Air layered hornbeam - should I shorten the trunk at separation?

I air layered a European hornbeam this season, it has a massive amount of foliage and a really tall trunk that I’m going to chop off to get better taper. My question now is, should I shorten the trunk at the same time I separate the air layer?
I left the whole trunk to give the tree maximum amount of leaf mass to produce new roots. But once the leaves have fallen later this fall, should I chop off the unwanted part of the trunk? My thinking is that because there probably isn’t enough roots to support new leaves on the whole tree the lower branches that I want to keep will suffer as the top (which is useless) will get most of the resources the tree has stored in its new roots.


I would agree Micke, cut it off after separation, assuming you have lower branches above the air-layer.

There’s quite a few branches and another, more slender trunk above the air layer that I will leave untouched. Those are the parts of the tree I want to develop.

I’m not an air layer expert. Why wouldn’t you remove the parts you don’t want before you even start the air layer?

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General info…
Healthy tree, healthy roots.
The lower (below the arilayer) foliage supports the original roots. The top leaves suck the water from the roots through respiration and evaporation.
The top leaves try to pump processed liquids, carbohydrates, down to the roots. The cambium layer has been removed. IF the tree has the aability to form roots, this carbohydrate soup supports the growth.
NOT all plants can grow roots on the trunk. Some easier/better than others (pines are very slow…). The above process is ideal. Some trees (maples…) CAN grow a root pad at the very base, without lower leaf mass to support it. They grow very fast and the roots don’t die before the air layer is separated; AND will probably put out new leaf growth from the cut off stump…
My Korean hornbeams did NOT sprout from the stumps…

As @KurtP wrote, the foliage above the air layer produces energy that can’t reach the original roots, thus they feed the new roots with energy. More foliage = more energy = faster root generation. That’s the reason I didn’t want to remove unwanted parts before the layering process.

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Thanks for pointing that out.

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Hi Micke,
This is actually my first post in this forum, so hello everybody else, too!
I would say it’s pretty much impossible to answer your question without knowing the individual tree and the individual situation / conditions. Could you maybe post a picture of your tree? Generally, it’ true that after separating an air layer you want as much foliage mass as possible to allow for optimal root development, but by no means do you want more foliage than your new and very juvenile root system can sustain. So what is that perfect ratio? As I said, impossible to tell from afar… You’ll have to make a good guess initially and then see how the tree is doing and adjust when you see it’s struggling. And since you can only adjust towards less foliage your initial guess should probably a bit on the ‘more foliage’ side.
However, I wouldn’t go as far as to remove a major part of the trunk after separating an air layer. I would use that part of the tree to manage foliage mass and (based on where I live, in Central Europe, north of the Alps) remove it before winter, after the tree has lost its leaves. Make sure to leave a stump of about an inch or so to allow for die back, then cut back to living tissue in spring and make the final cut to allow for a nice and clean scar after the prime of summer.
Hope this helps, its based on what I think I have figured out so far… If somebody detects any flaws in my reasoning, I would very much like to be corrected.

Hi @Obi,

and welcome to Mirai and thank you for you insights! I’ll take some pictures of the tree and the air layer later today when I get back from work.
I’m thinking of keeping the air layer on the parent tree until the leaves start turning their fall color and the foliage has had the maximum amount of time to pump energy to the new roots. I will then overwinter the tree in a cold cellar, so it’s protected from wind and big freezes. Last winter the cellar went below freezing during the coldest part of winter (-20 C). I’m planning on building a small insulated box for the hornbeam to further protect the delicate roots this winter. Is there any reason why it would be beneficial to remove the air layer earlier than after fall color change?

Frankly I don’t know. I’ve heard of air layers being left on the parent plant over the winter but never of one being separated while the parent plant was in a dormant state. So if you don’t want to separate now, I’d wait for the start of the next growing season. You could cut back the trunk this fall and separate in spring. The way I understand the process of ‘waking up’ in spring is that the plant will use its reserves to push out the first badge of growth (instead of nutrients gathered and provided directly by the root system) so this could be beneficial. Because the new roots wouldn’t have to feed the plant right away but could get kind of a transitional period in which they start growing and after that take over. But if the new roots are looking good by now, I guess I’d separate now rather than later. Have you checked the progress of new root development?

