The science of air-layering

Bonsai Nut

Nuttier than your average Nut
Messages
8,325
Reaction score
14,028
Location
OC, CA
USDA Zone
10A
Introduction
Air-layering as a way of propagating bonsai material that has some unique advantages. First, the material being layered is a genetic clone of the mother plant. Second, you can often be successful layering mature and large material that cannot be successfully separated via any other means. Third, layering will often create a ring of fine surface roots that yield an excellent nebari in a short period of time.

Because of the number of questions regarding air-layering on the site, I have attempted to pull together an overview of the science behind air-layering which will directly lead to a list of "best practices" and "frequently asked questions". I am going to try to keep the science really simple (because it gets extremely complicated very quickly) while at the same time ensure that everything I write is scientifically grounded.

Background
All plants sense and interact with their environment. Survival and growth is critically influenced by abiotic factors including water, wind and light. If we understand how a plant senses such factors, and how it translates them into optimal physiological or growth responses, we can better guide the development of our trees. Hormones play an important role in the internal signaling that brings about environment responses, and are keyed in many ways to the environment. Precisely regulated responses to the environment not only allow a plant to survive from day to day, but also determine when a flowering plant will produce a flower, or when a tree will produce roots or buds.

Hormones
Hormones are chemical substances produced in small, often minute quantities in one part of an organism, and when transported to another part, they can bring about physiological or developmental responses. They can both stimulate a response, or inhibit a response. They can work by themselves, or work in concert with other hormones. How they act in a particular instance is influenced by both the hormone(s) and the tissue that receives the message.

In animals, hormones are usually produced at definite sites, usually organs. In plants, hormones are not produced in specialized tissues, but instead, in tissues that also carry out other, usually more obvious functions (like roots, or leaf buds).

There are seven major kinds of plant hormones, but for the purposes of air-layering, we are primarily interested in two: auxin and cytokinin. But we'll get back to these later.

Translocation
Before we start discussing the action of the two hormones mentioned above, we need to review the concept of translocation in plants. Translocation is the movement of materials from leaves and roots to other tissues throughout the plant. Plants create carbohydrates in their leaves via photosynthesis, but other non-photosynthetic parts of plants also require carbohydrates as well as water and nutrients. For this reason, materials are translocated from sources (leaves or roots) to sinks (which could be flowers or nuts or stems). Parts of the plant can be a source of one material (roots absorb water) and a sink of another (roots consume carbohydrates). To facilitate the movement of materials around a plant, there are specialized tissues called the xylem and the phloem. These tissues are made of long continuous strands of specialized cells, and extend all the way from the roots to veins in the leaves. The xylem carries water, nutrients and hormones from the roots all the way up to the leaves. The phloem carries carbohydrates and hormones from the leaves back down the tree all the way to the roots.

In the case of a woody tree, the xylem is the "wood" of the tree, and it consists of older, inactive heartwood, and the part of the xylem that still actively transports materials called the sapwood. The phloem is the living layer just below the bark. Separating the xylem and phloem is the vascular cambium - which generates new layers of xylem in the interior of the tree (making the tree thicker) and also new layers of phloem under the bark. The vascular cambium is the part of the tree trunk that heals damage to the trunk.

trunk.jpg

Ok so I know this is getting a little complicated, but in simple terms think: sapwood carries water, nutrients and hormones up from the roots, while phloem carries carbohydrates and hormones down from the leaves.

trunk2.jpg

So let's get back to the two hormones we introduced earlier in this article: auxin and cytokinins. These are both "growth" hormones, but they each trigger different responses in the tree based on their presence and interaction. It is important to note that even today (2017) we are just beginning to understand many of the actions of these growth hormones at the cellular and molecular level, and how hormones regulate growth and development.

Auxin
Auxin acts to adapt the plant to its environment in a highly advantageous way. It promotes growth and elongation of cells, and triggers root generation when it is present in certain concentrations. It will migrate away from the presence of light, and will generate stronger growth on the shady side of a plant - which is why plant stems will bend towards light if only lit from one side. Interestingly, the presence of auxin suppresses bud development. Auxins have other impacts on growth and development depending on the presence and concentration of other hormones. These impacts are usually species specific.

