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Air Layering Bonsai

March 08, 2024Care, General, Plants, Styling

An example of air layering with roots developing from the wound.

Air layering is a technique that prompts roots to grow at a chosen point on a living tree. In bonsai, it is used to correct flaws in existing trees, as well as to create new trees.

Air layering is a horticultural propagation technique often used by bonsai (and non-bonsai) nurseries and hobbyists alike. It works by encouraging a selected area of a tree to sprout new roots at the chosen location. Once the location has developed a sufficient supply of new roots, the portion of the plant above the air layer is separated from the parent and planted as a new tree.

How Air Layering Is Used in Bonsai

Air layering is an intermediate-level technique that offers bonsai hobbyists several options for acquiring new bonsai material or improving existing material. These include:

Selecting an interesting branch to make a separate tree

Sometimes a species growing in the hobbyist’s backyard – a Japanese maple, perhaps – is one that makes good bonsai subjects. Or maybe the bonsai enthusiast has spotted a specific branch on a landscape tree or an existing bonsai that would make an interesting bonsai trunk. In these cases, air layering is an excellent way to get a mature-looking cutting to put into training as a bonsai.

Reducing the length of the trunk

Sometimes a rather gangly tree would make an excellent bonsai specimen if it just weren’t so tall. Air layering the trunk allows the hobbyist to shorten the tree by developing a new root system at a strategic point. As a bonus, the hobbyist gains a second tree from the air layer to train as a new bonsai!

A too-tall Brazilian rain tree bonsai that could benefit from air layering.
Plans to reduce the height of this Brazilian rain tree through air layering will make it a better bonsai specimen.

Correcting a flaw in trunk or roots

Sometimes a tree that otherwise would qualify as a nice bonsai has anomalies in its trunk or nebari. Perhaps the trunk has a thickened or bulbous area that disrupts its lines. Perhaps it suffers from the dreaded reverse or inverse taper. Using air layering to establish the trunk base at a new point can often greatly improve the bonsai’s structure.

An example of reverse taper, where a tree trunk widens above a thinner section. Air layering bonsai with reverse taper can correct the problem.
Reverse trunk taper is a condition that can be corrected through air layering.

Maybe the nebari – the portion of a bonsai’s roots that are visible above the ground – are oddly misshapen. Air-layering the trunk at a strategic location can establish a whole new system of roots. New roots tend to grow radially around an air layer wound, offering good potential for developing into a nice set of nebari.

A bonsai with numerous radial roots. Nebari like this can be developed through air layering bonsai.
Air layering bonsai can create nice, radial nebari as seen in this tree.

The Science Behind Air Layering 

To understand the science behind how air layering works, let’s first explore the layers of a tree trunk and their functions. We’ll start from the outside and work our way in.

Tree Layers and Function

The bark, as we know, is the outer, protective covering of the tree. It regulates moisture within the tree and helps protect against weather, disease, and pests.  Some tree barks have bitter-tasting compounds that discourage snacking by wildlife. 

The tough outer bark of a tree protects from insects, disease, and weather. In air layering, the outer bark is removed.
The tough, outer bark offers a layer of protection for the tree.

A corky layer of material known as the inner bark, or phloem, lies just inside the outer bark. The phloem has the important job of transporting food – in the form of sugars produced by the tree’s leaves through photosynthesis – back down through the tree to its roots.

Just inside the phloem layer is the very thin, but very crucial, cambium layer.  Depending on the plant, the cambium layer might be only a few cells thick. Nevertheless, it functions as the growth engine of the tree. During the growing season, the cambium creates new cambium cells and much more. It also produces new phloem cells on the outside and new xylem cells on the inside of its layer. 

Illustration of a cross-section of wood showing the five layers.
The five layers of a tree trunk.

The xylem, or sapwood, layer of the tree, lies just inside the cambium. It serves the important role of funneling water and minerals absorbed by the roots up the trunk to the rest of the tree. Each growing season produces a new sapwood layer. This creates the rings that we count to determine the age of the tree.

As each layer of sapwood ages, its ability to perform its job diminishes. Eventually, the layer of sapwood stops working altogether and turns into heartwood. 

The aptly named heartwood lies at the very center of a tree. Heartwood is non-living tissue, however it adds rigidity and helps balance and stabilize the tree. In a cross-section of a tree trunk, heartwood is the darker area in the middle.

How Air Layering Works

Now that we’ve reviewed the layers of the tree, let’s delve into how air layering actually works.

As we just mentioned, the roots of the tree absorb water and transport it to the leaves through the sapwood or xylem layer. The leaves produce food for the tree through the process of photosynthesis, then send the food down through the tree via the inner bark or phloem layer. 

The all-important cambium lies between the xylem and phloem layers. As we learned earlier, it is responsible for making both xylem and phloem cells. It can also generate roots in response to hormone signals produced by leaf buds further up the tree. The hormones pass down the phloem, along with food from the leaves. 

