Native to south east Asia with some plants now grown in northern Australia and other tropical countries worldwide. In addition to backyard plants in 2007, Thailand had 5500Ha under cultivation and fruit production was 30kT for domestic and export markets. The Philippines have more than 2 million trees under cultivation.
It is a relatively cold- and drought-tolerant tropical plant that can survive and produce fruit up to elevations of 1200m and only 800mm annual precipitation within 8 degrees of the Equator. Light frosts will cause serious damage and may kill young plants without protection. The ideal climate is hot monsoonal with dry winters and a mean annual temperature of 22°C; in cooler conditions trees are deciduous. Mature trees can withstand 40°C.
Seedling trees can grow to 30m high in their natural environment but usually only half this in cultivation in sub-tropical areas or when asexually propagated. Santol has a compact canopy and bark is normally smooth grey-brown and exudes a milky latex when damaged. The spiral entire leaves are compound trifoliate, glabrous glossy green above and light green below with 15-20cm long elliptic-ovate leaflets. There are two varieties in the species where senescing leaves become either red or yellow before abscission in dry or cool periods. From above, leaves have a quilted appearance.
It is a member of the Meliaceae family, which includes langsat, duku and longkong. There are only 10 species in the genus.
Highly fertile soils are not necessary but there should be good drainage. A pH of 6.5 is preferred with alkaline soils causing micronutrient deficiencies, particularly for iron.
Seeds are recalcitrant and only retain limited viability if stored in a moist medium such as sphagnum moss. Seedlings grow quickly but are highly variable in tree and fruiting characteristics so should preferably be used only for grafting. Inarching, cleft and whip grafts and shield budding all give good results, producing trees with a more bushy canopy. Marcotting is slow (5-6 months). Seedling rootstocks should be 1-2 years old with stem diameter >5-6mm before grafting.
Thailand has been the leader in selection of superior cvs, with Pui Fai being the most popular. There are several others with equally foreign-sounding names, all difficult to obtain in WA. Most selections have come from plants with yellow senescent leaves as these are usually sweet, whereas the red type produces more sour fruit. One of the Thai cvs grown in the Philippines is called Bangkok (a tetraploid, with 44 chromosomes) and this variety has then been given yet another derivative name in the US - Manila. The other principal variety grown in the Philippines is Native (a diploid) but it is smaller, more sour and has a lower flesh/seed ratio than Bangkok.
Axillary inflorescences are formed on young shoots as 20 to 30cm-long drooping panicles with numerous individual perfect flowers, 10mm wide and 5mm long. These are pale green to greenish white, 1cm wide with 5 sepals, 5 reflexed petals and united stamens forming a tube with 10 anthers. The ovary is superior with a central style and 4-5 locules each with two ovules; the style becomes elongated as the flower matures. Anthesis normally occurs in the early evening but peak anther dehiscence does not overlap with stigma receptivity so self-incompatibility is common, resulting in low fruit set (1% or less) by the natural means of insect pollination. Flowering can last 2-3 months.
Adequate water is essential when plants are young and during flowering and fruit development. Nutrition needs have not been extensively studied, but a complete fertilizer with trace elements should be applied twice annually, commensurate with tree size. Mature producing plants may require 4kg pa of fertilizer, and mulching is recommended. Santol forms a mycorrhizal association which benefits the plant and enriches the surrounding soil.
Branches are easily broken so trees should either be planted in protected locations or with windbreaks.
During the first 2-3 years, plants should be pruned to form a well-spaced scaffold of 3-4 main branches with the goal of producing a low and spreading canopy. Pruning of mature trees is focussed on size containment and removal of dead, diseased or crossing branches.
The firm oblate velvety fruit is a berry with slight longitudinal indentations, 5-10cm wide and green when immature maturing to golden-yellow. Damaged skin is latiferous. The pericarp is leathery with edible sub-acid mesocarp and endocarp. The edible inner fleshy arils are soft, translucent white, juicy and often fibrous and firmly (more rarely not) adherent to the 3-5 seeds which are triangular in cross-section, 15-20mm long. The edible parts of ripe fruit have reasonable levels of iron and carbohydrate content is about 14%.
Seedlings may take 5-7 years to fruit whereas asexually propagated plants may bear in 3-5. Fruit takes 6-7 months to mature and is picked when becoming soft and the skin turns yellowish; immature fruit will not ripen properly. In the final 6 weeks of maturation, fruit weight can double, acidity decreases and sugar content can increase more than 10-fold. Retail fruit are often scuffed from the assumption that with their leathery rind they can be handled roughly with impunity. Storage life in a crisper is 2-3 weeks. Good mature trees can produce more than 200kg each.
They are usually eaten fresh by cutting equatorially and then spooning out the edible inner part of the pericarp and the arils not adhering tightly to the seeds. Alternatively, they can be processed into jam, jelly, chutney and other preserves.
Some pests and diseases reported in the tropics include gall-forming mites, Chinese wax scale, and pink and blight diseases. The thick rind helps insulate it from attack by many insects.
If you manage to source one of these it will probably be a seedling, so try to ensure it's one from the yellow senescent leaf form to increase the chances of it being sweet. Although santol prefers a hot humid climate it will grow in cooler frost-free environments. A challenge for us in south west WA is our dry summers and wet winters whereas the reverse is favoured. Reported self-incompatibility may not be an absolute stumbling block as some single trees are known to fruit satisfactorily. Size will have to be contained from the beginning or it can become a big tree. All up, a challenge but maybe doable.