Longan can be propagated from seed, air-layering, budding, grafting, cutting and inarching. Propagation by seed is not advisable since the seedling takes a long time to bear fruit (7-8 years) and the planting material is not true to type. Vegetative propagations are, therefore, recommended means of propagating the planting material. Among the vegetative propagations, air layering (marcottage) is the most popular method and has been widely used for a long time in China, Taiwan Province of China and Thailand. This method yields a high percentage of successful planting material as the marcotted branches produce roots readily. Claims of success rates of 80 to 90 percent with air-layering are very common.
Marcotting is usually carried out during the wet season. A strong healthy branch with matured leaves is chosen. The branch is first girdled or a strip of bark (2 to 2.5 cm wide) is completely removed to the cambium layer. The exposed surface is then scraped to remove the phloem, and cambial tissue to prevent premature healing. IBA or any other rooting hormone is usually applied onto the cut surfaces of the cincture (towards the tip of the branch) to encourage rooting. A rootball made up of a suitable medium which holds moisture and is well aerated (e.g. wet peat, sphagnum moss or suitable soil mixture) is then wrapped around the cincture. This is followed by wrapping a polythene film, 20 to 25 cm square, around the rootball making sure that it is completely covered and the two ends of the wrapping are tied securely to prevent water from seeping inside. The marcot is removed after two to four months (Figure 5), when the roots have turned from white to creamy brown. In Thailand, the marcotted branches produce roots in about a month when propagation is carried out during the wet season. Pruning of the rooted marcot to reduce the top in proportion to the roots is usually necessary. The rooted marcot is potted in a suitable container and placed under warm, humid and partially shaded conditions in the nursery to allow the plant to acclimatize prior to planting out in the field. Under normal conditions, the marcotted tree is planted out in the field after 6 to 12 months in the nursery.
Trees obtained by marcottage are more susceptible to wind damage when compared to grafted trees. This is because tap roots are absent in the marcotted trees. To prevent wind damage in the field, the marcotted trees are either supported by permanent bamboo props, by soil mounded around the trunk, or by rooted seedlings planted close by for later inarching.
Beside marcotting, longan can also be propagated by grafting. In China, whip-and-tongue graft has been practiced as far back as the late 1970s. Approach grafting is also carried out using seedlings of the same cultivar as the rootstock. Eight to twelve months old seedlings (commonly cultivar 'Wuyuan') are used as rootstocks. They are approach-grafted to a similar sized branch of the scion cultivar. After a union has occurred within 40 - 60 days, the top of the rootstock plant is removed above the graft and the base of the scion plant is removed below the graft. The grafted plant is nursed in a pot or polythene bag for 2-4 weeks before planting out in the field. Sometimes, double or triple graftings (successive grafting operations on top of the previous graft) are carried out by the Chinese growers as they believe that such graftings can give added strength to the tree and produce larger and sweeter fruit. Currently, propagation by budding (patch budding with a single bud) has become popular and the success rate has been reported to be over 85 percent.
In Thailand, grafting is commonly done in winter when vegetative growth has ceased. One year old longan seedlings which show signs of good growth are chosen as rootstocks. In this respect fast growing longan such as the native longan (or 'Kradook') can be used as rootstock. After the grafting operation, the grafted plants are covered by plastic bags to provide a high humidity condition around the graft union. The plants are well watered for about one month before the plastic covers are removed. The newly grafted plants are usually nursed in the nursery for about 2 months before planting out in the field.
Forket budding and cuttings have been found to be successful with longan cultivars introduced into Australia (Menzel et al., 1990).
Work from Thailand and Queensland suggest that some scions/rootstocks are incompatible. In China, graft incompatibility has recently been reported to occur in Guangdong. The rate of incompatibility for cultivar 'Chuliang' has attained 36 percent (Liu and Ma, 2000). Researchers recommend grafting a cultivar onto seedlings of the same cultivar. In Australia, grafting is preferred onto seedlings of the same cultivar or at least not Thai cultivars on Chinese rootstocks or vice versa (Menzel et al., 1990).