Auxin is generated naturally in a plant at the branch tips (apical meristems) in the buds, and secondarily, in the roots. In its natural form it is known as IAA (Indoleacetic Acid). From the tips of the branches and the tips of the roots, auxin will travel towards the base of the trunk.

Synethic auxins such as NAA (naphthalene acetic acid) and IBA (indolebutyric acid) have been discovered that generate a stronger response than IAA. They have many uses in agriculture and horticulture, and can be used to prevent premature fruit drop in trees, hold berries on holly, or induce root formation in cuttings.

It should be noted that while one range of auxins can have a positive effect on plants, too much can have an extremely negative (if not fatal) impact. Synthetic auxins are routinely used in weed control products, where they are applied in higher concentrations than IAA would normally occur in plants. The herbicide 2,4,5-T is a synthetic auxin that was a component of Agent Orange.

Cytokinins
A cytokinin is a type of plant hormone that, in combination with auxin, stimulates cell division and differentiation in plants. Unlike auxin, most cytokinin is produced in the root tips (apical meristems) and transported throughout the plant. Cytokinins, working with other hormones, seem to regulate growth patterns. They promote growth of lateral buds (while auxins suppress them).

auxin.jpg
(in this graphic, image A shows lateral buds being suppressed by auxin generation in the apical meristem (branch tip). B shows how, when the apical bud is removed (and auxin generation reduced) the plant sprouts lateral buds. However, if an artificial source of auxin is applied, as in image C, bud growth continues to be suppressed.

Conversely, cytokinins inhibit the formation of lateral roots, while auxins promote their formation. It is this balance between cytokinins and auxins that determine whether a plant will favor root development, shoot development, or undifferentiated cells. Lots of plant buds (lots of auxin) and minimal roots (less cytokinin) and the plant will favor root development. Lots of roots (lots of cytokinin) and few buds (less auxin) and the plant will favor shoot development.

auxin_cyt-ratios.jpg

Air-Layering
Now, let's put it all together. Remember what we learned about sapwood - which carries water, nutrients and cytokinin up from the roots. Then we have the phloem - which carries carbs and auxin down from the branch tips. When we air-layer a tree, we are interrupting one of these flows, while keeping the other intact, in order to trigger a specific growth response in our tree.

I will assume people know the process of air-layering, in general. You start by "girdling" the tree, which means that you move a strip of material entirely around the trunk or branch that you wish to layer. You want to ensure three things: First, that you completely sever the phloem, which will cause a break in the flow of carbohydrates and auxins down the trunk/branch. Second, that you completely sever the vascular cambium, since this is the healing layer of the trunk. Third, that you keep the sapwood more or less intact, since you want to continue to provide water and nutrients to the upper part of the air-layer.

girdle.jpg

As soon as you girdle the tree, auxins will start to build up at the location of the wound site. Additionally, the vascular cambium will respond to the wound by generating a special type of fast-expanding non-discriminant growth cell that will form a callus, which will physically protect the wound site and try to close it. Finally, the level of auxins below the girdle will start to drop because they are no longer being supplied via the upstream phloem.

girdle2.jpg

Within a short period of time (as soon as 48-72 hours, if all conditions are optimal) the auxin level above the wound site will reach the level where the balance between auxins and cytokinins shifts decidedly to the auxin side. Because of the elevated presence of auxin in the area, the activity levels of all cell functions are elevated. Cell growth is stimulated, but instead of general non-discriminant growth, the tree is being signaled to generate roots. Meanwhile below the girdle, as auxin levels drop, the auxin/cytokinin balance shifts to the cytokinin side, and the tree is being signaled to produce shoots and develop buds.

girdle3.jpg

Because the sapwood remains in place, the roots can continue to supply water and nutrients to the upper part of the layer, which is actually in very little distress.

Once an adequate volume of roots has grown at the top of the girdle, the layer may be separated from the parent tree. Usually this is accompanied by two effects - first, the reduction of water, nutrients and cytokinin from the established roots in the parent plant (via the sapwood) will cause a short term drop in shoot development. This will be accompanied by a short term push in root development in the layer due to elevated auxin levels. Once the roots develop to the point that they balance the upper part of the layer, cytokinin levels will rise, auxin / cytokinin balance will be restored, and growth levels will balance in the layer/new tree.

