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Australian Native Plant Profile: Soap Tree (Alphitonia Excelsa)

Looking into the canopy of Alphitonia excelsa, the Red Ash or Soap Tree.
Looking into the canopy of Alphitonia excelsa, the Red Ash or Soap Tree. | Source

Common Names: Red Ash, Soap Tree, Leatherjacket, Coopers Wood
Scientific Name: Alphitonia excelsa
Synonyms: Colubrina excelsa
Family: RHAMNACEAE

Alphitonia excelsa, commonly called Red Ash or Soap Tree, is a generally evergreen, medium tree growing up to 35m tall in wild rainforest settings but usually only reaching about 10m under cultivation. The trunk can reach 1.25m wide in the wild but generally grows to about 0.6m wide under cultivation. Alphitonia excelsa has an umbrella-shaped, round, spreading canopyto 5m across. Alphitonia excelsa is semi-deciduous during particularly dry winters and may drop most of its leaves.

Features of Alphitonia Excelsa, the Soap Tree or Red Ash

Alphitonia excelsa produces cream flowers that although small are pleasantly fragrant. New shoots smell of sarsaparilla when crushed. The leaves have a glossy green upper-surface and dull silver under-surface providing an attractive color contrast. Alphitonia excelsa is an excellent wildlife tree. Native bird species are attracted to both the fleshly blue-black fruits and the high numbers of insect pests, in particular the looper moth caterpillars like to feed on the foliage of this tree en masse. It is a fast growing pioneer species capable of growing as much as 3m a year under ideal growing conditions.

Its bark provides the right environment for an attractive and varied richness of different lichen growths which can an additional point of interest. It is the sole host-plant of the Green-Banded Blue (Psychonotis caelius taygetus) butterfly. Additionally, the Soap Tree is a host-plant of the Blue Jewel (Hypochrysops delicia), Indigo Flash (Rapala varuna simsoni) and Fiery Jewel (Hypochrysops ignita) butterflies. Alphitonia excelsa is an adaptable tree capable of growing across a wide range of conditions. It is both drought resistant and will tolerate moderate frost once established. This tree is quite disease resistant, although insect damage will nearly always be present on the leaves due to the wide range of species that use it as a host plant. Such chewing damage if present is really just an indication of a rich, bio-diverse ecosystem in your garden and is nothing you have to try to control or correct.

How to Use Alphitonia Excelsa in Your Garden

Alphitonia excelsa is an ideal shade tree with a good natural shape. The foliage can be left to extend to the ground or lifted to expose the trunk as desired. It can be used as a street or park tree where space will allow for its mature size and overhead power-lines don't pose a risk. It has been suggested as a replacement plant for exotic privets (Ligustrum sinense & Ligustrum lucidu) which can become weeds in local bushland.

Cultural and Traditional Indigenous Uses of Alphitonia Excelsa

The have been many traditional indigenous uses recorded for Alphitonia excelsa.

  • Indigenous people used it for soapy baths and liniments. The crushed leaves contain saponin and create a lather when rubbed in water, which is how the tree gained its common name.
  • Water infused with crushed leaves was applied across the head to reduce head aches and sore eyes.
  • Insect bites and stings were also treated by applying a water infusion of the crushed leaves to the affected area.
  • Infusions of the bark and root were rubbed on the body to relieve muscle ache or gargled to relieve tooth ache.
  • Young leaves were also chewed to aid gastric upset.
  • Crushed leaves and berries were used as a fish poison. The saponin breaks down the surface tension of the water, leading to deoxygenation which stuns the fish and causes them to float to the surface.

The effectiveness of these remedies appears to be due to anti-inflammatory chemicals within the plant. The timber of Alphitonia excelsa is straight grained and darkens with age to a bright red colour useful for cabinetmaking, tool handles and building. The timber was once so widely used for barrels and buckets that it earned the name 'Coopers Wood'. Alphitonia excelsa was also used in the past for boat-building. It has also been used as a fodder tree for sheep and cattle and is not astringent.

Gardening Limitations

Ornamental use may be limited due to its raggedy appearance that caused by insect damage. It is not suited for extremely small gardens due to its potentially large size when mature.

Natural Growing Conditions

Alphitonia excelsa can be naturally found growing in open eucalypt forests and adjoining rainforests of New South Wales, Queensland, the Northern Terrortory and northern Western Australia. It's generally found along the coast, on lower tablelands and on slopes with a western aspect. It is best suited for growing in climate zones that have a high humidity in summer & a warm winter, a warm humid summer & a mild winter or in warmer temperate environments.

Tips for Growing the Soap Tree or Red Ash

Alphitonia excelsa grows best in a sunny open position on well drained soils. Ensure that ample moisture for young trees is provided throughout summer. Applying fertiliser during the growing season is beneficial, especially when the Soap Trees are young. Remove any dead branches and prune to a single leader when young. Once established Alphitonia excelsa requires little in the way of maintenance.

Propagate from scarified seed soaked in warm water overnight. Germination of seed may be erratic. Cuttings strike readily but will develop weaker root-systems than seed grown plants.

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