Acacia confusa

Common name: Formosa acacia

Other common names: False koa, Formosan koa, Fine leaved wattle, Taiwan acacia

Names in non-English languages: Philippines

Description

Native to the Philippines, this fast-growing ornamental and forestry tree is commonly 10 to 15 m (30 to 50 ft) tall with a single or forked trunk supporting a moderately dense, rounded crown. The bark is grey or grey-brown and is usually smooth.

The leaves are long and narrow with a slight curve or sickle-shape, medium green in colour and remain on the tree year-round.

The flowers are small, bright yellow and round, in the shape of small puff-balls. They bloom in profusion from spring to summer, in clusters at the ends of the branches, creating a showy display. Masses of flat, brown seedpods follow and persist on the tree where they eventually become dry and then fall to the ground.

Use

A resilient forestry tree, it is able to withstand adverse weather events on account of its far-reaching root system, which also has nitrogen-fixing abilities, helping to enrich the soil as it grows. This has led to its widespread use in forest regeneration projects, particularly on degraded soil and sloping land, in efforts to minimise soil erosion and protect watershed areas.

It produces a medium-weight to heavy wood, averaging out at around 750 kgs per cubic meter (47 lbs per cubic ft), which puts it in the hardwood class. However, many trees develop low-branching or forked trunks of a small diameter, which makes sawing them into lumber impractical. The trunk and main branches are often only suitable for use in their roundwood form, for cutting-up into lengths for firewood or for making charcoal.

It is cultivated as a landscape tree in Hawaii, mainly for its showy flowering display. However, the leaves emit an unpleasant odour when they are brushed against or ruffled by wind.

Climate

Grows naturally in humid subtropical and tropical lowland to mid-elevation climates, generally in areas with annual lows of 16 to 24 °C, annual highs of 25 to 33 °C, annual rainfall of 1200 to 3000 mm and a dry season of 2 to 6 months.

Growing

New plants are usually raised from seed that have undergone pre-treatment by immersing them in near boiling water for around a minute.

Performs best on free-draining clay, loam and sand soils of a moderately acid to slightly alkaline nature, generally with a pH of 5.5 to 7.5 and on sites with full to partial sun exposure. It has good tolerance to strong winds and drought conditions.

Problem features

There does not appear to be any reports of seed dispersal by birds or other animals. However, many of the seed that fall to the ground germinate readily, creating dense, impenetrable stands or thickets over time, eventually crowding out other plant species.  It is recorded as a major weed in most areas where it is introduced and is assessed to be a high weed risk species for Hawaii by the Hawaii Pacific Weed Risk Assessment project (HPWRA).

The roots are quite vigorous and wide-spreading, which can lead to damage to building foundations, pavements, footpaths or sidewalks if the tree is planted too close to these structures.

Where it will grow


References

Books

  • Allen, O. N. & Allen, E. K. 1981, The Leguminosae : a source book of characteristics, uses, and nodulation, University of Wisconsin Press, Madison, Wisconsin

  • Barwick, M., et al. 2004, Tropical & subtropical trees : a worldwide encyclopaedic guide, Thames and Hudson, London

  • Holttum, R. E. & Enoch, I. C. 2010, Gardening in the tropics : the definitive guide for gardeners, Marshall Cavendish Editions, Singapore

  • Jim, C.Y. 1990, Trees in Hong Kong: Species for Landscape Planting, Hong Kong University Press, Hong Kong

  • Little, E. L. & Skolmen, R. G. 1989, Common forest trees of Hawaii (native and introduced), Agricultuural Handbook No. 679, Forest Service, U.S. Department of Agriculture, Washington, D.C.

  • Randall, R. P. 2002, A global compendium of weeds, R.G. and F.J. Richardson Press, Melbourne

  • Rauch, F. D. & Weissich, P. R. 2000, Plants for tropical landscapes : a gardener's guide, University of Hawaii Press, Honolulu

  • Reyes, G. 1992, Wood densities of tropical tree species, U.S. Department of Agriculture, Forest Service, Southern Forest Experiment Station, New Orleans, Louisiana

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