and second most common, is well known. This large to very large tree becomes 100 ft (30 ) tall and 5 ft (1.5 ) or more in trunk diameter but may be only half that size. Trunk straight and tall or becoming crooked and branched; spreading, rounded to dark green. Bark light gray, smooth on small trunks, becoming very rough, thick, and deeply furrowed, scaly and shaggy. Twigs brown, becoming hairless. Mimosa subfamily (Mimosoideae).

©2002-2013 J. B. Friday
Leaves mostly modified as sickle-shaped leaf-stalks or (phyllodes), narrowly lance-shaped, curved, 4–6 inches (10–15 ) long and 1⁄4–1 inch (6–25 ) wide, slightly thickened and leathery, hairless, very long-pointed at both ends, with dot-like near base, several fine parallel veins from base, dull green to dark green. True (juvenile) leaves on seedlings and young twigs twice or divided (), 6–7 inches (15–18 ) long, with 5–7 pairs of axes, each with 24–30 paired stalkless oblong 1⁄4 inch (6 ) long.

Flower clusters of light yellow balls (heads) 3⁄8 inch (1 ) in diameter, one or few on slender stalks about 1⁄2 inch (13 ) long at leaf base. Flowers tiny, numerous, stalkless in balls, nearly 1⁄4 inch (6 ) long, consisting of cup-shaped five narrow petals slightly united at base, many spreading threadlike separate ending in dot and narrow with threadlike Flowering mostly in late winter and early spring.

(pods) are broad, flat, 3–6 inches (7.5–15 ) long, 5⁄8–1 inch (1.5–2.5 ) wide, brown, mostly not splitting open. Seeds are several, beanlike, 5⁄16 inch (8 ) long, oblong, flattened, straight, dark brown or blackish, slightly shiny.

Koa is an excellent cabinet wood of reddish brown color that is often highly figured. It is moderately heavy wood ( gr. 0.55) identical in weight and strength properties with black walnut (Skolmen 1968). This stable wood works and seasons well and takes a high polish. It is not resistant to decay and is quite susceptible to drywood termites.

The wood is used for furniture, cabinet work, carved bowls and turnery, gunstocks, and veneer; formerly for construction and surfboards. Many large offices as well as homes in Hawaii have paneling and furniture of koa. It is Hawaii’s best known wood. Native Hawaiians had many uses, such as house timbers, carved dugout canoes, and paddles. Koa canoes are prized for competitive paddling by outrigger canoe clubs today and logs suitable for making them are extremely scarce and costly. The bark served in tanning.

Koa is widely distributed in both dry and rain forests at 600–7000 ft (183–2134 ) altitude. Koa forests are an important habitat for rare birds. This species may be seen on all the larger islands, near Kokee on Kauai, along Likelike and Nuuanu Pali Highways on Oahu, the Hana Road near Keanae, Maui, and Hawaii Volcanoes National Park in Hawaii. There are 2.5 million cu ft (0.7 million cu ) of sawtimber in Hawaii. Only Hawaii and Kauai have tall straight-stemmed koa suitable for lumber. The trees on other islands have short crooked stems. Trees on the Island of Hawaii growing in closed forest at 4,000–6000 ft (1219–1829 ) are by far the largest.

Special areas
Waimea, Arboretum, Tantalus, Haleakala, City, Volcanoes, Kipuka Puaulu

Former champion
Height 140 ft (42.7 ), c.b.h. above bulge, 37.3 ft (11.4 ), spread 148 ft (45.1 ). District of Ka’u, Hawaii (1969). This giant was probably the largest native tree in Hawaii, the tallest as well as greatest in trunk circumference. Unfortunately, it split in two from the groundline in the late 1970s and is now quite small. A new champion has not been selected.

Known only from Hawaiian Islands

A few botanical varieties have been named from different islands, distinguished by the shape of the “leaves.” Kauai koa (Acacia kauaiensis Hillebr.) is a closely related species to Kauai and common at Kokee. This large tree is easily recognized by the flower clusters of many light yellow balls (heads) at ends of twigs). Koaia (Acacia koaia Hillebr., another closely related species, is to dry areas of Molokai, Maui, and Hawaii is now rare and considered endangered. It is distinguished by seeds that lie parallel rather than perpendicular to the pod axis. Wood is heavier, harder, and more finely textured than koa.

An evergreen tree retains a large portion of its green leaves all year.

stamen -- the pollen-producing reproductive organ of a flower; The stamen consists of an anther supported by a filament.

style -- This is a long and thread-like structure that connects the stigma with the ovary. A flower may have a single style, or several of them.

sp. -- The abbreviation for "species". The plural is "spp". When used it sometimes means that the exact species is unknown. For example, "Aster sp" would mean some species within the Aster genus but the writer may not know exactly which species.

cm -- A centimeter which is about 0.4 inches.

Irregular flowers, such as those of the violet or the pea, are often bilaterally symmeteric. These flowers typically have petals of unequal size or shape.

mm -- millimeter. About 1/25th of an inch.

alternate -- leaves alternate along the main stem and are attached singly.

m -- A meter is about 10% larger than a yard.

Glands are plant structures that secrete liquids, salts or other substances. Glands often appear as hairs with a drop of liquid at the end.

calyx -- the sepals of a flower, typically forming a whorl that encloses the petals and forms a protective layer around a flower in bud.

fruit -- any seed-bearing structure in flowering plants. It is formed from the ovary after flowering.

leaflets -- Each little leaf-like thing in a compound leaf is a leaflet.

The anther is a pad at the end of the stamen that holds the pollen.

A leaf is compound when multiple leaflets are on the same stem.

petiole -- the stalk of a leaf.

canopy -- The foliage of a tree; the crown. Also the upper layer of a forest.

A pistil is the female structure of many flowers. It contains one or more carpels. Each carpel contins an ovary, style and stigma. The stigma receives the pollen which grows thru the style to reach the ovary.

endemic -- when restricted to a certain country or area.

Bipinnate -- A compound leaf with two rows of leaflets where those leaflets are again compound with two rows of leafelets.