Colubrina oppositifolia has smooth leaves with prominent veins, small star-shaped flowers, and dry fruit capsules that turn from green to brown or black when ripe (Photograph courtesy of G.D. Carr©). The cross-section of this dead kauila trunk shows the color pattern of its extremely dense wood. This 15-year-old cultivated kauila is already about 20 ft (6 m) tall.

Growth Requirements

General

    Colubrina oppositifolia is a beautiful tree for dry or mesic sites. Use it as an accent, for shade, or for screening in your landscape. Kauila can also be kept in a container where it will eventually become stunted if not periodically moved into larger and larger pots. While there are some small morphological differences between the Oʻahu and Hawaiʻi Island forms, both are easy to grow. 

    Kauila does best in a site with full sunlight for all or most of the day,  and poorly in shaded locations. If possible, plant your kauila in a windy site since this seems to reduce attacks by black stem borers (See Pest and Diseases below). When first placed in the ground, water your young tree regularly (i.e., about once a week) until it's established and starts putting out new stems and leaves. Then, cut back on your watering. In very dry locations, watering once or twice a month is normally best; in wetter locations, anywhere from once every couple of months to not at all will keep your kauila "happy." Kauila prefers long deep waterings rather than short showers, with the surface soil being dry most of the time. And, kauila is good at telling you how much water it needs. Too little, and its leaves will wilt and start to fall off; too much, and it will start producing water shoots from the base of its trunk.    

    Colubrina oppositifolia grows at a moderate rate of about one foot a year (in contrast to the other kauila, Alphitonia ponderosa, which grows very slowly). Young trees (less than ten years old) are often shrub-like with many side branches, while older kauila develop into proper-looking trees with one or two main trunks. I (Koebele) have often been tempted to prune away the side branches of a young kauila to discover if this would hasten vertical growth and a widening of the main trunk — but have always lacked the courage. In any case, kauila tends to self-prune over time. Kauila begins flowering and fruiting after only a couple of years (i.e., two to three feet in height). However, these early fruits often contain nonviable seeds. Even mature trees can produce a large percentage of non-viable seeds; therefore, if you are collecting seeds, test them in water before storing them. Viable seeds will sink in the water while inviable seeds will float.

    Colubrina oppositifolia can endure heat, drought and strong winds, and will grow in a variety of soils, including lava/cinder, clay, and organic-dominant; we have not tried to grow it in sand or coralline soils. However, regardless of type, the soil must drain well because kauila will die (or be unhealthy) in soil (or media) that is consistently saturated with water (i.e., waterlogged). Young (and possibly older) trees in the ground benefit from (but don't require) applications of a controlled-release fertilizer every six months. Kauila still in a container should be regularly fertilized with either a controlled-release fertilizer every six months, or with a water-soluble organic or inorganic fertilizer every couple of months. Hand-prune your tree to remove dead branches and any water shoots.

Pests and Diseases

    Black twig borers are a serious pest for kauila, requiring vigilance to keep them controlled. Young plants (i.e., two years old or less) are particularly vulnerable since a lone borer can kill the main stem. If this happens, don't be dismayed (yet). Often, the young tree is able to put out new shoots from the remaining live stem (even if it's slightly below ground). For older trees, borers can slow their growth by killing their thinner side branches; this damage also makes the tree unsightly.

    Several factors regulate the amount of damage caused by the black twig borer to kauila. First, there is the plant’s location. Expect the most damage in damp, shady, windless sites and the least in dry, sunny, windy locations. Second is the health of the tree. Occasionally watering your tree to avoid drought stress will reduce or eliminate borer attacks. However, do not overwater your plant, or you’re likely to face new problems from unfriendly soil microbes. Third is the prompt removal of infested branches. Black twig borer larvae within the dead branch take time to mature and then spread to new healthy branches. If you prune away and dispose of the infested branches quickly, you can break the cycle of infestation. Disposing of the branches means taking them far off-site — don’t just toss them onto your compost pile.

    Occasionally, whiteflies will infest the underside of a kauila's leaves and, if left untreated, can quickly overwhelm small trees. Refer to the Pests & Diseases page for more information on whiteflies and the black twig borer, and how to deal with each.

Special Features and Information

General

Rhamnaceae is a large family of mostly trees and shrubs (and some vines) with about 950 species within 55 genera. They have a worldwide distribution and are most common in tropical and subtropical regions [WikipediA;Rhamnaceae]. In addition to Colubrina oppositifolia, other native members of the Buckthorn family include: the indigenous climbing shrub ānapanapa (Colubrina asiatica); the other endemic kauila or kauwila (Alphitonia ponderosa); and three endangered endemic species of Gouania [Flora of the Hawaiian Islands].    

    Colubrina oppositifolia is one of the hardest of all native woods and was used by early Hawaiians as a metal substitute since there were no readily available sources of iron, etc., in the Hawaiian Islands. Joseph F. Rock (1974) notes it being even harder than its close relative Alphitonia ponderosa. When the wood is freshly cut, Colubrina oppositifolia has a yellowish sapwood and a light reddish brown heartwood. In contrast, Alphitonia ponderosa is red throughout with light red sapwood and dark red heartwood. Unfortunately, when dried and aged the color differences fade over the years making them visually indistinguishable. Microscopic analysis offers a way to tell the two woods apart, but scientists and museum curators are reluctant to shave off the needed samples from rare artifacts. Therefore, for the time being, there will remain uncertainty in many cases as to which species was used for any particular artifact [Abbott 1992].

Hawaiian Name

    The names kauila and kauwila are different spellings of the same name for this species and its close relative, Alphitonia ponderosa. Pukui & Elbert (1986) state that o‘a is a "Maui name for kauila (Colubrina oppositifolia), a tree. (Neal 541.)" However, there is no mention of o‘a on page 541 of Neal (1965). Therefore, it is more likely that Rock (1974) is correct, stating that o‘a is a Maui name for Alphitonia ponderosa. 

Etymology

    Colubrina is derived from the Latin word coluber, meaning "snake" or "like a serpent," referring to the snake-like stems or stamens [Quattrocchi 2000]. The species name, oppositifolia, refers to the opposite leaf (-folia) arrangement found in this species [Gledhill 2008].