Hennie Nel, a bonsai friend of mine, and myself recently had lunch together and decided that we should make full use of the time together and document an air layering he wanted to do on a Chinese elm bonsai which he has had for many years.
The tree is nice as is, and many people would perhaps stop there and be happy with what they had achieved. However Hennie and I agreed that the tree could be improved by shortening it quite drastically and using the current apex as the future tree. Another problem which Hennie told me about was that he had neglected to remove moss which had grown up the trunk quickly enough. As this is a cork bark elm the moss resulted in the bark rotting. Although the cork bark will develop again in time, it will probably take longer than it will to simply re-create the tree.
Removing the bark
Once Hennie had identified the level and angle at which he wanted future roots to emerge he carefully and deliberately made a clean cut into and all around the trunk. This was repeated a little further down. You can use any very sharp implement for this step but my recommendation would be a grafting knife and not a blade. The force that is required to make the cut through the bark will bend a blade and you might get hurt if it slips or breaks. A grafting knife is sharp yet made of much thicker steel and so it will not give under the pressure.
After making both the top and bottom cuts, Hennie removed the ring of bark from the tree. The next step is to remove absolutely every last trace of live, green cambium to reveal the wood beneath.
Failure to remove the cambium entirely may result in your layering not producing any roots as the tree will instead of issuing roots, simply heal over.
It is important that these cuts be clean and crisp as a blunt knife will crush the living cells and reduce the probability of rooting.
Its highly recommended to paint the top cut with a rooting hormone for hardwood cuttings, which is the strongest form of the hormones found at your local garden center. We used Dynaroot 3 which I have had a lot of success with.
Rooting medium and holder
Hennie has a pretty clever idea for a growing container for the future roots. He uses a clear plastic tub which he purchases with a lid on. He makes a cut down the side and also a round hole in the bottom, sufficient for the trunk to fit into. He then inserts the trunk through the side, through the cut, and it locates into place in the bottom hole. He then wraps the container with clingfilm. Whats nice about it being clear is that you can see when it has filled with roots (Black plastic will be wrapped around the whole container and it is only when you want to check on it that you will reveal the clear container below).
As a rooting medium you can use various components but Hennie has used vermiculitefor this layering. The main criteria for what you use is that it must retain as much water as possible but be as open structured as possible to allow any water you give the layering to drain away quickly and for there to be very good oxygen penetration as it is this combination of moisture and oxygen which will result in root formation. After filling the container with rooting medium Hennie put the lid on which will help to reduce moisture loss.
After a thorough watering the tree was moved to a shady spot. Hennie will now need to make sure that once new roots do start to develop that the rooting medium is never allowed to dry out as this will kill the new roots. I have also found it to be beneficial to rotate the tree every few weeks because I believe this will give you more even distribution of roots, especially if it is only one side of the tree which is warmed by the sun. No fertilizing is needed at this stage.
The layering should probably take a few weeks before roots will begin to form and it will most likely be around 3 months before you will see them, when you remove the outer plastic screen. Although it is possible at that stage to separate the layering from the mother plant, I usually wait till the following season. Let's wait and see what Hennie decides.