I haven’t checked root development, but I’m planning on doing that today also. I’m afraid the new roots wouldn’t survive the harsh Finnish winter if I leave the air layer on the parent tree until spring. The European Hornbeam used as bonsai definitely need winter protection where I live and I doubt the roots of the air layer would survive either, that’s why I want to separate in the fall.
If there’s good root development when I check them today I will separate soon and leave the unwanted trunk for now and see how the tree responds to having to rely on the new roots for water.

Here’s a couple of pictures of the tree.
The whole tree:

The tree above the air layer. The thick middle trunk is the one I’m thinking of removing:

And here’s how the air layer looks. Good callous all around the cut sight and there seems to be a lot of root tips emerging (I assume the light colored blobs are roots), but definitely not time to separate yet.

Hi Micke,

I was somehow assuming the parent plant would be potted also. When did you start the air layer? I don’t know when winter starts where you are (pretty early I’m guessing?) but honestly it seems to me there’s not too much time left this season for those roots to get to a point where you can safely separate the top part. Now that I see the tree I would also definitely say that you need to reduce it after separating. If no roots or not enough roots will appear in time I’d suggest you try again next year a bit further up the trunk.

It will be fine to leave it over winter, the wound is loaded with sugars/starches and is at least as winter hardy as the rest of the tree.

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The layer was started in early June once the new growth had hardened off, so it’s been a bit over 2 months. We usually get the first cold weather towards the end of October and winter usually starts properly in November/December. The parent tree will stay in the ground a few more years until it’s grown a new canopy.

Ok, I guess you’ll just have to wait and see. If the tree hasn’t produced roots after two month in summer, I honestly doubt it will produce enough, if any, in the remaining season. But as I said, if not, just try again next year. Though I don’t know whether it’s best to cut off the callus that’s been produced or to cut a new ring above it or to just leave it be and give it more time…

I agree, but since

I’d say better safe than sorry. Conditions 1 ft above ground can be entirely different than below ground.

You’re right that the tree hasn’t produced almost any roots so far, but it has produced a lot of callus and the root tips are now clearly visible so I really hope that root growth will speed up now. If it’ll be able to produce enough roots, I don’t know.

If I’ve understood things right, root growth will continue, though slowly, even after leaf drop as long as temperatures are above about +5C (the threshold for metabolic activity). Thus, I think I have two options:

  1. Separate the layering once leaves have dropped (to allow maximum time for the tree to produce energy using the parent tree’s roots for water), and then place it in my cold basement (to try to keep temperatures above +5C as long as possible). Then chop the unwanted trunk very early spring and hope the tree has enough roots to push new growth and sustain it.
  2. Leave the air layer on the tree and try to protect it as well as I can during winter. If the roots survive the winter they will continue to grow next spring. If they don’t make it I have to rescar the tissue and try again.

I’m not sure which option I’ll choose. But I’ll check the roots again in a month or so to see how much they’ve grown and make my decision based on what I see.

It’s not a bonsai though, it’s a wound with callus on a tree. I think if anything, the survival of the base is at more risk. I guess separating and as it is basically a hardwood cutting, might be a good option, thinking about it.

Either way, will be interesting.

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Nothing lost. Just takes a little patience. Slower than maples…
Treat the callus with rootone, maybe scrape the seperation clean , just in case. Moss / soil the air layer back up. Cover, wet down and leave it. If it gets real cold (Finland!), wrap the whole thing with an insulation. Check for roots in the spring. About when the new shoots on the main tree start showing.
NOT gonna kill the tree base. Even if it appears dead–Cut it off in spring and into a fine rooting type soil. It’s gonna be real slow to wake up…
Yes, after, shorten the center trunk down to 2-4 new limbs. Eventually below the s.
.Good luck.
Post next spring. I’m interested to hear the outcome.!

Thanks for your insights Kurt! I’m leaning towards leaving it until spring and separating it then.

I was thinking of cutting the center trunk at the red line. As there’s no branches along the orange line and only two smaller branches before the ‘S’. I’m interested to know if there’s any benefit of leaving some branches on the central trunk if I’m going to remove it later anyway.

There’s a really clear collar just below the red line so I will do a angled cut leaving a small stub above the collar. Once the stub has died back to the collar I’ll go back in and remove the stub and hopefully get the cleanest possible scar.

I’m not at all worried about the survival of the parent trunk/base, there’s several healthy branches below the air layer feeding those roots and it’s pushed new growth lower on the trunk after air layering.

I’ll keep you posted on the progress, don’t worry :wink:

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