Star fruit or Carambola is a cultivated fruit tree originating from Southeast Asia, in the humid tropical forests of Malaysia and Indonesia.
It is a small tree up to 10 m (33 ft) tall under ideal conditions, though is more typically 3 to 5 m (9 to 16 ft) tall with a slender trunk and low-branching habit, forming a dense, rounded or dome-shaped crown. The bark varies from grey to chocolate brown and from smooth to slightly cracked.
The leaves are large and compound, with up to thirteen glossy green, oval leaflets arranged in pairs along the length and with an extra leaflet at the tip.
The flowers are small with pink to lavender petals, borne in clusters arising on the branches and trunk, where they bloom steadily throughout the year.
The fertilised flowers are followed by curious oval fruit with five prominent wings that when cut cross-wise reveal a star-shape. Green-skinned when young, they become yellow-green to yellow-orange when ripe, depending on the variety and are typically 7 to 12 cm (2.7 to 4.7 in) long. Some have flat brown seeds at the centre and some not. The yellow-orange varieties tend to be larger and sweeter than those that are yellow-green.
Star fruit cut in half
The fruit are mostly eaten fresh out of hand or are cut into star-shaped slices for use as a decorative garnish. The skin is thin, glossy and is usually left on to be eaten with the pulp, which is firm, juicy, refreshing and varies in taste from sour to mildly sweet or bland, depending on the variety. They are also suitable for processing into dried fruit and are commonly stewed with sugar and cloves in its native range or are juiced to make a refreshing beverage.
The acidic juice has tarnish and stain removal properties and is used for cleaning metal surfaces as well as removing stains from white cloth.
The fruit pulp and juice contains good amounts of Vitamins A, B1 (Thiamine), B2 (Riboflavin), B3 (Niacin) and C (Ascorbic acid).
Grows naturally in humid subtropical and tropical climates, generally in frost-free areas with annual lows of 14 to 25 °C, annual highs of 23 to 35 °C, annual rainfall of 1200 to 4500 mm and a dry season of 5 months or less, extending to 12 months with irrigation or groundwater. However, the most flavoursome fruit tends to come from warm tropical lowland climates.
New plants are usually created by grafting selected cultivars onto seedling rootstock, this is the preferred method because seed raised plants do not always come true to type, often resulting in trees with inferior fruit.
Performs best on free-draining clay and loam soils of a moderately acid to neutral nature, generally with a pH of 5.0 to 7.5 and on sites with partial sun or light shade exposure, especially in warm, low humidity environments. It has good tolerance to seasonal flooding.
The fruit of old cultivars or types are small and sour and have been replaced with new cultivars which are larger and more flavoursome. It is a heavy bearer, with reports of some exceptional trees bearing up to 440 pounds (200 kg) of fruit per year.
It is recorded as having escaped cultivation somewhere in the world, but there does not appear to be any record of it anywhere as a serious weed. It is assessed as a low weed risk species for Hawaii and Florida, respectively by the Hawaii Pacific Weed Risk Assessment project (HPWRA) and the IFAS Assessment of Non-Native Plants in Florida's Natural Areas.
This fruit have a high oxalic acid content which may adversely affect kidney function in some.
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