In an air layer, the outer bark, phloem, and cambium layers are all removed from a narrow strip around the tree’s circumference. Removing just these outer three layers leaves the sapwood and heartwood intact. Therefore, the upward flow of water and minerals through the sapwood in the wound to the tree above continues unaffected. 

However, with the phloem layer now gone, food cannot flow downward to the roots. Since the cambium layer is also gone, the phloem cells will not grow back. The tree senses the interrupted flow of food at the site where the phloem is missing. It signals the cambium layer above the wound to develop new roots at the point where the phloem ends.

Air Layering Bonsai Versus Cuttings

Bonsai enthusiasts often keep cuttings from pruning work to root and start new plants.  Small cuttings root more easily after a dusting of rooting hormone. Cuttings, however, take several years to grow big enough to make nice bonsai.

Unfortunately, rooting from cuttings doesn’t work well for thick branches. Hobbyists planning to perform a major restyling in which a thick branch or trunk will be cut off will often air layer prior to restyling. The girth and mature bark of the air layered section puts development of a new bonsai on a faster track compared to rooted cuttings.

When to Air Layer

The best time to perform an air layer is when the tree is actively growing, but recent new growth has matured. Look for new leaves which have transitioned from their early, tender, light-green state. As leaves mature, they grow to their full size, turn a darker green, and develop a waxy coating that adds stiffness to the body of the leaf.

For most trees, that means that late spring into early summer is the best time to perform air layering. Here in Ohio, as well as in many other locales, April and May are ideal for the technique. The separated air layered tree with its fresh, new roots benefits from having plenty of time to get established prior to winter.

How to Air Layer

Air layering is not difficult, and requires just a few supplies that most bonsai practitioners already have on hand. Success lies in removing the tree layers properly, making sure the air layer stays moist and giving the tree enough time to develop new roots.

Gathering Supplies

You will need the following supplies:

  • A sharp knife
  • Rooting hormone powder
  • Sphagnum moss
  • Clear plastic (Using plastic wrap or a sandwich bag to wrap the wound allows you to check the progress of root growth.)
  • String or twist ties
  • Aluminum foil

How to Perform an Air Layer

Follow these steps for your air layer:

  1. Place a handful of sphagnum moss in a bucket of water to soak.
  2. Select the spot on the tree for the air layer. You might want to use a marker on the spot to help guide your cuts. Allow around an inch between the top and bottom marks for the cuts.
  3. Cut a ring all around the bark of the tree on the top mark. Then, cut another ring around the bottom mark. 
  4. Use a knife to peel away the outer three layers of the tree in the area between the two cuts. You’ll remove the outer bark, inner bark, and the green-colored cambium layer to expose the sapwood of the tree. It’s important to remove the entire cambium layer from the cut in order to trigger the tree’s root-forming response. Otherwise, the tree will form a scar over the area and the cambium layer will regenerate underneath the scar instead of forming roots.
  5. Next, secure one end of your plastic wrap above or below the girdle using a twist tie or string.
  6. Dust the girdled area with rooting hormone powder
  7. Squeeze the water out of the soaking sphagnum moss, then use a generous amount of it to cover the entire girdled area.
  8. Position the previously secured plastic over the girdled area and its moss bundle.
  9. Once the moss and plastic is properly in place, use another twist tie or string to close the open end of the plastic.
  10. Cover the plastic wrap and twist ties with a generous round of aluminum foil. The idea is to prevent light from reaching the wound, which can interfere with the tree’s process of root creation.
  11. Check the moss periodically for moisture, and do not allow it to dry out.

Separating the air layer

You should be able to see new roots growing within the sphagnum moss by the 30-day mark. Most air layers need from 30-90 days to grow enough roots to fill the plastic, at which time they are ready for separation and planting. When the moss has a nice volume of roots, use sharp cutters or a saw to remove the air layered branch or trunk right below the new roots.  

Plant the new tree into a container. For a while, it will need to be protected from wind, temperature extremes and other harsh conditions. Monitor it closely, and do not allow it to dry out. The new roots will need time to further develop and increase their ability to provide adequate water to the tree. Depending on local weather conditions, the tree might benefit by being kept under a humidity tent.

It’s best to refrain from trimming the tree branches right away. This is because hormones produced in the branch tips aid in prompting the growth and development of the new roots. Once the new tree has recovered and its roots are working adequately, it will begin to produce new buds and growth.

At this point, it is safe to prune unwanted growth, but do not over-prune or wire the tree. Lightly fertilize for the rest of the growing season. Overwinter the tree, then put it into training  as a bonsai the following year.

Interested in more information aimed at advanced beginners to the hobby? Check out our articles on “beyond basic” styles and the Brazilian rain tree.

Air layering is useful in bonsai as a way to correct structural flaws, as well as to create new trees from existing stock. It works by prompting roots to grow from a chosen area of a living tree. After roots are sufficiently developed, the portion above the roots is removed from the parent and planted as a separate tree.

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