Information collected from: http://nativeplants.hawaii.edu/
Koa is the largest native tree in the Hawaiian Islands reaching heights of about 115 feet (33 m)! Commercially, koa is one of the most expensive woods in the world. It is used to make furniture, veneer, and crafts. Most koa is harvested from remnant individuals or stands in pasture lands. 
Plant Form / Growth Habit
Mature Size, Height (in feet)
- Tree, Small, 15 to 30
- Tree, Medium, 30 to 50
- Tree, Large, Greater than 50
- Mature Size, Width
Koa can have a canopy spread of 40 feet or more. But typically, with a canopy spread of 20-40 feet. 
- Long lived (Greater than 5 years)
- Erosion Control
- Provides Shade
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.  Though koa can grow at lower elevations, some varieties may succumb to diseases after a decade of growth. Since there is so much variety in koa habit from shrubby, multi-branched forms to tall straight trees, it is good to inquire of the source so as to suit your landscaping needs.
While koa can grow to heights of over 100 feet in their natural undisturbed environment, it would take many decades, if ever, to reach such heights in the urban landscape. They are more likely to grow to about 20 to 30 feet in the landscape at low elevations. [Rick Barboza, Hui Kū Maoli Ola]
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.
Recommended planting is above 2000 feet [610 m]. 
Koa are easily damaged by lawnmowers and gas trimmers, a.k.a. “weed wackers.” J. B. Friday, Extension Forester with the University of Hawaiʻi Cooperative Extension Service, comments that once koa injured, “rot tends to set in and the trees day are numbered.” How to prevent unnecessary injury for these fragile trees? J. B. says that “a wide band of mulch and keep weed-eating implements away.” 
J. B. Friday concludes an article with encouraging words for planting koa. He states: “Although koa may only live 5 to 20 years in urban lowland settings and never develop into the huge spreading tree seen in the forest, there is still educational value in planting this endemic tree. At schoolyard or demonstration plantings especially, generations of children will be able to see what koa [is] and develop a connection with our forests.” 
Leaf Growth Requirements
Lightly fertilize seedlings 2 or 3 weeks after secondary growth. Since koa are nitrogen fixing trees, additional nitrogen is usually not necessary.
Pruning koa often does more harm than good. Wounds from pruning may not heal, exposing the heartwood to rot and greatly increases their suspeceptability to disease and pests. Additionally, pruning slows the growth of the trees. Therefore, koa should not be pruned if it can be avoided. Lower branches do self prune. If pruning is needed, it should be absolutely minimal, done properly, and without excessive injury to the tree. [1,16]
Koa seedling roots should not be pruned, trimmed, or otherwise “fluffed out.”
Additional Water Information
- Water once a month during dry months, more often for “coastal” trees.
- Soil must be well drained
- Full sun / Partial sun
- Trees should be spaced 30 to 40 ft. apart.
Natural Zones (Elevation in feet, Rainfall in inches)
- 150 to 1000, 0 to 50 (Dry)
- 150 to 1000, 50 to 100 (Mesic)
- 1000 to 1999, 0 to 50 (Dry)
- 1000 to 1999, 50 to 100 (Mesic)
- 2000 to 2999, 0 to 50 (Dry)
- 2000 to 2999, 50 to 100 (Mesic)
- 3000 to 3999, 0 to 50 (Dry)
- 3000 to 3999, 50 to 100 (Mesic)
- 4000 to 4999, 0 to 50 (Dry)
- 4000 to 4999, 50 to 100 (Mesic)