Koa is a large tree, typically attaining a height of 15–25 m (49–82 ft) and a spread of 6–12 m (20–39 ft). In deep volcanic ash, a koa tree can reach a height of 30 m (98 ft), a circumference of 6 m (20 ft), and a spread of 38 m (125 ft).
Initially, bipinnately compound leaves with 12–24 pairs of leaflets grow on the koa plant, much like other members of the pea family. At about 6–9 months of age, however, thick sickle-shaped "leaves" that are not compound 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 7–25 cm (2.8–9.8 in) long, 0.5–2.5 cm (0.20–0.98 in) wide phyllodes on an adult tree.
Koa are fast growing trees at 5 feet per year for the first five years, and can reach impressive heights in several decades in upper elevation landscapes.
Is good for erosion control.
It can survive cooler temperatures and the occasional frost.
Koa is a large evergreen broadleaf tree (it can reach 35 m, but more commonly reaches 20-25 m in height, and some populations are much smaller, with a shrub-like form), which can grow in pure stands, but usually is found in mesic forest. Koa is found on all volcanic soil types of all geologic ages.
It grows well in moderately to well-drained, medium to very strongly acid soils on both flatland and steep slopes.
Occurs in a variety of habitats, has a large elevation range and is often a dominant plant in dry to wet forests at elevations ranging from 60 to 2,300 m (Wagner et al. 1990). Morphological differences in koa have been observed on several islands. Seeds are contained within a pod 15–20 cm long, containing 6–12 seeds. The species is a fast-growing tree.
Growth is in virtue of symbiosis with special bacteria called rhizobia that live associated with the roots. The bacteria convert, or fix, nitrogen from the air into usable nitrogen fertilizer for plants. The leaves, flowers and branches also provide nitrogen for understory and plants in the area. Koa inoculated with rhizobia tend to be more vigorous trees.
The bark may be ground into a powder and saturated into water to create a tea, or may be spread onto various foods as a spice and taste enhancer. The wood has a density of about 0.75 g/cm³
To induce sleep during fever or when the patient was sick long in bed, young koa leaves that had been pounded and crushed were spread to completely cover a sleeping pallet made of mats piled one upon another. This caused the patient to perspire, and he or she was able to fall asleep.
Upon waking, the patient was given a drink of fresh spring water or perhaps a tea such as one made from ko'oko'olau leaves. One 'opihi shell full of ashes obtained by burning koa and one 'opihi shell full of ashes obtained from burning the dried flesh of a mature niu were mixed with the milky sap of four green kukui nut fruits and well blended.
This material was smeared on the lesions in the child's mouth twice a day, once in the morning and once in the evening. This medicine was used primarily for children from six months to one year of age.