Bonsai can be grown via a number of different methods and air layering is one of the very common methods. While this method of growing bonsai has been practised for over 1500 years, it continues to be used extensively even today. It is believed that the ancient Chinese could have possibly discovered the air layering process accidentally, for instance, by observing a tree or a branch that was partially snapped and subsequently developed roots on the ground on its own. This process is very effective in the tropical regions where the climatic conditions are hot and humid. In fact, air layering is rapid and, hence, a frequently practised process. For instance, in India, people propagate various types of fruit trees, including guava and mango, by air layering. The usual process involves wrapping a lump of damp clay or manure around a branch that has been severed partially. Subsequently, the clay ball is tied at the severed end of the branch using a burlap sack and new roots will develop just within some weeks. Propagating plants by air layering has a number of advantages and the main among them is that it gives rise to a mature tree, which has the ability to bear fruits from a very early age and of course in a very short period of time. As a result, this process is perfect for growing bonsai, because air layering facilitates the tree to become mature in just a small part of the time actually needed by the tree to have a similar trunk size or thickness when grown from its seeds or wood cuttings. Moreover, air layering helps the grower to choose a branch having the most desirable and attractive character and shape. Hence, it would appear to be surprising that in spite of having several attractions, in contemporary times, air layering is usually not considered to be a method that is commercially viable for growing bonsai. One of the reasons put forth by the nurseries is that the air layering process involves a lot of manual labor. Secondly, as far as materials gathered from the wild is concerned, the supply is more or less very limited and when the existing supplies are used up, it takes a long time to avail new supplies having the same quality. However, in the case of amateur bonsai enthusiasts, air layering continues to be a perfect propagation method, because this process is simple, inexpensive as well as quick.
The air layering process basically involves purposely interrupting the sap flow to a branch. When this is achieved, the concerned branch will struggle for survival. It will either bridge the restriction or develop new roots to absorb moisture and nutrients from the environment closest to it. Practically speaking, the flow of sap to a branch can be interrupted using two basic methods. However, even these two methods have several variations. The first method involves cutting a ring of the bark encircling the trunk or branch of a tree. On the other hand, the second method involves applying a tourniquet (a device that temporarily constricts the flow of sap) encircling a tree trunk or branch. In both cases, the aim is to interrupt the sap flow via the bark. In fact, the process of breaking off the flow of sap is a traumatic one. Therefore, it is usually advised that you should allow a tiny shard of bark to remain on the branch so that it serves as a feeder or bridge, ensuring that the branch terminals continue to receive nourishments, but, of course, at a very slowly rate.
The appropriate air layering method that you need to adopt will largely depend on the tree variety, for instance, some tree varieties will respond well even when a ring of the bark is removed completely from the branch, while there are other varieties that will find the shock of cutting the ring around the bark too much to endure and will eventually die. Some of the trees that are able to endure the complete removal of the bark include Chinese elm, trident maple, Japanese maple, all types of junipers, cotoneaster, willows and zelkova. Air layering method is best suited for lofty nursery trees that are normally grown as standards. Usually, all the shoots of these trees are removed up to a height of about 1.8 meters (6 feet), just allowing a "head" of branches to grow from an elongated stem. Normally, a tree with a height of 1.8 meters (6 feet) can produce about nine to ten trees just from one stock plant when you layer it from its base to the top. If you choose the tree varieties that air layer readily and by air layering a few sections simultaneously, it is possible to develop about six to seven new plants just in one growing season of the plants.
Most of the time, deciduous trees respond most excellently to the air layering techniques that entail removing the bark completely. On the other hand, it has been found that evergreen conifers are more suited to either the wire tourniquet method or the bridge method. However, the junipers are different and can be considered to be an exemption. In fact, all varieties of junipers air layer very easily even when the complete ring bark is removed. The time taken between the beginning of the air layering process and the emergence of roots differs, subject to the tree species being used. It has been observed that junipers usually develop new roots within just two weeks from the start of air layering. Then again, pines are very slow in producing roots after air layering. In fact, white pines may take as long as one year and, in some cases, even two years to produce roots that would be able to sustain the new tree appropriately. Hence, it is probably best to apply the wire tourniquet method when you air layering pines. This is mainly because air layering by removing the complete bark ring is a very harsh method. Alternatively, one may also apply the sliver variant of ring bark technique or the multiple bridge method of air layering in case of pines.
