Carambola (Averrhoa carambola)

Carambola (Averrhoa carambola) is a tree that’s indigenous to lush and steamy regions of Southeast Asia. Another name for it is star fruit. People call it this because when you slice it in half, you’ll see an unmistakable star shape. Not only is the fruit tasty, but it also reportedly has health benefits.

I. Appearance and Characteristics

Averrhoa carambola is a species of tree in the family Oxalidaceae native to tropical Southeast Asia; it has a number of common names, including carambola, star fruit and five-corner.

The genus was named after Abū ‘l-Walīd Muḥammad bin Aḥmad bin Rushd (better known just as Ibn Rushd), who was called Averroes in European literature – a famous Arabian physician, astronomer and philosopher of the 12th century.

Averrhoa carambola is a small, slow-growing evergreen tree with a short-trunk or a shrub. The branches are drooping and the wood is white and turns reddish. It has a bushy shape with many branches producing a broad, rounded crown.

The compound leaves are soft, medium-green, they are spirally arranged around the branches in an alternate fashion. The pinnate leaves have a single terminal leaflet and 5 to 11 nearly opposite leaflets, each leaf is 15–20 cm long, and the 3.8–9 cm long leaflets are ovate or ovate-oblong in shape. The top sides of the leaves are smooth and the undersides are finely hairy and whitish. The leaflets are reactive to light and tend to fold together at night, they are also sensitive to abrupt shock and when shaken tend to close up also.

The lilac or purple-streaked, downy, flowers are produced in the axils of leaves at the end of twigs. The flowers are arranged in small clusters on the ends of the branches or sometimes on the larger stems and trunk, each cluster is attached to the tree with red stalks. The bell-shaped, perfect flowers, are produced in loose panicles that are much-branched with pedicellate flowers; each flower is around 6 mm wide, with 5 petals that have recurved ends.

The fruits are showy with an oblong shape: they are longitudinally 5- to 6-angled and 6.35–15 cm long and up to 9 cm wide. The fruits have a thin, waxy skin that is orange-yellow colored. The juicy fruits are yellow inside when ripe and have a crisp texture and when cut in cross-section are star-shaped. The fruits have an oxalic acid odor, which varies between plants from strong to mild, the taste also varies from very sour to mildly sweetish. Each fruit may have up to twelve 6–12.5 mm long seeds, which are flat, thin and brown. Some cultivated forms produce fruits with no seeds.

Toxicity

Like several other plants of the family Oxalidaceae, its fruits are rich in oxalic acid, which is toxic in high concentrations.

There have been reports of intoxication in dialysis and uremic patients caused by a neurotoxin called caramboxin present in the fruit. Such toxin is normally filtered by the kidneys, but patients in dialysis or suffering from kidney deficiencies may show severe symptoms, in a few cases fatal, after drinking the fruit juice.

II. How to Grow and Care

Carambola is a tropical fruiting tree known for its distinctive star-shaped fruit and relatively easy care requirements. Key care points include ensuring carambola has ample sunlight and consistent watering, while avoiding overwatering to prevent root rot. Carambola thrives in well-draining soil and benefits from periodic fertilizing during its growing season. These elements, along with its resilience, categorize carambola as an easy-care plant suitable for both novice and seasoned gardeners.

Sunlight

Carambola thrives best in full sunlight, which is essential for its growth and fruit production. Ideally, it should receive at least 6-8 hours of direct light daily, enhancing photosynthesis and energy acquisition for healthy development. While carambola exhibits a degree of light tolerance, it prefers consistent exposure to bright conditions. Insufficient light can lead to reduced fruit quality and poor growth.

Outdoors, carambola should be planted in locations with unobstructed access to sunlight, avoiding shaded areas that can compromise its vigor. Adapting to intense light, carambola’s foliage may display protective responses to mitigate stress effects, ensuring its resilience in varying light environments.

Watering

Originating from the humid environments of Southeast Asia, carambola has adapted to thrive in conditions where moisture is abundant. This species exhibits a preference for consistent hydration but demonstrates resilience with moderate drought tolerance. Watering practices should mimic its natural setting, equating to once every 1-2 weeks. Given its tropical origins, carambola flourishes best when grown outdoors, where it can benefit from the seasonal rains that align with its growth cycles and enhance fruit production.

Fertilizing

Fertilize carambola with high phosphorus fertilizer to stimulate blooming and fruiting, switching to balanced nutrition during active growth. Apply quarterly, proportionate to the plant’s size, avoiding over-fertilization which can harm the roots. Seasonal changes affect carambola’s feeding needs; less in winter, more in growing season. For safety, water carambola after applying and protect foliage from direct contact with fertilizers. Using slow-release formulations can provide consistent nutrients, enhancing carambola’s health and productivity.

Propagation

Carambola is propagated effectively through grafting, a technique which promises genetic similarity with the parent plant and tends to ensure a higher fruit quality. Successful grafting requires a healthy rootstock and scion with compatible vascular tissues. The union should be kept under appropriate humidity and temperature until the graft has taken, usually evident by the growth of new shoots. Care should be taken to limit infection risks at the graft site, often achieved by using a sterilized blade and sealing the wound with grafting tape or wax.

Transplanting

Transplant carambola in the vitality of mid-to-late spring or early summer, harnessing the season’s consistent growth conditions. Choose sunny, well-draining sites, and when necessary, enrich the soil to give carambola a robust start.

Repotting

Repot carambola every 2-3 years to accommodate its medium-sized, bushy growth habit. Best repotted in early spring to align with its growth cycle, ensuring a smooth transition. Select a pot 2-3 inches larger in diameter for optimal space. Post-repotting, provide indirect light and maintain soil moisture for recovery. These tips encourage carambola’s vibrant, star-shaped fruit and lush foliage.

III. Uses and Benefits

Averrhoa carambola is best known as the star fruit, as it is best known for the star-like shape it has when cut. In addition to being eaten, the fruit is used in traditional Asian medicine to treat chickenpox, intestinal parasites, headaches, and other illnesses. Star fruit contains oxalate, too much of which can be dangerous.

Starfruit juice can be used to remove stains from linen.