A castanospermum is a tree in the genus Castanospermum, which has only one known species, Castanospermum australe. These trees are native to the eastern coast of Australia and can be cultivated in temperate to semi-tropical climates in other regions of the world. In addition to being grown as an ornamental in gardens, this tree can also be cultivated as a houseplant.
These trees are known by common names like Moreton Bay Chestnut and Black Bean. They have tall, slender trunks topped with branching crowns of oval, glossy leaves. Castanospermum trees produce bright red to orange flowers, followed by seedpods filled with large beans. The roots and beans are both toxic, an important characteristic for gardeners to be aware of, as they can be dangerous for pets and young children.
In nature, castanospermum trees can be found in coastal rainforests and along Australia's beaches. The trees can grow quite large and are often found in clumps or clusters. The trees famously have an extensive root system designed to take advantage of poor soil conditions. Gardeners planting castanospermum trees may want to consider this when deciding on where to place a seedling, as the trees should not be grown close to foundations, wells, and underground utility lines, or their sprawling roots may cause problems.
Cultivated as a houseplant, trees in this genus can grow into small, shrubby plants. Size can be controlled by keeping the plant in a relatively small pot, and people can also shape their plants with pruning, trimming, and training. Known as lucky bean plants in some regions of the world, these houseplants prefer bright, indirect light and moderate humidity. Like their outdoor counterparts, they need moist, loose, well-drained soil worked with some enriching materials to keep the plant healthy.
A castanospermum tree can make an excellent standalone garden feature, or it may be used with a cluster of trees to create a privacy screen or shaded area in the garden. Generally, these trees prefer full sun and they do not tolerate frost well.
In Australia, Aboriginal people have historically treated the beans with a special process to remove the toxins and ground them into flour for consumption. Products made with the flour can be obtained in some parts of Australia. People interested in processing their own beans can find resources describing the process. Special care needs to be taken because if the beans are not properly handled, some of the toxins can remain and people may get sick.