The resulting plant will be a cultivar, the same as the parent one, so you know exactly what characteristics you are getting — specifically what colour, texture, taste, etc, the fruit it bears will have. By Air-Layering you avoid the variation that you can get when growing from seed. It is used particularly on trees such as lychee and longan. A lychee tree “born” through air-layering will start bearing fruit immediately (from seed, you have to wait 10 to 15 years.) The fruit, though, will be small, so a few years of growth still needs to be allowed.
To do Air-Layering, the bark, and layers just below the bark (called the “cambium” and below that the “phloem “) are removed from a section of a branch to expose the inside of the branch (called the “xylem.”) The section should be about 1 inch (2 1/2 cm) wide. You make a 3 inch or so (7 1/2 cm) pad of a growing material such as moistened sphagnum or peat moss, and place the pad around the exposed area. You hold the growing medium in place by means of a piece of plastic film or tin foil (using plastic film allows you to visually monitor progress), and seal it tightly at both ends to stop insects from entering.
Carbohydrates and photosynthesized materials (called ” photosynthates”) flow from the leaves to the trunk via the “phloem”; nutrients from the soil flow from the roots to the leaves via the “xylem.” The cut allows the branch to still receive all the nutrients the rest of the plant is sending it, but disrupts the outward flow, keeping all the collected nutrients in the branch. These build up at the cut out section, and cause roots to form in the growing medium. In 8 to 12 weeks, the growing medium will be full of roots.
You then saw the branch off 1 or 2 inches (3 to 5 cm) below where the root mass has formed (below, as in the direction of where the branch meets the trunk of the tree), remove the wrapping, and soak the root mass in water for a bit. Trim off 50% of the foliage on the branch (professionals do this with an eye to shaping the tree that this will become.) Plant it either in the ground, or in the case of professionals, in a container for shipping.
The root systems of trees grown by air-layering may not have the depth or spread of those trees grown by grafting.
In China, Air-Layering has been practised for at least 800 years in Fujian Province.