Moreton Bay chestnut
In the wild Castanospermum australe grows in moist soils in lowland, subtropical and tropical rainforests in north eastern Australia and New Caledonia. Outside of these high rainfall habitats, it is also native to coastal rainforests and the gallery forests that occur along water courses. This highly ornamental, evergreen tree provides important forage for insects, mammals and people, and has the potential for significant medicinal value.
Castanospermum australe reaches 40 m at maturity and is the only species in the genus. It has glossy, dark-green, compound leaves which each have 11 to 15 leaflets. The leaves are rich in iminosugars, whilst the seeds are poisonous. The young foliage is a food source for the larvae of the pencilled-blue butterfly (Candalides absimilis) which occurs along the Australian east coast.
Castanospermum australe produces masses of beautiful pea-like flowers, forming dense clusters in inflorescences up to 15 cm long. Individual flowers are about four centimetres across and as they age, turn from yellow and orange to red. In the wild, both the pollen and nectar are important forage for bees. The nectar also attracts species of butterflies and bats, whilst lorikeet parrots have been reported to get intoxicated by the nectar.
The fruit, a pod up to 30 cm long, contains one to five large edible seeds. However, before they can be safely consumed the seeds must be carefully processed by roasting and soaking to remove the toxins. Flour milled from the seed is a traditional staple food of Australia's Aboriginal peoples, who have been partially responsible for the tree's spread through Australia.
The extensive root systems formed by Castanospermum and its preference for moist soils, makes it a useful tree for stabilising the banks of watercourses. Castanospermum australe also fixes atmospheric nitrogen using soil bacteria contained within nodules on the tree's roots.
Moreton Bay chestnut is favoured for the colour of its showy sprays of bright flowers and for the shade of its canopy. Consequently, it can be found planted as an ornamental around the world, particularly in India and South Africa. As a pot plant it is popular in Europe and the Americas but its roots must be restricted in a container to prevent it growing too tall. The tree produces a high-value decorative timber that has an appearance similar to walnut.
Castanospermum australe yields compounds, such as the alkaloid castanospermine, which have been shown to have powerful antiviral properties in the laboratory.
Further reading
Huxley A 1999. The New RHS dictionary of gardening. Macmillan Ltd.
Rossetto M et al. 2017. From Songlines to genomes: Prehistoric assisted migration of a rain forest tree by Australian Aboriginal people. PLoS One 12: e0186663.
Whitby K et al. Castanospermine, a potent inhibitor of dengue virus infection in vitro and in vivo. Journal of Virology 79: 8698-8706.
Kate Pritchard