It is a little surprising to me, but one of the more frequent questions I get asked about is air layering – the process of re-growing roots on a tree, either to create a bonsai from a landscape tree, reposition existing roots and planting angle, or making more a more attractive radial root base. I have successfully performed the procedure on many trees including Japanese Maples and Crabapples. Sometimes I keep both the top and bottom after separation, but my primary goal is always the top, it seems.
All of the trees I have air-layered have been existing bonsai, and were always in a pot already. I haven’t actually performed the procedure in a landscape tree. These are essential tips to make you successful. For a more complete explanation of the process, consult another resource such as the excellent magazine articles and many bonsai books in the library, or even online.
The first thing we have to understand is what is going on inside the tree itself. Water is transported up to the leaves through the center core of the tree – the xylem, and nutrients flow down to the roots via a thin layer on the outside of the tree just under the bark - the phloem. With air layering, we are interrupting the flow of nutrients from the leaves down to the roots. With no place for the nutrients to go and with the optimal environment provided, the tree begins to send out new roots to feed itself. Since we leave the xylem intact, the tree has plenty of moisture to maintain itself.
Taking this information into account, the best time to begin an air layer is when the leaves have hardened off and are pumping nutrients downward. This tends to be about May. With many species, we would then be able to separate the tree later in the fall. Some species like conifers may take more than one season to root. Some trees will not root at all, so do your research.
Here then, are the tips. Be sure to remove a bark ring with a width at least the diameter of the trunk. Also critical is making the cut at the new potting angle you wish the tree to be at. The new roots will grow right on this upper cut line. After the cut, an option is to wrap a wire snug up against the top cut line. Wrap this wire around twice and tighten well so that you can completely cut off the sap flow. I like aluminum because it’s thicker. I had a beech jump the cut and wire both, and then re-fuse together. So make sure you get it cleaned out adequately.
I use rooting hormone in a powder form. But it doesn’t stay so well. The solution is to make a paste by adding a little water and then spread it on the wound. It only needs to go on the upper cut line.
The next hint is a biggie. I slice a plastic pot down one side and then cut a hole the diameter of the trunk in the bottom so that I can fit this new pot around the air layer area. I make plans to either wire the pot back together once on the tree, or duct tape it together. I recommend wire all the way around the pot as the easiest method with little risk of failure.
Now that my pot is ready, I apply the hormone to the wound. Then I wrap a layer of sphagnum moss around the wound and tie it on tightly with raffia. This keeps constant moisture on the cut site and the extra pressure seems to help. Once this is done, I mount my pot in the proper position, usually with wire hanging from branches, and sometimes propped up with chopsticks. It must be immobile. The remainder of the pot is then filled with regular potting soil. This setup means that I can separate the tree at the appropriate time and not repot it for another year, allowing the new roots to grow undisturbed. This contraption may look really odd for a year, but it works great.
When I go to repot in the future, I use a piece of plywood and screw it into the base of the new air layer. This stabilizes the tree and gives me something to wire the tree into the new pot, and is invaluable in protecting the fragile new roots. If you don’t have a quite enough roots filling the pot at separation time, you might even do this outside, under the plastic pot. And then, when the roots have grown two seasons, I can safely pick out the sphagnum moss and introduce regular bonsai soil.
And now, here are just a few quick tips on repotting in general. There are several posts from previous year’s Branch Tips that you can review on our website that have more detailed information. There is also a summarized step-by-step procedure under Documents and Articles in the Member Services section.
The most important thing that I wanted to mention is all of the rain that we have received. Once we take the tree out and prune off the small feeder roots, they have a diminished capacity to uptake water. That means that the tree can literally drown in all this nasty Portland wetness. We want to provide an evenly moist environment for the tree to recover and begin growing new roots. To do this, I try and keep my trees under cover for several weeks after repotting and water them only when dry – which right now with foliage barely emerging means only once a week or so.
If you are new to bonsai, beg, grovel, or become an indentured servant and repot with a more experienced member. There is no way to adequately explain what roots to cut, or how much on this tree and how much on that tree. Or, how do I fasten the tree into the pot? Repotting must be learned by doing it over and over. So volunteer your set of hands for a Saturday morning and see how fast you learn.