The star fruit, or carambola, is a member of the oxalis family. Only one other species is commonly cultivated for its fruit, the less esteemed, more frost sensitive Averrhoa bilimbi. The star fruit is native to Sri Lanka and popular throughout southeast Asia, India, southern China, New Zealand, and Australia. The tree is well adapted to a variety of tropical and subtropical climates and is now cultivated in Mexico, the Caribbean, Central and South America and to a lesser extent, Israel. The main restricting factor in California is frost, the tree being about as cold hardy as limes. The golden-yellow fruit is 3 to 8 inches long and 2 to 4 inches in width with an star shaped cross section. The thin skin, appears waxy but is edible. The sweet to acid flesh is juicy, crisp and mild flavored and usually eaten fresh. It makes an extremely attractive fresh or cooked garnish when cut to reveal its 5 or 6 pointed star cross section. It is also made into relishes and chutneys. The sourer varieties contain more oxalic acid and can be used for polishing brass and removing rust stains. The wood is used for furniture and construction. The fruit is used medicinally as a folk remedy for a large number of maladies including hangover.
Botany
The oxalis family, Oxalidaceae, includes about 1,000 species which are mainly subtropical and tropical and usually herbs and shrubs. The carambola is a slow growing tree eventually reaches 20 to 30 feet under ideal conditions. The tree is deciduous. The 6 to 10 inch leaves are spirally arranged and divided into 5 to 11 opposite 1 to 3.5 inch leaflets. The lilac pink .3 inch flowers are borne from the axils of the leaves. Cross pollination sometimes improves fruit set. The longitudinally ribbed, ellipsoidal fruits are borne 6 to 9 months after pollination, generally in late fall and winter. Fruits very greatly in acidity, from very acid to sweet. The cross section of the fruit ranges from a distinct star to a more roundish shape. The fruit can be seedless or contain up to a dozen .25 to .5 inch, flat, brown seeds.
Climatic requirements
Young trees can be killed if temperatures go below 32oF. Mature trees can withstand short periods of 27oF.
Soil Requirements
The carambola prefers well draining slightly acid soils. Salty soil conditions should be avoided.
Cultural Requirements
Spacing and training
Trees should be planted on 15 to 20 foot centers in a sunny location. No special training is recommended.
Watering
The star fruit needs moist conditions and must receive regular watering in the summer and even during dry periods in the winter.
Fertilization
Regular applications of nitrogen rich fertilizer should be applied every 60 to 90 days. Deficiencies of minors can be a problem and can be treated as with citrus.
Pruning
The main purpose of pruning is to control size and secondarily shape.
Pests and Diseases
In California the tree is generally pest free.
Propagation
Seedlings are considered too variable to be reliable producers of large, sweet fruit. Clonal reproduction is normally done by cleft or veneer grating. Grafted trees bear in 2 to 4 years. Air layering is difficult because of poor root formation.
Harvesting and Storage
The handling characteristics of the fruit are not well established. Bruising could be be a problem; however, fruits are shipped successfully from the United States to Europe. The fruits are sensitive to storage temperatures, 50o F being the optimal.
Fiscal Orchard costs should be approximately the same as oranges.
Market
A market for star fruits is well established in the Asian community. Elsewhere, star fruits are not well known and are only available in stores with gourmet produce counters. Competition from importation into California would appear to be limited due to the severe fruit fly problems associated with star fruit and the inability to retain the desired crispness characteristic of star fruit through sterilization processing. Development of more frost resistant varieties would decrease the financial risks associated with this crop.
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