Averrhoa carambola Seeds | Star Fruit

Flat, brown Star Fruit seeds, roughly triangular and papery, on a plain background — Star Fruit (*Averrhoa carambola*): Oxalidaceae, native to Southeast Asia and grown throughout the tropics.

LATIN NAME: Averrhoa carambola

COMMON NAME: Star Fruit, Carambola

Plant Profile

Star Fruit is a small, bushy evergreen tree, usually five to eight metres in cultivation and sometimes twice that in the forest, with pinnate leaves that fold together at dusk and open again at dawn. The flowers are small, pink to purple, and appear in clusters on the trunk and older branches; the fruit is waxy, yellow-green, five-ribbed, and slices into perfect stars.

It is native to the Moluccas and probably to other parts of Southeast Asia, and has been carried through the tropics for so long that its exact origin is uncertain. It fruits more or less continuously in the humid lowlands, with a main crop in the cooler months.

Science & Biotechnology

The species is unusual in the wood-sorrel family for its cauliflory, flowering directly on the trunk, and for the nyctinastic leaf movement that has made it a subject in studies of plant circadian rhythms. Its fruit is rich in oxalic acid and in caramboxin, a neurotoxin that is normally harmless but accumulates dangerously in people whose kidneys cannot clear it.

Research on the fruit has focused on two things: the pharmacology of caramboxin, which can cause hiccups, confusion and seizures in dialysis patients, and the polyphenol content of the fruit and leaves, which is high enough to interest food scientists. The tree itself is grown in the tropics as much for its ornamental habit as for its crop. The tree is also sensitive to frost when young, which confines it to the warm tropics and to glasshouses elsewhere.

Uses & Benefits

The fruit is eaten fresh, sliced into salads and used as a garnish, and it makes a sharp, refreshing juice in Southeast Asia. It is also cooked into chutneys and preserves, and the unripe fruit is used as a souring agent in some regional dishes.

The caution is not a formality. Anyone with kidney disease, or with reduced kidney function from any cause, should avoid the fruit entirely, and healthy people should not eat large amounts on an empty stomach. In the garden the tree is grown for shade and ornament in the tropics and as a conservatory specimen elsewhere.

Growing from Seed

Star fruit seed loses viability quickly and should be sown within a few days of extraction. Clean off the pulp, sow the seed flat in a loose, free-draining tropical mix, and keep it at 25 to 30 °C; germination takes two to four weeks and is usually reliable with fresh seed.

Grow the seedlings in bright, humid conditions and pot them on as they fill the container. Plant out in a sheltered, frost-free position with deep soil and regular water; the tree will begin fruiting in three or four years from seed. In cool climates keep it in a large pot and move it indoors for winter.

Our Take

A small tropical tree with striking fruit and a serious medical caveat. Easy from fresh seed, quick to bear, and best grown where the winter is warm.

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