The carambola is a medium height (7 10 m), spreading (6-7.6 m wide) single or multi-trunked tree that flowers and fruits all year round in the tropics. Carambola trees do not need to be shaded but is tolerant of shade up to 40-50%. The carambola has small leaves that are arranged opposite each other along a centre spine. The leaves are soft, light green and a slightly hairy and whitish underside. The fragrant pink or lavender flowers are about 1 cm in diameter and grow in clusters at the base of the leaves on both young and older branches (in fact fruits born on very young twigs are of poorer quality). They bloom profusely several times a year.
Grafted starfruit plants begin to flower at 9 months while those grown from seeds may not flower until they are 4-6 years old.
The carambola is a fleshy fruit with a waxy surface. It is usually yellow. When cut in a cross-section, it has a star shape, hence the common name, starfruit. Fruits are 5-15 cm long, usually with 5 prongs. The flesh is yellow, crispy and very juicy. They may range in taste from sour to sweet. There are usually only about 10-12 seeds per fruit though there are seedless cultivars as well. The seeds are brown, thin and usually have a gelatinous membrane.
Growth Habit: The starfruit tree is small to medium in height (7 to 10 m), spreading (6 to 7.6 m), evergreen, and single or multi-trunked. Trees grow rapidly in locations protected from strong winds. The mid-canopy area (3 to 7 ft height; 0.9 to 2.1 m) is the major fruit-producing area of mature trees. The young plant has a shape of a pyramid whereas the older plant has a round shape.
Foliage: The spirally arranged, alternate leaves are 15 to 30 cm long, with 5 - 11 nearly opposite, ovate-oblong leaflets that are 1.5 to 9 cm long. They are soft, medium-green, and smooth on the upper surface, faintly hairy and whitish on the underside. The leaflets are sensitive to light and more or less inclined to fold together at night or when the tree is shaken or abruptly shocked.
Flowers: The fragrant, pink to lavender flowers are 1 cm in diameter, perfect, and borne in clusters in axils of leaves on young branches, or on older branches without leaves. There are several flushes of bloom throughout the year. The flowers are heterodistylous, i.e. the flowers have styles of two different lengths. The flowers have five stamens which are partially joined at the base and five pistils. There are two types of pistils, i.e those with short styles and long styles. Hence there are two floral types: one with stamens above the pistil and the other below the pistil. Clones may have one or the other floral type. Pollination is usually conducted by insect such as bees. Grafted plants produce flowers within 9 months, while seedlings may not flower until 4-6 years old. Flowering may be induced by pruning followed by the application of high potassium fertilisers with a short period of water stress. Continuous flowering may occur depending on the fruit load.
Fruit: The fruit is a fleshy 4- to 5-celled berry with a waxy surface. Fruits are 5-15 cm in length, with 5 (rarely 4-8) prominent longitudinal ribs; star-shaped in cross section. The fruit skin is thin, light to dark yellow, and smooth, with a waxy cuticle. The flesh is light to dark yellow in colour, translucent, crisp, very juicy, and without fibre. Good cultivars have an agreeable, sub-acid to sweet flavour. Fruit are sweetest when allowed to ripen on the tree. Green fruit will slowly turn yellow if picked before fully ripe. It takes about 60 to 75 days from fruit set to maturity depending upon cultivar, production practices, and weather. Green and ripe fruits are easily damaged and must be handled with great care. Starfruit fruit is low in calories (36-57 cal/100 grams), a good source of potassium, and a moderate source of vitamin C.
Seeds: The fruit may contain between 1 23 seeds but usually has between 4 5 seeds. The seeds are 0.6-1.3 cm long, thin, light brown, and enclosed by a gelatinous aril. The seeds maintain viability for 1 2 months if properly air-dried and stored in a cool dry place.
Clonal trees bear in 2-3 years, seedlings after 5-6 years. The trees grow, flower and fruit continuously, but usually there are three to four pronounced harvest seasons, each lasting about 3 months from pruning to harvest. Trees flower abundantly, and overbearing is common (for a good crop only about 0.5% of the flowers need to be pollinated). Flowers open from 08:00-10:00 h and reach peak anthesis at around 09:30 h. The pollen is shed at around 08:30 10:00.
Fruit set is usually good but later fruit fall tends to limit the crop. Starfruit flowers with long styles are self-fertile, but trees with short-style d flowers as a rule require pollen from long- styled flowers. Depending upon variety fruit ripens 80-110 days after anthesis but harvesting may begin about 65 days after anthesis .
Clonal Compatability: The fruitset of starfruit is rather low (2.8 3.0%) and planting 2 clones of different style length will improve pollination and fruit set. Cross pollination between clones with short styles and long styles are required to achieve good fruit set. Insect pollinators such as bees also help to improve fruit set and ensure fruits have a good shape. Fruit set is usually good and fruit thinning is done to ensure that only good, well-fertilised and well-formed fruits are kept while curved and not fully fertilised fruits are thinned out during bagging. This will also ensure fruits achieve the right size and grade. The presence of curved fruit is caused by incomplete pollination or temperature effects on pollen tube development. It may also be the result of incompatible clones and in turn, cause low yield. Hence, polliniser clones need to be planted to ensure high yields and the production of good quality fruit. Some prefer to grow at least one polliniser tree around 6 8 commercial clones.
The Starfruit plant is well-suited to the tropical climate from sea level to 2000 ft. It needs plenty of rainfall, between 1500 and 3000 mm annually. A dry season of more than two months will have a negative effect on plant growth and production. Starfruit will grow very well in friable and well-drained soil, pH 5.5-6.5. Starfruit can live well in other various types of soil right from sandy soil to clayey soil but it needs a good soil improvement and management especially irrigation system and application of fertiliser. Drought, flooding and salinity are not tolerated.
Windbreaks are recommended on exposed sites. Starfruit cultivars vary in their susceptibility to wind damage. Symptoms of wind damage include defoliation, desiccation, twig dieback, stunted growth, and fruit damage (wind scar). Heavy rainfall during flowering may inhibit pollination and fruit production. Starfruits are susceptible to root rot under wet conditions, but they generally do well with moderate, year-round rainfall. Trees planted in humid environments are susceptible to algae and the fruits are prone to anthracnose fruit spot.