Image: Girdling a Chinese cork elm.
air3.jpg

Image: Root development at separation of the layer from the parent tree (33 days after girdling).
air1.jpg

Image: Root development one year after separation (after minor trimming and raking). In the case of easy to root species like Chinese elm, the development of roots may be so robust that they extend from the wound site to some distance above the wound - projecting through the bark in places.
air2.jpg

Best Practices
When girdling, make sure you cut all the way through the phloem and the vascular cambium to the sapwood. It is better to even scrape into the sapwood than to fall short and leave strips of vascular cambium behind. Even a small layer of vascular cambium cells can quickly heal into a bridge between upper and lower parts of the girdle, restoring flow to the phloem and preventing the accumulation of auxin above the girdle.

Cover your air-layer location with aluminum foil. Because auxins migrate away from light, there is a possibility that allowing light to shine on one side of the girdle may suppress root development on that side.

FAQ
Q: Is there a best time to air-layer?
A: Yes. The best time to air-layer is during late spring / early summer when new growth has hardened but while the tree is still in growth mode.

Q: Should I defoliate / prune a branch before air-layering it?
A: Generally no. Because auxins are generated at the tips of the branches, it is best to leave as many apical buds as possible.

Q: Should I use artificial auxin on the upper edge of the girdle?
A: Usually not required, as long as the airlayer has a lot of healthy growth (and buds) above the girdle. Will not harm your airlayer attempt, regardless, as long as you don't use too strong of an auxin concentration.

Q: What concentration of artificial auxin should I use?
A: Depends on the species of the tree. I recommend further research, or reference to Dirr and Hueser's The Reference Manual of Woody Plant Propagation.

Q: Is it better to use sphagnum moss or bonsai mix in the air-layer?
A: Whichever you personally prefer. The function of the propagation mix in the air-layer bundle is simply to prevent the new roots from drying out. It serves no other functional purpose, since the water and nutrient needs of the air-layer are still being met by the translocation flow in the sapwood.

Q: Can I do more than one air-layer on a tree at a time?
A: Yes, but you should only try to air-layer a branch that has an uninterrupted path to terminal buds. In other words, you can air-layer two branches next to each other, but do not try two air-layers on the same branch (directly above and below each other).

Q: After I separate the air-layer, should I prune the branches?
A: It is better to leave them unpruned, since the branch tips are providing auxin to continue to develop the new roots. If you need to protect the foliage until the new roots increase their capacity to provide adequate water, you can use a humidity tent over the tree. The sign that it is "safe" to prune the tree is when you see it start to pop new buds and push new growth.
 
Last edited:

Anthony

Imperial Masterpiece
Messages
5,814
Reaction score
7,418
Location
West Indies [ Caribbean ]
USDA Zone
13
Have Fukien teas in the ground, bird planted, no colander for root aid ----------------- airlayer.
Thank you mucho Bnut.
Good Day
Anthony
 

Bonsai Nut

Nuttier than your average Nut
Messages
8,325
Reaction score
14,028
Location
OC, CA
USDA Zone
10A
But! If you ground layer and it is unsuccessful, the roots haven't received food back and the rootstock will probably die.
Not necessarily. I certainly don't view it as any more stressful than what happens if you try a trunk layer and are not successful. A tree typically has energy stores that allow it to get through pretty dramatic short-term stress and injury. It does however highlight two things:

(1) The healthier your parent material, the more likely your air-layer attempts will be successful.
(2) Branch air-layers are less stressful to the tree than trunk air-layers, since with branch air-layers there are still carbohydrates flowing to the trunk and roots via other branches.
 

barrosinc

Masterpiece
Messages
3,912
Reaction score
4,121
Location
Santiago, Chile
USDA Zone
9b
Not necessarily. I certainly don't view it as any more stressful than what happens if you try a trunk layer and are not successful. A tree typically has energy stores that allow it to get through pretty dramatic short-term stress and injury. It does however highlight two things:

(1) The healthier your parent material, the more likely your air-layer attempts will be successful.
(2) Branch air-layers are less stressful to the tree than trunk air-layers, since with branch air-layers there are still carbohydrates flowing to the trunk and roots via other branches.
My experience (with trees that do not send suckers) is that if no leaves feed the roots, these die. (Obviously above a node)

I did write probably, do it isn't certain and I have only done some 15 layers only.
 
