Koa In The Wild
Koa (Acacia koa) is a species of flowering tree in the pea family, Fabaceae. It is endemic to the Hawaiian Islands. Koa is a large tree; in deep volcanic ash, a koa tree can reach a height of 30 metres (98 ft), a circumference of 6 metres (20 ft), and a spread of 38 metres (120 ft). It is one of the tallest and fastest-growing Hawaiian trees, capable of reaching 6–9 metres (20–30 ft) in five years on a good site.

Leaves
Koa leaves are unusual in that the juvenile plants first leaves look totally different than a mature plant. Initially, 12–24 pairs of leaflets grow on the koa plant, much like other members of the pea family. At about 6 months of age, however, thick sickle-shaped "leaves" begin to grow. These are phyllodes, blades that develop as an expansion of the leaf petiole. The vertically flattened orientation of the phyllodes allows sunlight to pass to lower levels of the tree. True leaves are entirely replaced by phyllodes on an adult tree.

Flowers and Fruits
Flowers of the koa tree are pale-yellow spherical racemes with a diameter of 8–10 millimetres (0.31–0.39 in). Flowering may be seasonal or year round depending on the location. Fruit production occurs when a koa tree is between 5 and 30 years old. The fruit take the form of pods with an average of 12 seeds. The seeds are flattened ellipsoids and range from dark brown to black in color. Seeds are covered with a hard seed coat which allows them to remain dormant for up to 25 years.

Habitat
Koa is endemic to the islands of Hawaiʻi, Molokaʻi, Maui, Lānaʻi, Oʻahu, and Kauaʻi, where it grows at elevations of 100–2,300 metres (330–7,500 ft). It requires 850–5,000 millimetres (33–197 in) of annual rainfall. Acidic to neutral soils (pH of 4-7.4), that are typically derived from volcanic ash. Its ability to fix nitrogen allows it to grow in very young volcanic soils and also help other native plants flourish. Koa and ʻōhiʻa lehua dominate the canopy of Hawaiian forests.

Uses
Though both the bark and the crescent leaves were used in various medicinal preparations, it is the Koa wood which stands out in its utility to Hawaiians. The wood characteristics are amazingly variable, from blonde low density to deep red to very heavy and dark, Koa wood comes in all hues and texture. The koa's trunk was used by ancient Hawaiians to build waʻa (dugout outrigger canoes), and papa heʻe nalu (surfboards). Even today it is prized by Hawaiians families as a wood of preference for their homes, crafts and outdoors. Commercial silviculture of koa is difficult because it takes 20 to 25 years before a tree is of useful size. The wood is similar in strength and weight to that of black walnut and is sought for use in wood carving and furniture. Koa is also a tonewood, often used in the construction of ukuleles, acoustic guitars, and Weissenborn-style Hawaiian steel guitars.

Conservation
Because the Koa wood is so prized, it is now illegal to kill any living Koa tree, and only dead trees may be used for any type of woodworking. The koa population has suffered from grazing and logging. Although formerly used for outrigger canoes, there are few koa remaining which are both large and straight enough to do so today. In areas where cattle are present, koa regeneration is almost completely suppressed. However, if the cattle are removed, koa are among the few native Hawaiian plants able to germinate in grassland, and can be instrumental in restoring native forest. For instance, experiments at the Hakalau National Wildlife Refuge have shown that ʻōhiʻa lehua (Metrosideros polymorpha) survives best in pasture when planted under koa. This is because koa trees reduce radiative cooling, preventing frost damage to ʻōhiʻa lehua seedlings.