The longan is an evergreen tree which can grow up to 20 m and possesses a spreading or erect habit, depending on the cultivars. The brittle trunk and branches have a corky bark which gives a split and peel appearance, unlike those of lychee which are smooth. The compound leaves are alternate and paeipinnate with 6-9 pairs of leaflets which are dark glossy green on the upper surface and paler green on the lower surface. The young leaf flushes are reddish brown in colour and with maturity turn to light green. Inflorescences are terminal (Figure 3), 8-40 cm long, densely tufted-tomentose, leafless and greatly branched (Figure 4). They are borne on new growth produced during midsummer or autumn, although sometimes they are borne on shoots produced in spring on terminals which have not set panicles. Cymules 3 - 5 flowered and normally only the central flower developed into fruit. Flowers are small and yellowish brown, calyx lobes 2 - 5 mm x 1 - 3 mm; petals 5, 1.5-6 mm x 0.6 - 2 mm, densely woolly to glabrous.
The flowers of longan within a panicle are made up of staminate (pistil nonfunctional), pistillate (stamens non-functional) and hermophrodite flowers. The staminate flower has 8 or less hairy stamens arranged in a single row on a light brown disc. The pistillate flower has anthers which are sterile and non-functional. Hermaphrodite flower contains bicarpellated and densely-hairy ovary with bilobed stigma. Normally, only one carpel develops into fruit. The stamens of the hermophrodite flower consist of 8 sessile filaments with anthers producing viable pollens.
The longan is a cross-pollinated species. In order to achieve cross-pollination it has duodichogamy, that is, the tree has three stages of flowers, which open directly after each other, with a certain degree of overlapping. Generally within a panicle the first phase in the sequence of opening is the staminate flowers, follow by the pistillate flowers and then the hermophrodite flowers and finally the staminate flowers again. Male and female phases of flowering overlap 4-6 weeks depending on cultivars. Pollination is mainly carried out by insects and is most effective between 08.00 - 14.00 hours. Fruit set per panicle improves greatly with bloom rating for the tree, leading to a sharp progression in yield per tree (and an obvious risk of biennial bearing). The period from bloom to harvest is 5 - 7 months, depending on cultivars and climate. In Thailand a panicle may carry up to 80 individual fruits which vary in weight from 5 to 20 g. The premium commercial grades of longan fruits are in the range of 14 to 18 g. The fruit rind is thin but tough and leathery and changes colour from greenish yellow to yellowish brown with advancing maturity. Tubercles are typically flattened or indistinct. However, in related species, the rind tubercles are very distinct. The aril has total soluble solid values ranging from 15 to 25 percent. It is translucent white to off-white and may constitute from 60 to 75 percent of the total fruit weight. Texture of the aril ranges from juicy to very crispy and flavour ranges from bland to sweet and aromatic, but seldom acidic. The seed is small, round to ovoid in shape and glossy reddish brown to black in colour and easily detached from the aril. Only one seed is present in each fruit and in some cultivars there are a certain percentage of small-seeded fruits.
Longan seeds are recalcitrant and, therefore, short-lived and best sown fresh. Germination takes 7-10 days. Seedling growth is slow and the juvenile phase lasts about 7 years. Longan trees grown from air layers come into bearing during the third or fourth year and yields tend to increase with tree size over a very long trajectory.