Technically, the so-called mangosteen "seeds" are not true seeds. They are adventitious embryos, or hypocotyl tubercles, in as much as there has been no sexual fertilization. When growth begins, a mangosteen shoot emerges from one end of the mangosteen seed and a root from the other end. This original mangosteen root is short-lived and is replaced by roots that develop at the base of the shoot. The process of reproduction being vegetative, there is naturally little variation in the resulting trees and their mangosteen fruits. Some of the mangosteen seeds are polyembryonic, producing more than one shoot. The individual nucellar embryos can be separated, when desired, before planting.
In as much as the percentage of germination is directly related to the weight of the mangosteen seed, only plump, fully developed mangosteen seeds should be chosen for planting. Even these will lose viability in 5 days after removal from the mangosteen fruit, though they are viable for 3 to 5 weeks in the mangosteen fruit. Seeds packed in lightly dampened peat moss, sphagnum moss or coconut fiber, in airtight containers, have remained viable for 3 months. Only 22% germination has been realized in seeds packed in ground charcoal (for 15 days). Soaking in water for 24 hours expedites and enhances the rate of germination. Generally, sprouting occurs in 20 to 22 days and is complete in 43 days.
Because of the long, delicate taproot and poor lateral root development, transplanting is notoriously difficult. It must not be attempted after the mangosteen plants reach 2 ft (60 cm). At that time, the depth of the taproot may exceed the height of the mangosteen tree. There is greater seedling survival when seeds are planted directly in the nursery row than when they're grown in containers first and then transplanted to the nursery. The nursery soil should be at least 3 ft (1 m) deep. The young mangosteen plants take 2 years or more to reach a height of 12 in (30 cm) which is when they can be taken up with a deep ball of earth and set out. Fruiting may take place in 7 to 9 years from planting, but usually not for 10 or even 20 years.
Conventional vegetative mangosteen propagation is difficult. Various methods of grafting have failed. Cuttings and air-layers, with or without growth-promoting chemicals, usually fail to root or result in deformed, short-lived mangosteen plants. Inarching on different rootstocks appeared promising at first, but incompatibility has become evident with all except G. xanthochymus Hook. f. (G tinctoria Dunn.) or G. lateriflora Bl., now commonly employed in the Philippines.
In Florida, approach-grafting has succeeded only by planting a seed of G. xanthochymus about 1 1/4 in (3 cm) from the base of a mangosteen seedling in a container and, when the stem of the G. xanthochymus seedling has become 1/8 in (3 mm) thick, joining it onto the 3/16 to 1/4 in (5-6 mm) thick stem of the mangosteen at a point about 4 in (10 cm) above the soil. When the graft has healed, the G. xanthochymus seedling is beheaded. The mangosteen will make good progress having both root systems to grow on, while the G. xanthochymus rootstock will develop very little.