Let's use the word layer as a synonym for air-layer, even though
there are several other methods of layering. Also, I did a search for
and gathered comments I've made here on the forums about layering to
draw from, and I can see this is going to be a long post, even if I
try to leave plenty of room (by not going into every detail) for
questions. I guess we'll see how interested you are in layering and
whether or not I can provide info in a way that holds your attention.
I'll be as brief as I can and still try to make sure everything is
tied together so it's easy to digest. I apologize for any errors I made. I'll try to
correct them as I find them - for as long as the edit feature works. I've been at this since about 2p.m., and
it's after 10 now - lots of formatting issues trying to get it all in one text box..

propagule = the part of the plant you're propagating – cuttings
and layers are both propagules

viable/ viability = alive, loss of viability is death

photosynthate = sugar/ carbs - plant's true food/ source of energy
- the product of photosynthesis

vigor = a measure of the plant's genetic potential – different
from vitality

vitality = health – how a plant is dealing with the cultural
hand it's been dealt - different from vigor

distal = away from the root to shoot transition. A branch is
distal to the trunk and a leaf to the branch

proximal = opposite of distal. Twigs are proximal to the leaves
that grow on them and the trunk is proximal to twigs and branches

strike = successfully grow new roots

polar = upward or downward – the flow of the hormone, auxin, is
'polar' and downward only

blanching = blocking light from specific parts or the entire plant
for the purpose of enhancing propagation

There are many ways of propagating/ cloning plants. Layering is an
inexpensive way of producing new plants with exactly the same genetic
characteristics as the plant from which the layer was obtained. The
same is true of plants grown from cuttings, but layering warrants an
added measure of certainty your efforts will be successful because,
unlike a cutting, the main plant will still be supporting/ supplying
the layered part with water and nutrients while we wait for roots to
form on the propagule. For inexperienced propagators, the odds of a
layer being successful (for plants that lend themselves to layering)
is much greater than the odds of cuttings striking (growing roots).

Interestingly, there are several reasons a grower might wish to
utilize layering other than to increase the number of plants with the
same genetic sequence. As a bonsai practitioner, many of the layers I
perform are designed to rid a plant of an ugly root system, so the
plant is 'layered off' immediately above its existing roots; this,
because an attractive root system with roots radiating evenly and
horizontally from the base of the trunk is highly desirable – ugly/
uneven/ misshapen root systems are not. It's also possible to layer
large branches or tops of mature trees that have interesting
movement, which can immediately produce bonsai material that would
otherwise have taken years to produce.

Tip: Unless you are just having fun, looking for practice, or
something to bolster your confidence, don't bother layering plants
that come easily from cuttings. Most ivies/ climbing plants, and many
other self supporting plants have preformed adventitious roots that
make propagation by cuttings so simple it doesn't make sense to layer
unless you want to propagate a thick stem/ branch. For the most part,
success %s of cuttings and layers decreases as the diameter of the
propagule increases, but cutting success diminishes much faster than
layering success with the increase in diameter. Plants endowed with a
high level of genetic vigor are generally easier to propagate from
cuttings or layers, and are good candidates to consider trying to
propagate from easier cutting methods. I'll offer some not well-known
tips on how to increase the chances of success later in the thread.

It wouldn't be right to offer directions or tips relative to
layering w/o explaining what makes layering possible. Dicot stems
branches are covered by bark, immediately under the bark is the
cambium, which is made up of phloem tissues toward the outside and
xylem tissues toward the inside. Water and nutrients move upward in
the living xylem tissues growing toward the center of branches/
stems, so intentionally wounding the plant by cutting through the
cambium does not interrupt the upward movement of water and nutrients
that supply living tissues above the layer; thus, the branch/ stem
being layered is not isolated from these life-sustaining necessities
as would occur in propagation via cuttings. The types of wounds used
to form a layer all disrupt the polar flow of a hormone (auxin) that
suppresses lateral branching and stimulates root formation and
division. When we cut through the vascular cambium of a plant, all
the way through to the xylem, photosynthate cannot move below the
cut, so both the hormone that stimulates root formation and
photosynthate collect in the tissues immediately above the cut to
provide conditions that facilitate new root formation. If we provide
a medium surrounding the layer that has moisture and plenty of
aeration, odds are extremely high that roots will form and the grower
will eventually be able to separate from the main plant, a
self-sustaining propagule on its own root system.

