Air layering is a propagation method dating back over 2,000 years! Layering is often done when a portion of a branch or a trunk is desirable for bonsai but the rest of the plant may not be.
Layering shares some things in common with rooting cuttings but had a more complicated process. One advantage it has over a cutting, though, is that it continues to be supported by the parent plant while new roots are being created. This not only allows for a larger stem to be used but also gives the opportunity to “try again” if the first attempt doesn’t work (unless the layer is below the first branch).
There are primarily two types of layering: simple layering (also known as ground layering) and air layering.
With simple layering, a stem or branch is bent down and staked or partially buried. It can naturally occur when a branch touches the ground for a long period of time and can also be the result of excessive mulching. Once sufficient roots have formed, the stem or branch is separated from the parent plant.
For air layering, the stem (trunk) or branch is wounded and then protected from dying out so that new roots can form. Like a simple layer, once there are sufficient roots to support the new plant then it is separated. Layers generally take around 90 days but can take several years with some slow-to-root species.
One method of performing an air layer is known as the slit method.A long slit is made in the branch or stem and wedged open. It is then usually wrapped in sphagnum moss and plastic to keep it moist so new roots can grow. Without this moisture, as the plant sends out new roots they would quickly desiccate and die. This method is not generally used for bonsai because it tends to result in a one-sided root system that requires additional work to create pleasing nebari (above ground roots).
Another method is the tourniquet method where a wire is twisted into the branch or stem until it bites deeply.
The third method of air layering is the most commonly used and is known as the girdle or ring method.
With this method a ring of bank is removed from completely around the stem or branch. This is usually done by using a sharp knife to make a circular cut all the way around the branch. A second circular cut is made approximately 1″ below the first one. These two cuts are then joined by a vertical slit and then the bark is peeled away.
Sometimes a wire is added to the top of the wound in order to help prevent “bridging”. Some people will also cut upwards under the remaining bark at the top to flare it out a bit with the thought that it might help improve the taper.
To understand how this all works it helps to review a little bit of plant biology.
All woody plants have stems and branches composed of various type of cells arranged in layers. The outermost part is the bark that everyone is familiar with. The bark helps protect the tree from insects, disease, and desiccation.
Under the bark is the phloem, which carries the sugars and starches the foliage has produced via photosynthesis down the branch to the stem to the roots which feeds the plant.
The next layer below the phloem is the vascular cambium. This layer produces the phloem and the xylem cells.
The innermost portion of the tree is the xylem which is separated into the sapwood and the heartwood. The sapwood is the portion of the xylem that transports the sap containing water and nutrients up from the roots to feed and hydrate the plant. The inner layers of xylem are dead cells that support the tree like bones.
It is this upward and downward flow that we are most concerned with when it comes to air layering. We want to stop the downward flow but not the upward flow. This keeps the section above the air layer alive while it forms new roots.
So, we want to sever these layers as shown in the cross-section on the right. The tourniquet method crushes and cuts into the bark, phloem, and cambium to sever this downward path. However, the cambium is very good at repairing damage and can often heal around the wire without sending out roots. With the ring method, all three layers (bark, phloem, and cambium) are removed. Without cambium the other layers cannot regenerate. However, it can be difficult to remove all the cambium which, again, can cause enough healing to bridge the gap. Because of this it is advisable to really scrape the cut and be sure all of the cambium is removed. Some people advocate using rubbing alcohol to kill all the cambium, others will leave the would open for a couple of days to be sure all the cambium cells are dead before covering the wound.
It can be tricky to tell exactly where the cambium stops so you know where to stop cutting but in general it is difficult to cut too deeply (provided a reasonably large branch or stem is used). As you go deeper into the tree the layers will change color until becoming a very light brown that is almost white. The cambium is usually a greenish color (and is what you see when you do a “scratch test” to see if a tree or branch is still alive).
Air layers are best started in the spring or early summer and since plenty of sap is flowing the bark tends to be pretty easy to remove. Often you’ll be able to just peel it off and that makes it fairly easy to see where to stop. Later in the season it can require more work to get the bark removed but again, as long as the branch or trunk isn’t very small (usually 1/4″ is the smallest diameter you’d want to attempt an air layer but 1/2″ is more reasonable as a minimum) it is hard to mistakenly remove too much xylem (wood) to kill the tree. Beginners usually remove too little rather than too much!
The way all of this works is that once you sever the downward flow of nutrients those will start to build up at the top of the cut site. Auxins, a plant hormone responsible for growth, will also begin to accumulate in higher concentrations and trigger growth.
The tree will first create a callus (the bumpy growth you see in the picture on the right) and then roots will emerge from or above the callus.
Since the upward flow of water and nutrients is still intact the upper portion of the tree is usually not harmed at all. This is why some layers can survive for years while they slowly form new roots. An exception to this, though, is if you do the layer below the first branch (often incorrectly referred to as a ground layer when that term is more correctly applied to a simple layer).
If your layer is below the first branch then there is no foliage to feed the roots. It is now a race against time to create the new roots before the old roots die. If the old roots die before there are enough new roots to sustain the tree then the entire tree will die. Such drastic layering should only be attempted with deciduous as evergreen can rarely win this race.
As mentioned earlier, the new roots must be kept moist to keep them from drying out and dying.
This is usually done by wrapping the wound in sphagnum moss (not peat moss) and wrapping that with plastic. Some people prefer to wrap the plastic with aluminum foil to keep it dark and simulate below-ground conditions. Foil is used as opposed to dark plastic because dark plastic risks absorbing too much heat and cooking the new tender roots. Care must also be taken that the bottom is not wrapped too tightly as this would not drain properly and any new roots would drown. The plastic is left on until sufficient roots can be seen.
Another method sometimes used is the “pot method” where a container is fit around the branch or trunk and filled with bonsai soil. It is thought that this results in stronger roots that are less tangled than the “bag method” described above. This method doesn’t work quite as well on branches that are more horizontal but there are special containers specifically made for air layering that do not have an open top so can secure soil when horizontal.
Regardless of which method is used you will need to check on the progress of the layer periodically. Some plants can have sufficient roots in as little as two weeks but most take about a month an a half. You should plan on having the layer complete at least 8 weeks prior to the first frost so the roots have time to get established before winter. Interestingly, layered roots that are still attached to the mother plant tend to be more winter hardy while attached so most layers can easily survive harsh winters. So, if you don’t have sufficient roots by the 8 week mark it is best to leave the layer attached until spring.
With all layers the final step is separation! After verifying you have sufficient roots you’ll want to separate the layer as close to the new roots as possible. If grown in sphagnum moss some people like to plant the tree into pure sphagnum to give the new roots a chance to toughen up before being planted into a training pot or bonsai pot. Whichever you plant into you should be cautious handling the roots as they may be a little fragile until they mature.
You can now enjoy your new tree!