The commercial longan is a subtropical tree that grows well in the tropics but requires a prominent change of seasons for satisfactory flowering. A short (2-3 months) but cool (mean temperature 15 - 22°C) winter season brings out a prolific bloom; in this respect longan is less demanding and more predictable than lychee. However, longan is sensitive to frost and is likely to be killed or badly injured by prolonged temperatures below freezing. From fruit set onwards high night temperature beyond 25°C is detrimental for fruit development and temperature above 40°C causes fruit damage and fruit drop. In Thailand the best temperatures for flowering and fruit set are 20 - 25°C.
Ample soil moisture is needed from fruit set until maturity. Suitable annual precipitation is about 1,500 mm. Drought during the flowering and fruit set period can reduce fruit productivity. However, excessive rainfall during flowering can result in poor pollination and increase in flower drop, while overcast weather prior to harvest leads to fruit shedding, possibly due to poor production of photosynthate.
Longan is sensitive to wind damage and strong cyclonic winds can cause branch splitting and fruit shedding. Trees in China and Thailand are sometimes mounded after planting and/or branches supported by posts and bamboo pole fences to reduce wind damage, especially when the tree is carrying a heavy crop. The mounds are built up gradually over time around the trunk of the tree to about 1 m high (Menzel et al., 1990).
Longan thrives on rich sandy loams, it does well on oolitic limestone; moderately acid sandy soils are more marginal and on organic muck soils flowering is deficient, probably because shoot growth continues for too long. In Thailand, the soils yielding high fruit production are the heavy alluvial soils with access to water table. The roots grow down 2-4 m to the water table. In eastern Australia, the preferred soils for longan growing are heavy, fine textured soils and red loams with high fertility and good water holding capacity. In general wet lowlands or heavy clay soils are best avoided.
Figure 3. Longan tree with terminal inflorescences.
Figure 4. Terminal, greatly branched and leafless inflorescence of longan.