Last edited:

Bonsai Nut

Nuttier than your average Nut
Messages
8,325
Reaction score
14,028
Location
OC, CA
USDA Zone
10A
My experience (with trees that do not send suckers) is that if no leaves feed the roots, these die. (Obviously above a node)

I did write probably, do it isn't certain and I have only done some 15 layers only.
I have only done trunk air-layers on elms, which are one of the easier genera. I have separated air-layers leaving just 4" of trunk with no buds or branches at the soil line and after separation the tree sprang back none the worse for wear.

However for difficult to root or bud genera (like pines), I assume a successful trunk layer would mean you most likely will end up disposing of the lower section of the trunk and the old roots (if it did not already have other branches).

So it may just depend on the tree species...
 

bonhe

Masterpiece
Messages
3,210
Reaction score
5,478
Location
Riverside, CA
USDA Zone
11
Thank you very much for taking time to create this interesting subject.
I have some questions:

Auxin
. Interestingly, the presence of auxin suppresses bud development. Auxins have other impacts on growth and development depending on the presence and concentration of other hormones. These impacts are usually species specific.
Could you elaborate the bold part?

The herbicide 2,4,5-T is a synthetic auxin that was a component of Agent Orange.
It reminds me Vietnam war!!!


Conversely, cytokinins inhibit the formation of lateral roots, while auxins promote their formation. It is this balance between cytokinins and auxins that determine whether a plant will favor root development, shoot development, or undifferentiated cells. Lots of plant buds (lots of auxin) and minimal roots (less cytokinin) and the plant will favor root development. Lots of roots (lots of cytokinin) and few buds (less auxin) and the plant will favor shoot development.
So, when auxin = cytokinin -----> callus = healing process ?

Air-Layering

Within a short period of time (as soon as 48-72 hours, if all conditions are optimal) the auxin level above the wound site will reach the level where the balance between auxins and cytokinins shifts decidedly to the auxin side. Because of the elevated presence of auxin in the area, the activity levels of all cell functions are elevated. Cell growth is stimulated, but instead of general non-discriminant growth, the tree is being signaled to generate roots.
and this is why some gardener use tourniquet technique to increase the flower and fruit production!

Meanwhile below the girdle, as auxin levels drop, the auxin/cytokinin balance shifts to the cytokinin side, and the tree is being signaled to produce shoots and develop buds.
So we can get the new shoot wherever we want one.


Q: Can I do more than one air-layer on a tree at a time?
A: Yes, but you should only try to air-layer a branch that has an uninterrupted path to terminal buds. In other words, you can air-layer two branches next to each other, but do not try two air-layers on the same branch (directly above and below each other).

I did that with my flowering quince and pomegranate without any problem at all. If you look at the anatomy of trunk, you can see the xylem tract will not be disrupted, so the lower and upper parts of airlayering still receive enough water to survive.
Bonhe
 
  • Like
Reactions: Vin

susieq14114

Yamadori
Messages
55
Reaction score
27
Location
Gulf Coast of Florida
I had 2 trees that had lost important branches and lost all hope of being restyled, but they each had very pretty tops with a lot of potential. They were both junipers of the variety called procumbens nana.... I think the correct name is squamata prostrata? Anyhow, it's very forgiving to work with. They have these little protrusions all up and down the trunk, that have the ability to either be a new bud/branch, or..if covered with dirt, a new root. I was googling how to air layer and ended up cutting a small plastic nursery pot, to fit around the trunk at the level where new roots were desired. I had poked holes in the pot around the top edge and made hooks of copper wire, to hang on the upper branches.

At the level where I hoped to have roots, I cut horizontal, around the trunk, through the outer layer. I had also read that it was good to cut vertically, up from the original cut, probably 1/2 inch, every half inch or so, all the way around the trunk, creating a fringe of flaps. I read that this helped create the start of a nice flare of roots ....Then I wrapped copper wire around the horizontal cut and twisted it very tightly. I'm trying to remember if I used a rooting powder on this area, after this was done. (It's been a few years) Then I wrapped the cut nursery pots around that level and closed the cut sides with duct tape.....as per Red Green....LOL I filled the pots with a bonsai soil mix that I had added some actual soil to.....and waited. My trees are all on a drip system so I added a dripper to the air layer pot on each tree also. I now have a lovely, tiny cascade juniper off of one of these and a "chunky" little informal upright out of the other one.

