Koaia (Acacia koaia)

KingdomPlantae
PhylumTracheophyta
ClassMagnoliopsida
OrderFabales
FamilyLeguminosae
GenusAcacia (1)
SizeHeight: up to 5 m (2)

Classified as Vulnerable (VU) on the IUCN Red List (1).

The koaia is a small, hardy tree of the pea family which, due its grey-silver tinted foliage and prominent yellow flowers, is an attractive part of the Hawaiian landscape (2). It has a domed canopy that is usually as wide as the tree is tall when grown in the open (2) and produces pale yellow flowers, which grow arranged in pretty clusters (2) (3). The koaia is somewhat unusual in that it produces two distinct types of leaves in its lifespan; young seedlings have leaves made up from many smaller leafs, or ‘leaflets’ (known as ‘bipinnate compound true leaves’) (2), while mature leaves are narrow, sickle-shaped and measure up to 15 centimetres long (3).

The koaia is similar in appearance to the more common and widespread Acacia koa and, in fact, there is some debate as to whether the two are actually distinct species. However, the koaia can be distinguished by its smaller stature, more gnarled appearance (2), its flattened, narrower seed pods (4), and harder wood (2). This hard, durable wood has been used by Hawaiians for many years to make items such as spears and canoe paddles, and the leaves are used to cover shelters (5).

The koaia is native to the Hawaiian islands of Hawai‘i, Moloka‘i, Maui, Lāna‘i, and Kaua‘i (2). It is also found on Oahu, but may have been planted there (1).

The koaia occurs in the forests and shrublands of the Hawaiian Islands. It prefers dry, windy and open conditions, but sometimes also grows in relatively moist habitats (2). 

The koaia grows rapidly in its first few years, typically at a rate of 1 to 1.5 metres per year (2). After a few years the growth rate slows down, but it grows continually unless there is an extended dry period, at which point the canopy broadens and the stems increase in diameter (2).

The koaia starts flowering after two or three years, after which it will flower throughout each year, with a peak in the autumn. The pale yellow flowers provide pollen and nectar for the native insects of Hawaii (2), which are assumed to also pollinate this plant. The koaia produces long pods containing 6 to 12 seeds, and in keeping with its preference for dry conditions, in dry climates the seed quality is high, whilst in wetter climates the seed quality is often poor (2). When ripe, the brown seed pods split open to release the dark brown to black seeds (6).

The roots of this small tree form extensive, shallow, spreading systems which may often be partially exposed at the soil surface (2). The koaia (as a legume) has a special symbiotic relationship with bacteria (Rhizobia) which live in nodules in the plant’s roots. This bacteria is ‘nitrogen-fixing’, meaning it is able to convert the nitrogen in the air to make it available to the plant (2).

Avariety of threats have resulted in a decline in the number of koaia, and it is now considered at risk of extinction. Due to the koaia growing at relatively low elevations, often close to inhabited areas, it has been severely impacted by ranching, agriculture and human-ignited fires (1). Other threats to the species are over-browsing by deer, feral goats and pigs, and competition with invasive alien plants (1).

The koaia is also susceptible to ‘koa wilt’, a common, fatal disease that causes the leaves to wilt, turn yellow and die; a small tree may be killed by the disease in as little as a few weeks. Koa wilt is caused by infection by a fungus (Fusarium species), which was thought to be introduced to the islands within the past few decades (2).

The Hawai'i Forest Stewardship Program (Kalopi Dryland Forest Restoration) is restoring the ridges of Kohala Mountain in Hawai’i, which includes the planting of koaia (4). Ex-situ conservation work is also being conducted at the National Tropical Botanical Garden, where the koaia is being grown and its seeds are being preserved (4).

For more information on the conservation of Hawaiian plants see:

For more information on the koaia, see:

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
arkive@wildscreen.org.uk

  1. IUCN Red List (May, 2010)
    http://www.iucnredlist.org
  2. Elevitch, C.R., Wilkinson, K.M. and Friday, J.B. (2006) Acacia koa (koa) and Acacia koaia (koai‘a). In: Elevitch, C.R. (Ed) Species Profiles for Pacific Island Agroforestry. Permanent Agriculture Resources, Hōlualoa, Hawai‘i.
  3. Krauss, B.H. (1993) Plants in Hawaiian Culture. University of Hawaii Press, Hawaii.
  4. Centre for Plant Conservation (May, 2010)
    http://www.centerforplantconservation.org/Collection/CPC_ViewProfile.asp?CPCNum=17
  5. Medeiros, A.C., Davenport, C.F. and Chimera, C.G. (1998) Auwahi: Ethnobotany of a Hawaiian Dryland Forest.Cooperative National Park Resources Studies Unit, University of Hawaiʻi, Mānoa.
  6. Culliney, J.L. and Koebele, B.P. (1999) A Native Hawaiian Garden: How to Grow and Care for Island Plants. University of Hawaii Press, Hawaii.