The growing season of the plants, especially early spring, is the best time to undertake air layering. This is the time when the tree sap starts rising strongly. Starting air layering early in the season has other advantages too. When you begin early you can actually continue to air layer the branches of the same tree continuously from early spring to the beginning of fall. Some air layering experts suggest that you wrap the sphagnum moss ball on the bark ring with a clear plastic and then wrap it again using a black plastic sheet with a view to block light. Nevertheless, it seems that wrapping the ball of sphagnum moss with the black plastic is not necessary, as the sphagnum moss will itself prevent much light from the region of the branch or trunk, which is air layered. When you use a clear plastic it has its own advantages. For instance, the clear plastic will allow you to see the roots when after they develop and emerge from the sphagnum moss. Hence, you will also be able to know the appropriate time when you should cut off the branch. The success of an air layering greatly depends on the time when a branch is cut off from the parent tree. In fact, this is one of the most vital aspects of any air layering to be successful. In case, the branch is detached prematurely from the parent tree, the chances of the air layering's survival are dim. It is important to bear in mind that you should only cut off the branch that has been air layered after enough roots have filled the sphagnum moss ball. When this happens it is very visible, as the root ball is actually an accumulation of plump white roots. The chances of the air layering's survival are directly related to the number of roots in the root ball - the more the roots, the better the chances. While detaching the branch from the parent plant, you should ensure that the cut is clean. At the same time, it is important to handle the root ball as less as possible. Some experts suggest that it is best to saw half-way through the branch to allow the air layering to be removed gradually - in stages. After detaching the air layered branch, the sphagnum root ball should be placed in a strong flower pot and, subsequently, be filled with unadulterated peat. If you use moss peat in place of potting compost or sharp sand, the chances of the roots breaking will be lesser, thereby enhancing the chances of the air layerings. On the other hand, using grit, sharp sand, or a heavy compost will compress the roots owing to their weight, thereby often damaging the fragile new roots. In case the air layering with new roots has a large number of branches or leaves, it is essential to get rid of a number of them with a view to lessen the rate of transpiration or loss of water till the time when the air layering cut off from the parent plant becomes well established. In addition, it is important to place the pot containing the air layering in a shallow saucer containing water to enable the new plant to absorb sufficient moisture throughout this period. At the same time, feeding the potted air layering with vitamin B1 will help it to establish itself more quickly. However, be careful no to use fertilizers during this stage because it will harm the roots, which are still very young. If possible, it would be best to place the newly-potted air layerings in a moist, humid environment, like under mist propagation or in a cool greenhouse. When you do this, it will promote quicker root formation and also facilitate establishment of the air layering. It has been found that an air layering that has been severed from the parent tree and potted in unadulterated moss peat possesses the aptitude to pack a flower pot with new roots in a very short period - just within two to three weeks from the date of potting up. It is advisable that you never try to directly plant a freshly rooted bonsai from air layering into a bonsai pot. On the contrary, you should allow the plant developed by air layering to grow in a normal flower pot for no less than a year. Subsequently, you need to grow the air layering in a big seed tray for another year. Alternately, you may also grow the air layering outdoors in the ground with a view to help the plant to make its roots firmer, hereby allowing you to manoeuvre as well as plant it later in a suitable bonsai pot. When the air layering has established itself, you can start changing it into an attractive bonsai from the second year of its existence onwards. After you have planted the tree in a pot, you will be able to train as well as refine its branches and the general structure of the bonsai. After some time, it will be difficult to distinguish whether the bonsai has been propagated by means of air layering or any other more conventional methods like propagation from seed, grafting or cuttings.