I now have a larger juniper of the same variety, that is trying to go in 2 different directions, style wise. I'm hoping to do the same thing with the top of this tree and after removing it from the tree, making the lowest branch, the new trunk line and growing a new top on it. If I'm lucky, I'll have a "two-fer" out of it. I'm not sure if I did this right and it was a success...or if the material is just so darn forgiving, that it worked in spite of me...



Introduction
Air-layering as a way of propagating bonsai material that has some unique advantages. First, the material being layered is a genetic clone of the mother plant. Second, you can often be successful layering mature and large material that cannot be successfully separated via any other means. Third, layering will often create a ring of fine surface roots that yield an excellent nebari in a short period of time.

Because of the number of questions regarding air-layering on the site, I have attempted to pull together an overview of the science behind air-layering which will directly lead to a list of "best practices" and "frequently asked questions". I am going to try to keep the science really simple (because it gets extremely complicated very quickly) while at the same time ensure that everything I write is grounded in science.

Background
All plants sense and interact with their environment. Survival and growth is critically influenced by abiotic factors including water, wind and light. If we understand how a plant senses such factors, and how it translates them into optimal physiological or growth responses, we can better guide the development of our trees. Hormones play an important role in the internal signaling that brings about environment responses, and are keyed in many ways to the environment. Precisely regulated responses to the environment not only allow a plant to survive from day to day, but also determine when a flowering plant will produce a flower, or when a tree will produce roots or buds.

Hormones
Hormones are chemical substances produced in small, often minute quantities in one part of an organism, and when transported to another part, they can bring about physiological or developmental responses. They can both stimulate a response, or inhibit a response. They can work by themselves, or work in concert with other hormones. How they act in a particular instance is influenced by both the hormone(s) and the tissue that receives the message.

In animals, hormones are usually produced at definite sites, usually organs. In plants, hormones are not produced in specialized tissues, but instead, in tissues that also carry out other, usually more obvious functions (like roots, or leaf buds).

There are seven major kinds of plant hormones, but for the purposes of air-layering, we are primarily interested in two: auxin and cytokinin. But we'll get back to these later.

Translocation
Before we start discussing the action of the two hormones mentioned above, we need to review the concept of translocation in plants. Translocation is the movement of materials from leaves and roots to other tissues throughout the plant. Plants create carbohydrates in their leaves via photosynthesis, but other non-photosynthetic parts of plants also require carbohydrates as well as water and nutrients. For this reason, materials are translocated from sources (leaves or roots) to sinks (which could be flowers or nuts or stems). Parts of the plant can be a source of one material (roots absorb water) and a sink of another (roots consume carbohydrates). To facilitate the movement of materials around a plant, there are specialized tissues called the xylem and the phloem. These tissues are made of long continuous strands of specialized cells, and extend all the way from the roots to veins in the leaves. The xylem carries water, nutrients and hormones from the roots all the way up to the leaves. The phloem carries carbohydrates and hormones from the leaves back down the tree all the way to the roots.

In the case of a woody tree, the xylem is the "wood" of the tree, and it consists of older, inactive heartwood, and the part of the xylem that still actively transports materials called the sapwood. The phloem is the living layer just below the bark. Separating the xylem and phloem is the vascular cambium - which generates new layers of xylem in the interior of the tree (making the tree thicker) and also new layers of phloem under the bark. The vascular cambium is the part of the tree trunk that heals damage to the trunk.

View attachment 139462

Ok so I know this is getting a little complicated, but in simple terms think: sapwood carries water, nutrients and hormones up from the roots, while phloem carries carbohydrates and hormones down from the leaves.

View attachment 139463

So let's get back to the two hormones we introduced earlier in this article: auxin and cytokinins. These are both "growth" hormones, but they each trigger different responses in the tree based on their presence and interaction. It is important to note that even today (2017) we are just beginning to understand many of the actions of these growth hormones at the cellular and molecular level, and how hormones regulate growth and development.

