November 28, 2021

Tree Propagation Through Air Layering – Part 1, by T.S., PhD

It’s summer and you find yourself in a TEOTWAWKI situation. You wish you had access to more trees or shrubs that produce food. You realize things won’t be back to normal anytime soon, so investing the energy and time now seems like a good idea for the payoff in calories of fresh fruit or nuts a few years into the future. Propagation by seed is one easy way to get more trees, but you’ll have to wait until late summer or fall to harvest seeds, and then let them cold stratify (i.e., simulate winter conditions via subjecting them to cold temperatures) over winter. Additionally, most trees and shrubs won’t breed true (i.e., meaning the seeds will not grow into the delicious food-producing nursery cultivar tree you gather seed from). So how else can you propagate a tree or shrub? This article covers one of the easiest methods for tree or shrub propagation, air layering.

Materials needed (with possible substitutions discussed in this article):
1. Clear Kitchen Wrap ~ 18 inches
2. Peat Moss ~ 1 handful
3. Aluminum foil ~ 18 inches
4. Rooting hormone (~0.8% IBA strength)

Note: Items 1-3 on the above list are be easily substituted or improvised in a TEOTWAWKI scenario. Methods for improvising rooting hormone are not easy. Air layering can be performed without rooting hormone with much lower success rates. This article does discuss how to create your own rooting hormone solution using Salix spp. (i.e., trees in the willow genus) trees.

Air layering has been practiced for thousands of years. When a node (botany term for the part of a plant where leaves or branches form) containing part of many plant species finds itself tipped down into/onto the ground, roots form. For example, if a branch is bent over to the ground and kept in place using a rock, roots may form under the rock and over time the roots can support new shoots and live independently of the original plant. This process, which clones the original plant, is known as layering. Plants can be layered using the tip of a branch or mid branch. Air layering is simply what you call it when we artificially bring the ground up to the plant. Other common names for air layering include pot layering (because a special pot with premade holes is used to hold soil next to a branch) and Chinese layering (the method is believed to have been developed by the Chinese). Air layering can be done in many ways, which will be covered below.

But first, let’s understand why air layering is a good idea for a novice in plant propagation and well suited for a TEOTWAWKI situation. Note, several other plant propagation methods will be mentioned briefly, not as a how-to-guide, but to contrast them with air layering.

Why I prefer air layering over propagation by seed: Air layering can be performed in the spring or early to mid-summer, a huge window of time compared to the few weeks of time window you would have for optimal seed collection/preparation. Once collected and prepared for winter storage, seeds must be kept mold free and protected from rodents and insects. Preparing most seeds requires removal of some outer layer of husk/shell/flesh/etc., checking their viability, and then storage. In “normal times” this usually means packed in sand or moss in the fridge until spring. This can be a lot of work depending on the seed. I’ve lost a lot of seed due to fungal growth and rodent predation over the years. Without access to 1/4″ wire mesh from my local hardware store or refrigeration, I’d expect a much more difficult time storing seeds.

Layering produces a living plant that can both defend itself from fungal attacks and is shielded from pesky rodents via being off the ground. Those pesky rodents may still gnaw on your new plant over the winter which is why this article ends with a short discussion on stopping such predation. Once an air layering has formed roots, you simply cut it off and plant it. This article isn’t going to get into the specifics of seed propagation, but for some plants, especially nut trees that are not the best for short term food crop production (with the exception of chestnut which can produce a crop within a few years) propagation by seed is easier than air layering because most nuts are relatively easy to keep mold-free over the winter (e.g., many nuts lack a fleshy coating that would otherwise promote spoiling over winter.) So in summary, the key benefits of air layering versus seed propagation are: larger time window to perform the propagation, less work when one considers gathering/cleaning/storage of seeds, and air layering “breeds true”.

Why I prefer air layering over grafting: Grafting looks easy on internet tutorials, but like seed collection it has a short time window over which the graft can be made. Albeit, one can collect scion (the “top” of the graft where the fruit will form) after several hard freezes in the fall/winter and store them for months. The other half of the graft is called the root stock (the “bottom” of the graft joint where the roots will form). There are late-season techniques for grafting which can be done in mid to late summer. But for the most part, success will be best in early spring for grafting when buds are breaking open. Many more variations exist for grafting than any other propagation method I’m aware of, with specific techniques usually recommended for individual genera (related species of plants).

Grafting requires you have prepared scion and rootstock material on hand. In professional orchards, the rootstock is usually shipped in or grown specially on site; both are luxuries you won’t have. A freshly grafted scion requires high humidity, warmth, and a sterile environment. So this means you likely need to build some kind of miniature greenhouse to hold the newly grafted scions. Yes, some people successfully just put a plastic bag over a pot to produce a greenhouse-like environment, but such a solution will require frequent (i.e., more than once a day) monitoring for proper temperature and moisture levels. Without access to a light/climate controlled space, you’ll be spending a lot of time adjusting the amount of sun to keep your newly grafted plants happy. Without going into too much detail, there are also issues with size compatibility and species/genus compatibility between rootstock wood and scion wood that make grafting more complicated that air layering (in fact, some books recommend layering for some species because they are difficult to graft).

So, in summary, the key benefits of air layering versus grafting include: air layering is feasible for a longer season, air layering doesn’t require a greenhouse like structure, air layering doesn’t require you to have prepared rootstock wood, and air layering is simply more likely to succeed for more species than grafting. With all this said, grafting is the preferred method of commercial fruit tree propagation because one can pick a rootstock that keeps a fruit tree to a small physical size (think 8-foot apple tree versus 30-feet!) and is more disease resistant.

Why I prefer air layering to using cuttings: Growing a fruit tree or shrub from a cutting is probably the most difficult method. The cuttings will require special monitoring on a more frequent schedule than any other method to ensure they keep from getting too wet, cold, dry, or hot. The cuttings will require the most stringent sterile environment to avoid fungal growth (e.g., soil that you have heat treated) because the wounded part of the plant where roots are forming is actually in the soil. And lastly, the cuttings will require the construction of a special green-house like environment similar to a grafted scion. While it is true that this “special environment” can just be a plastic bag over a pot, it still needs constant attention to make sure it is staying at the correct temperature and humidity. A single afternoon left in full sun could kill weeks of work if the temperature becomes too hot. Lastly, air layering will produce an end result that is larger than a cutting. In other words, you are potentially a year closer to fruit with air layering relative to a cutting.

Why I prefer air layering over tip or “regular” layering: Layering passes a 100% genetic copy of a plant on to a new generation and only requires a branch touches the ground. So why air layer when the ground can be used? Some plants (trees especially) simply don’t have branches close enough to the ground to bend down into the soil, or their branches are too stiff to bend without breaking. This will be true for the likely tree candidates you’ll want to propagate (e.g., cherry and apple trees). Some shrubs, like a black currant, will naturally have low “running” branches making layering directly in the ground easy.

Regardless of if the access to the ground is easy or not, there are several advantages to air layering versus ground layering methods. First, air layering is largely immune to rodent predation. In my city yard, I’ve lost more plants to chipmunks (who will kill tender young shoots via biting then off without even eating any part of a plant) and rabbits (who will eat the tender young shoots from a tip layered branch) than I have grown successfully. Second, with air layering, you can control the amount of moisture at the rooting site to a higher degree than ground based layering (easy to get too wet if it rains, whereas air layering is protected from the rain as we will discuss). Lastly, with air layering, the soil can be sterilized which inhibits fungal growth whereas the ground soil will be laden with fungal spores.

(To be continued tomorrow, in Part 2.)

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