Hackberry (Celtis laevigata) growing next to cattle range
So you’ve found the perfect tree to collect. A beautiful twisted hackberry, the branches spiraling this way and that, the top munched down by cows over the years creating a dense nest of tiny ramified branches. It’s only four feet tall, but at least 50 years old, its growth having been kept in check by the constant cattle chomping the leaves. This is the one you’ve been waiting for. There’s only one problem. Looking down you notice the the ground beneath you is nothing but dried clay, as hard as cement. Taking your spade you dig a little next to the base of the tree. No roots to speak of.
The conditions have been so dry over the years that the tree has been forced to tunnel straight down in search of any moisture whatsoever. Instead of a mass of roots spread horizontally like a water seeking web, there’s only one root, a thick ugly taproot, and it goes south at a vertical line into the earth like a shot, who knows how many feet. This tree would be near impossible to collect and have survive, and doing so would cost you hours and blistered hands from brandishing a pick axe moving slowly inch by inch through the rock hard soil. When you did finally manage to retrieve it, hanging from the bottom would be one long root, as tall as the tree itself. Cutting the root in order to get the hackberry into a pot would spell certain death. In comes…the air layer.
Air layering in bonsai and general horticultural practices is a way, as well as ground layering, of forcing the tree to grow adventitious roots, or roots that are not at the standard position of a tree such as the base, in order to remove a branch for propagation. Or it can be used to correct poor existing root structure of the entire tree by “starting over” so to speak, such as the case with the hackberry example given above.
Layering of trees and plants occurs in nature often in the form of a ground layer. Take a walk through the woods and you can see this process in action, especially with vine like plants that spread outwards from a central origin. As the brambles and creepers touch the ground, adventitious roots form with soil contact, burrowing into the dirt and aiding propagation of the original plant.
How does this happen and how do we take advantage of this behavior for our own purposes? Let’s take a look at a plant’s structure and I’ll give you a quick botany lesson to understand more.
branch structure cross-section
In the above diagram it’s important to take note of a few things, the cambium, the phloem, and the xylem or “sapwood”. The xylem is the living wood of the tree. It contains cells that form vessels, transporting water and nutrients from the roots to the rest of the tree. Each year the xylem dies and becomes part of the heartwood. The phloem is always living, and is outside the xylem. It’s main purpose is transporting what’s created during photosynthesis by the leaves, mainly sugars used for energy, to the rest of the tree to ensure its survival. You can see what it transports in the form of sap when the tree is damaged.
The cambium is in between the phloem and xylem, and is responsible for producing both. As the cambium cells divide, interior cells differentiate into the xylem and exterior cells produce the phloem. Although the cambium is only a few cells wide, it is extremely important not only for xylem and phloem production, but it is also responsible for secondary stem and root production. Roots that you need to create a successful air layer.
A tree’s vascular system is an amazing thing, but we need to partially interrupt it to create an air layer. The goal is to stop the flow of nutrients or sap through the phloem from the leaves of our chosen branch we would like to create the layer from, but to make sure it remains alive by getting water through the xylem from the roots of the main tree until the cambium can respond by growing adventitious roots from the spot where we would like to remove the branch. Roots that are created using resources the leaves produce from photosynthesis, but are no longer going to the rest of the tree. Now, see how all of this botanic babble is coming together? I could have just as easily said cut here and here but not here, but that’s no fun! (yes, I have a weird idea of what fun is)
So now that you have some basic knowledge of your tree’s physiology, creating the air layer itself will make much more sense. Plus, if something goes wrong and the layer doesn’t succeed, you will be able to pinpoint why and correct your mistake the next attempt. In Part 2 of this series, I will show you what to look for when creating an air layer, what tools and materials you will need, and walk you step by step through the process with plenty of photos from start to finish.