Auxin
Auxin acts to adapt the plant to its environment in a highly advantageous way. It promotes growth and elongation of cells, and triggers root generation when it is present in certain concentrations. It will migrate away from the presence of light, and will generate stronger growth on the shady side of a plant - which is why plant stems will bend towards light if only lit from one side. Interestingly, the presence of auxin suppresses bud development. Auxins have other impacts on growth and development depending on the presence and concentration of other hormones. These impacts are usually species specific.

Auxin is generated naturally in a plant at the branch tips (apical meristems) in the buds, and secondarily, in the roots. In its natural form it is known as IAA (Indoleacetic Acid). From the tips of the branches and the tips of the roots, auxin will travel towards the base of the trunk.

Synethic auxins such as NAA (naphthalene acetic acid) and IBA (indolebutyric acid) have been discovered that generate a stronger response than IAA. They have many uses in agriculture and horticulture, and can be used to prevent premature fruit drop in trees, hold berries on holly, or induce root formation in cuttings.

It should be noted that while one range of auxins can have a positive effect on plants, too much can have an extremely negative (if not fatal) impact. Synthetic auxins are routinely used in weed control products, where they are applied in higher concentrations than IAA would normally occur in plants. The herbicide 2,4,5-T is a synthetic auxin that was a component of Agent Orange.

Cytokinins
A cytokinin is a type of plant hormone that, in combination with auxin, stimulates cell division and differentiation in plants. Unlike auxin, most cytokinin is produced in the root tips (apical meristems) and transported throughout the plant. Cytokinins, working with other hormones, seem to regulate growth patterns. They promote growth of lateral buds (while auxins suppress them).

View attachment 139469
(in this graphic, image A shows lateral buds being suppressed by auxin generation in the apical meristem (branch tip). B shows how, when the apical bud is removed (and auxin generation reduced) the plant sprouts lateral buds. However, if an artificial source of auxin is applied, as in image C, bud growth continues to be suppressed.

Conversely, cytokinins inhibit the formation of lateral roots, while auxins promote their formation. It is this balance between cytokinins and auxins that determine whether a plant will favor root development, shoot development, or undifferentiated cells. Lots of plant buds (lots of auxin) and minimal roots (less cytokinin) and the plant will favor root development. Lots of roots (lots of cytokinin) and few buds (less auxin) and the plant will favor shoot development.

View attachment 139470

Air-Layering
Now, let's put it all together. Remember what we learned about sapwood - which carries water, nutrients and cytokinin up from the roots. Then we have the phloem - which carries carbs and auxin down from the branch tips. When we air-layer a tree, we are interrupting one of these flows, while keeping the other intact, in order to trigger a specific growth response in our tree.

I will assume people know the process of air-layering, in general. You start by "girdling" the tree, which means that you move a strip of material entirely around the trunk or branch that you wish to layer. You want to ensure three things: First, that you completely sever the phloem, which will cause a break in the flow of carbohydrates and auxins down the trunk/branch. Second, that you completely sever the vascular cambium, since this is the healing layer of the trunk. Third, that you keep the sapwood more or less intact, since you want to continue to provide water and nutrients to the upper part of the air-layer.

View attachment 139525

As soon as you girdle the tree, auxins will start to build up at the location of the wound site. Additionally, the vascular cambium will respond to the wound by generating a special type of fast-expanding non-discriminant growth cell that will form a callus, which will physically protect the wound site and try to close it. Finally, the level of auxins below the girdle will start to drop because they are no longer being supplied via the upstream phloem.

View attachment 139526

Within a short period of time (as soon as 48-72 hours, if all conditions are optimal) the auxin level above the wound site will reach the level where the balance between auxins and cytokinins shifts decidedly to the auxin side. Because of the elevated presence of auxin in the area, the activity levels of all cell functions are elevated. Cell growth is stimulated, but instead of general non-discriminant growth, the tree is being signaled to generate roots. Meanwhile below the girdle, as auxin levels drop, the auxin/cytokinin balance shifts to the cytokinin side, and the tree is being signaled to produce shoots and develop buds.

View attachment 139527

Because the sapwood remains in place, the roots can continue to supply water and nutrients to the upper part of the layer, which is actually in very little distress.