You can start a layer any time on tropical plants most often grown
as houseplants, but layers strike fastest when performed at the point
in time when the plant has high energy levels and is about to enter
the portion of the growth cycle when the plant is approaching the
peak of its food-making ability; so for those in the Northern
Hemisphere, late May to early June will yield the fastest results and
highest likelihood of success for the lion's share of plants. I don't
like to use the words 'heal' or 'healing' in reference to plants
because plants don't 'heal' in the same sense that animals do by
replacing damaged or infected cells in their same spatial position,
but for the sake of imagery, let's use that descriptor when
considering that layers started 'out-of-season' take much longer to
strike, and increase the risk of failure due to the wound “healing”
before roots can form. This forms a bridge that restores the downward
movement of photosynthate and growth and hormones to roots, which
considerably decreases the probability of a successful layer. The
good news is it seldom results in the propagule's loss of viability.

The most common method of layering is probably the ringing method where all bark and cambial tissues are removed, leaving a longitudinal measure of sapwood equal to at least 1.5x the branch thickness exposed and scraped clean of all cambial traces. There is usually a white layer of inner-xylem tissue that sort of has a fuzzy-looking texture. This should be scraped off with a knife blade held 90* to the branch/ stem. See image A below for an image of the ring method. See image B for an illustration of the flap method.

often for plants that root easily. If you use this method, it's
important to use something to keep the flaps open so the wounds don't
'heal' over quickly. Pebbles, twine under the flaps, toothpicks, foam
beads are all potential items you can use to hold flaps open. Most
often, roots form on the flaps because that's where the auxin and
photosynthate collect. to hold flaps open. Most often, roots form on
the flaps because that's where the auxin and photosynthate collect.

Images C and D below illustrate use of the tourniquet method, which doesn't require wounding. Applying a tourniquet, then waiting for it to restrict the downward movement of auxin and photosynthate to collect above the restriction can be used as a long term layering method or to prepare a plant for a future layer. The plant with the wire tourniquet still visible below the root to shoot transition was layered off of an ugly root system. The
tourniquet method can be used on stems/ branches you wish to layer in
the future. You can apply a tourniquet and wait up to 2 years to
layer.

The image immediately above that shows two steps that I usually take a year apart. Normally, the tourniquet would be applied and a year of growth allowed to pass. As the layer site thickens above the tourniquet and restricts downward flow of nutrients, it swells. The next year I usually uncover the tourniquet and drill holes in the trunk, which I fill with an appropriate rooting aid. The result is an evenly spaced root system as seen in the plant with the wire tourniquet.

Blanching is another enhancement that can be used with much
success. Blanching is the blocking of light from the area on the
plant where you eventually want roots to grow. I use it very often in
conjunction with an appropriate rooting aid. Here's how: Sprinkle an
appropriate powdered rooting aid on a sheet of paper, then use it to
coat the end of a strip of electrical tape equal to the circumference
of the branch/ stem you intend to layer. Then wrap the tape around
the branch where you want roots to grow with the dusted part against
the bark and secure. Do this 3-4 weeks before starting the layer. The
blanching that occurs due to banding with the tape works together
with the rooting aid and higher moisture levels under the band to get
root initials started before you even prepare the layer. This is very
effective for a high % of plants that lend themselves to layering The
bottom of the tape will serve as a template for your top girdling cut
a month after you apply the tape with hormone. If you use a
tourniquet, apply the tape with hormone on it a month before you
start the layer. You can see that a bit of planning and the little
bit of patience it takes to wait until the most advantageous time to
start your layer can increase your chances for success and the
potential for things going south.

Sphagnum moss, which is different than peat moss approaches being
the traditional medium used for most layers hobby growers other than
bonsai practitioners are likely to use. Sphagnum moss is collected
while living, before it had died and partially decomposed, then
dried. It is very different than sphagnum peat moss, which is the
same plant but in an advanced state of decomposition. See images E
and F below.

Most growers are most familiar with layers that use the steps of
deliberate wounding, covering the wound with a clear pouch containing
sphagnum moss, then covering the plastic pouch with aluminum foil or
other material that blocks light and helps prevent heat build-up, but
containers can be used for layering as well. I frequently use them,
filled with gritty mix to layer branches or the entire top of a tree.
See images G and H in below.

I hope the thread is
helpful and gets lots of business as time passes, and I hope you'll bookmark it so you can direct others to it. I'm sure I missed a
few things I might have covered, but that's why questions were invented.

Al