Air layering, to me, is the closest horticulture gets to wizardry. Opening a successful layer feels like unwrapping a Christmas present - the excitement is palpable, and the satisfaction unparalleled. At the very least, air layering is a very useful technique to propagate woody plants that are difficult to do from cuttings. Once you get the hang of it, I find that it also has a higher success rate than hardwood cuttings, so its definitely a handy trick to have up your sleeve.
Essentially, layering refers to getting a branch/stem to produce roots while its still attached to the mother plant and drawing nutrients from it. Air layering is when this process is done in a completely self-contained little parcel on the plant, instead of by bending the branch down to the ground.
Today, many gadgets exist for layering, but I want to share the way I learned - done with materials you can find around the house. The basics are a sheet of clear plastic; broad packing tape; a length of string; a sharp sterile blade (I actually prefer a paper-knife because I find them very sharp and easily manoeuvrable, but a garden knife could work as well); a rooting substrate and a spoon; labels and a permanent marker.
To start, you want to select a branch that is approximately pencil-thick, and select a fairly straight, terminal section which bears two nodes which are within an inch-and-a-half of each-other. You will be making your cuts above and below this pair of nodes. Above the upper node, make a clean, single shallow cut girdling the branch. Do the same below the lower node. Now join these two girdling cuts with a straight, shallow cut running along the length of the branch - being careful to not make this cut through either of the nodes.
Now, with your fingernail, start carefully peeling back only the bark (usually brown or green) layer, leaving the cambium (usually white) layer exposed. Peel all the way around the circumference of the branch, so you end up with a short length of exposed cambium on your branch.
Now carefully wrap the plastic sheet around this exposed section, leaving plenty of space around the branch, and secure the free length with tape. Bunch it below the lower node and tie it very tightly with string. This should create a reservoir for the rooting substrate.
Next, fill the reservoir with the rooting substrate using a spoon. My preferred substrate is moistened (only very slightly, though) vermiculite, although I have also had decent results with moistened shredded sphagnum moss. Bunch and tie the plastic sheet above the upper node. Ultimately, you should end up with what looks like a kebab, skewered on your branch. This should be quite tight, so that it doesn't slip on the branch. I like to wrap it once more with packing tape just to make it air-tight (that's important), and reinforce it a bit.
Finally, label it with the date, and the expected date of cutting. Usually, a callus and roots should develop within three weeks (at least here in the tropics, thats how long it takes). More often than not, you can see the roots pressing up against the plastic sheet - a sure sign that you've succeeded! When roots have formed, you can make a clean, squared cut right below the layer, and unwrap it and plant it directly into a potting soil to encourage further root growth.
Plants I find that work really well with air layering are Ficus, Bauhinia (the plant in my example pics), Citrus, Gardenias, Magnolias, and actually a whole host of tropical trees and shrubs that are kind of hard to propagate from cuttings.
There are many variables with air layers - and I think everyone does them a little differently based on what works best for them, the golden rule is to experiment. If you have tried air layers, I would love to know how you do them, because my technique is also constantly being adjusted and fine-tuned. And if you haven't tried them yet, welcome to plant-wizardry! :)