Once an adequate volume of roots has grown at the top of the girdle, the layer may be separated from the parent tree. Usually this is accompanied by two effects - first, the reduction of water, nutrients and cytokinin from the established roots (via the sapwood) will cause a short term drop in shoot development. This will be accompanied by a short term push in root development due to elevated auxin levels. Once the roots develop to the point that they balance the upper part of the layer, cytokinin levels will rise, auxin / cytokinin balance will be restored, and growth levels will balance.

Image: Girdling a Chinese cork elm.
View attachment 139536

Image: Root development at separation of the layer from the parent tree (33 days after girdling).
View attachment 139537

Image: Root development one year after separation (after minor trimming and raking). In the case of easy to root species like Chinese elm, the development of roots may be so robust that they extend from the wound site to some distance above the wound - projecting through the bark in places.
View attachment 139538

Best Practices
When girdling, make sure you cut all the way through the phloem and the vascular cambium to the sapwood. It is better to even scrape into the sapwood than to fall short and leave strips of vascular cambium behind. Even a small layer of vascular cambium cells can quickly heal into a bridge between upper and lower parts of the girdle, restoring flow to the phloem and preventing the accumulation of auxin above the girdle.

Cover your air-layer location with aluminum foil. Because auxins migrate away from light, there is a possibility that allowing light to shine on one side of the girdle may suppress root development on that side.

FAQ
Q: Is there a best time to air-layer?
A: Yes. The best time to air-layer is during late spring / early summer when new growth has hardened but while the tree is still in growth mode.

Q: Should I defoliate / prune a branch before air-layering it?
A: Generally no. Because auxins are generated at the tips of the branches, it is best to leave as many apical buds as possible.

Q: Should I use artificial auxin on the upper edge of the girdle?
A: Usually not required, as long as the airlayer has a lot of healthy growth (and buds) above the girdle. Will not harm your airlayer attempt, regardless, as long as you don't use too strong of an auxin concentration.

Q: What concentration of artificial auxin should I use?
A: Depends on the species of the tree. I recommend further research, or reference to Dirr and Hueser's The Reference Manual of Woody Plant Propagation.

Q: Is it better to use sphagnum moss or bonsai mix in the air-layer?
A: Whichever you personally prefer. The function of the propagation mix in the air-layer bundle is simply to prevent the new roots from drying out. It serves no other functional purpose, since the water and nutrient needs of the air-layer are still being met by the translocation flow in the sapwood.

Q: Can I do more than one air-layer on a tree at a time?
A: Yes, but you should only try to air-layer a branch that has an uninterrupted path to terminal buds. In other words, you can air-layer two branches next to each other, but do not try two air-layers on the same branch (directly above and below each other).
 

Bonsai Nut

Nuttier than your average Nut
Messages
8,325
Reaction score
14,028
Location
OC, CA
USDA Zone
10A
Could you elaborate the bold part?
Auxins play a major role in many plant life and development functions. I didn't want to attempt to create a list because it would be incomplete and very confusing :) However some of the more important ones include:

(1) Phototropism; or the response of a plant to light. This includes the mechanism that causes plant stems and other parts of the plant to bend toward light.
(2) Gravitropism; or the response of a plant to gravity. This includes the mechanism that causes roots to grow downward (toward the center of the Earth) and stems to grow up, and is what causes a potted plant knocked on its side to reorient its direction of growth.
(3) Thigmotropism; which is the response of a plant to touch. This includes mechanisms that cause plant to react to an animal, other plants, or even the wind. It is what causes the tendril of a vine to perceive contact and promote uneven growth, causing the tendril to wrap around an object, sometimes within as little as 3 to 10 minutes.
(4) There are other tropisms as well, including electrotropism (response to electricity), chemotropism (response to chemicals), traumotropism (reponse to wounding), thermotropism (response to temperature), aerotropism (response to oxygen), skototropism (response to dark) and geomagnetropism (response to magnetic fields). There are other plant responses to stimuli that may or may not be true tropisms, and may or may not involve auxin, depending on species.

