Rambutan

Rambutan (Nephelium lappaceum var. lappaceum L.), a member of the soapberry[?] family (Sapindaceae), is believed to be native to the Malay archipelago[?] although its precise center of origin is unknown. It is closely related to other edible tropical fruits such as litchi, longan, and mamoncillo[?]. It is a popular back yard fruit tree and propagated commercially in small orchards. Rambutan, one of the best known fruits of Southeast Asia, is widely cultivated throughout the tropics including Africa, the Caribbean islands, Central America, India, Indonesia, Malaysia, the Philippines, and Sri Lanka. Thailand is the largest producer. Rambutan production is increasing in Australia and, in 1997, was one of the top three tropical fruits produced in Hawaii. The red, pink, or yellow fruit, about the size of a small egg, consists of a single seed covered by a translucent, juicy but firm, sweet aril or pulp. The fruit are usually sold fresh, used in making jams and jellies, or canned. Evergreen Rambutan trees with their abundant colored fruit make beautiful landscape specimens.

Rambutan, grown commercially within 15° of the equator, is adapted to warm tropical climates and sensitive to temperatures below 10C (50°F). The medium-sized trees have an erect, dense habit with a straight trunk and grow to a height of eight to ten meters (25 to 33 feet). Grafted cultivars are usually more compact reaching a height of only three to five meters (10 to 16 feet). The trees do best on deep soils that are high in organic matter and thrive on hilly terrain as they require good drainage. Rambutan is propagated by grafting, air-layering, and budding -- the latter is most common as trees grown from seed often produce sour fruit. Budded trees may fruit after 2-3 years with optimum production occurring after 8-10 years. Trees grown from seed bear after 5-6 years. There are well over 200 cultivars developed from selected clones available throughout tropical Asia.

The round to oval fruit, a drupe 3-8 by 2-4 centimeters (1.2-3.2 by 0.8-1.6 inches) are pendant in a loose cluster of 10-20 fruits. The leathery skin is covered with fleshy pliable spines, hence, the name Rambutan which is derived from the Malayan word rambut which means hairs. The aril is attached to the seed in some commercial varieties, but "freestone" varieties are available and in high demand. There is usually a single light brown seed which is high in certain fats and oils (primarily oleic acid[?] and eicosanoic acid[?]) valuable to industry, and used in cooking and the manufacture of soap. Rambutan roots, bark, and leaves have various uses in medicine and in the production of dyes.

Rambutan trees bear twice annually, once in late fall and early winter with a shorter season in late spring and early summer. The fragile nutritious fruit must ripen on the tree, then they are harvested over a 4-7 week period. The fresh fruit are easily bruised and have a limited shelf life. An average tree may produce 5,000 to 6,000 or more fruit (60-70 kilograms or 130-155 pounds per tree). Yields begin at 1.2 tonnes per hectare (2,360 pounds per acre) in young orchards and may reach 20 tonnes per hectare (39,360 pounds per acre) on mature trees. In Hawaii, 24 of 38 cultivated hectares (60 of 95 acres) were harvested producing 120 tonnes (264 thousand pounds) of fruit in 1997. It has been suggested that yields could be increased via improved orchard management, including pollination, and by planting high yielding compact cultivars.

The small 2.5 to 5 millimeter (0.1 to 0.2 inch), apetalous, discoidal flowers occur in erect terminal clusters (panicles) about 30 centimeters (12 inches) long. Rambutan trees are either male (producing only staminate flowers and, hence, produce no fruit), hermaphroditic (producing flowers that are only functionally female), or hermaphroditic (producing flowers that are female with a small percentage of male flowers). The latter is most commonly found in cultivar selections. Cultivars that produce only functionally female flowers require the presence of male trees. Male trees are seldom found as vegetative selection has favored hermaphroditic clones that produce a high proportion of functionally female flowers and a much lower number of flowers that produce pollen. There are over 3000 greenish-white flowers in male panicles, each with five to seven anthers and a non-functional ovary. Male flowers have yellow nectaries and 5-7 stamens. There are about 500 greenish-yellow flowers in each hermaphroditic panicle. Each flower has six anthers, usually a bi-lobed stigma, and one ovule in each of its two sections (locules). The flowers are receptive for about one day but may persist if pollinators are excluded.

In Malaysia, rambutan flowers from March to July and again between July and November, usually in response to rain following a dry period. Flowering periods differ for other localities. Most, but not all, flowers open early in the day. Up to 100 flowers in each female panicle may be open each day during peak bloom. Initial fruit set may approach 25 percent but a high level of abortion contributes to a much lower level of production at harvest (1-3%). The fruit matures 15 to 18 weeks after flowering.

Both male and female flowers are faintly sweet scented and have functional nectaries at the ovary base. Female flowers produce 2-3 times more nectar than male flowers. Nectar sugar concentration ranges between 18 and 47 percent and is similar between the flower types. Rambutan is an important nectar source for bees in Malaysia.

Cross-pollination is a necessity because pollen is absent in most functionally female flowers. Although apomixis may occur in some cultivars, research has shown that rambutan, like lychee, is dependent upon insects for pollination. In Malaysia, where only about one percent of the female flowers set fruit, research revealed that no fruit is set on bagged flowers while hand pollination resulted in 13 percent fruit set. These studies further suggest that pollinators may maintain a fidelity to either male or hermaphroditic flowers (trees), thus limiting pollination and fruit set under natural conditions where crossing between male and female flowers is required.

Aromatic rambutan flowers are highly attractive to many insects, especially bees. Those commonly found visiting rambutan flowers include bees (Apis spp. and Trigona spp.), butterflies, and flies (Eristalis sp. and Lucilia sp.). Apis cerana colonies foraging on rambutan flowers produce large quantities of honey. Bees foraging for nectar routinely contact the stigmata of female flowers and gather significant quantities of the sticky pollen from male blossoms. Little pollen has been seen on bees foraging female flowers. Although male flowers open at 6am, foraging by A. cerana is most intense between 8am and 11am, tapering off rather abruptly thereafter. In Thailand, A. cerana is the preferred species for small scale pollination of rambutan.

(a public-domain report from the Agricultural Research Service of the United States Department of Agriculture)