Also there is a long list of functions related to a plant's ability to adapt to harsh environmental conditions, and the ability to go into dormancy to protect itself during times of extended cold, or absence of water, and when to awaken from dormancy. Auxin is often involved, but the exact mechanism can be extremely different from plant to plant, even if the outcome (survival of the plant) is the same. Deciduous trees and shrubs respond differently to the signals of on-coming winter, compared, for example, to perennial landscape plants. The most extreme example is dormancy in seeds, which can (in some plants) last for decades.

Finally there are the reproductive functions of plants. Auxin is almost always involved, but the actual function varies substantially from species to species. Think of a grape vine, a pine tree, and a century plant, and how different their reproductive cycles are.

So when I said "function varies by species" what I meant to say is "plant species utilize auxin in many complex and varied ways which are too numerous to be listed here. And the list is by no means complete since much of the molecular basis of auxin function remains enigmatic."

I was trying to keep my comments extremely broad and to apply them specifically to auxin flow as it is related to air-layering :)
 
Last edited:

Bonsai Nut

Nuttier than your average Nut
Messages
8,325
Reaction score
14,028
Location
OC, CA
USDA Zone
10A
The herbicide 2,4,5-T is a synthetic auxin that was a component of Agent Orange.
It reminds me Vietnam war!!!
When 2,4,5-T is manufactured, it is unavoidably contaminated with minute amounts of dioxin. Dioxin, in doses as low as a few parts per billion, has produced liver and lung diseases, leukemia, miscarriages, birth defects, and even death in lab animals. The use of 2,4,5-T has been banned for most uses in the US since 1979.
 

Bonsai Nut

Nuttier than your average Nut
Messages
8,325
Reaction score
14,028
Location
OC, CA
USDA Zone
10A
So, when auxin = cytokinin -----> callus = healing process ?
I don't want to say "equals" because I doubt it is that simple, and it probably varies by species, and it definitely varies by location within the plant. What we are interested in is the BALANCE between auxin and cytokinin. When auxin and cytokinin are in balance (which could mean that the natural state of auxin is 2x as much, or .5 as much as cytokinin), the growth is non-discriminate, and does not favor either root or shoot development. The more the balance tilts in one direction or the other, the more the growth favors one type of specific development.

There are two other considerations which I did not get into during the article. They are:
(1) The balance between auxin and cytokinin may not be a linear or exponential function (in terms of the level needed to elicit a growth response) but it may be a step function. In other words, nothing may happen until levels reach a certain point, at which point root or shoot development is "triggered". I do not know, and cursory online research did not give me any clues.
(2) There is a time function involved, but I have no idea what it might be. In other words, auxin levels need to rise, and they needed to remain at elevated levels for a certain period of time to trigger the proper developmental response. So, for example, you might girdle a trunk, but not create a wide enough girdle, and if the girdle is closed by the tree shortly thereafter, you might not get root generation because auxin flows are restored and auxin levels above the girdle drop before the tree issues new roots.

This leads me to hypothesize that there might be an advantage to using artificial hormone to "jump-start" the process by helping auxin levels rise more quickly at the top edge of the girdle, but I have found no scientific information to support that hypothesis.

Also, callus growth is not (in and of itself) related to the air-layer process. Callus growth is a direct response of the tree to any bark injury that cuts into the vascular cambium or deeper. The vascular cambium starts generating indiscriminate cellular growth (neither phloem or xylem) that expands rapidly in order to close the wound. In the case of general wounds (not complete girdles) it appears that levels of both auxin and cytokinin are elevated, which causes accelerated cell division. However auxin and cytokinin balance is not disrupted such that the plant would favor root development.
 
Last edited:

Lars Grimm

Chumono
Messages
543
Reaction score
723
Location
Durham, North Carolina
USDA Zone
7
Q: Can I do more than one air-layer on a tree at a time?
A: Yes, but you should only try to air-layer a branch that has an uninterrupted path to terminal buds. In other words, you can air-layer two branches next to each other, but do not try two air-layers on the same branch (directly above and below each other).
Is it possible to do multiple air layers on the same trunk, provided there is foliage between each layer that could supply each new root system?
 

music~maker

Shohin
Messages
392
Reaction score
673
Location
Boston, MA
USDA Zone
6b
This is an outstanding write-up on this topic. I shared it with the good folks over at the /r/bonsai forum on reddit and it was well received.
 

Similar threads


Top Bottom