Question concerning larch air layer

[Javaman4373]

I collected a larch in 2020 and have kept it in a grow box since then. The tree is described more fully in a thread I started in the New to Bonsai forum. This past April I decided to air layer the top half of the tree to get a more manageable tree. About 4 wks ago I inspected the air layer and found one small root started. I had the air layer wrapped in black plastic and it occurred to me that perhaps that resulted in too much heat in our hot summer sun. I posted a question about this on that thread and Leo in IL suggested I wrap in in Aluminum foil instead, which I did. Today I find a fairly substantial new root on one side of the air layer, but I don't see any roots on the other sides yet. My question, which I repeat in this thread, is whether to separate the air layer this fall, or let it go into the next growing season and see what further roots might develop. The concern is whether the new roots in the air layer will survive the winter cold. They could be wrapped in bubble wrap to provide some insulation. I include some photos. The larch in 2023:
The air layer:

Roots on front side:

As a general question re deciduous trees: would root growth stop once the foliage drops off in the fall because the generation of auxin ceases?

 

[ponderingsage]

To me that looks like a good amount of roots for an airlayer. Your probably good to remove it.
I would think that after leaf fall there is no energy to feed root growth. I’m not sure if energy in the vascular part of the tree would keep developing roots. My hunch is not.
I have a similar project and will have to peek… my larch leaves turned last week.

 

[Wires_Guy_wires]

would root growth stop once the foliage drops off in the fall because the generation of auxin ceases?

No. Auxins can be produced locally and they are produced locally if a plant needs to.
Root growth can continue for quite some time even without foliage, as the process consumes carbohydrates but doesn't necessarily need photosynthesis or even auxin production to keep going. It will deplete stored starches and carbohydrates though, so it's recommended to not do a lot of pruning in the first year after separation.

If all the roots you're seeing originate from one root at the base, I suggest you wait until spring before separating. Your air layer roots will be exposed to the cold, but there's also a whole system of the parent tree protecting them.
If you separate too early and the tree compartimentalizes the sap flow, you can potentially lose the half without roots. So I believe it's better to keep it connected. That way you ensure that the anti-frost agents the plant will produce will be spread throughout the whole plant.

 

[Javaman4373]

I elected to separate the larch air layer today. My reasoning, rightly or wrongly, is that the roots are sending cytokines to the top and pushing the buds despite the coming onset of winter. I speculated that severing the air layer now would slow that push and my hope is that there is enough roots to sustain the air layer after the separation. I secured the air layer in a grow box, screwed the stump butt to the box, and because it is set at an angle. secured the top to a brace. It is solidly fixed in the grow box. I will learn next spring, if the tree is going to make it.

 

[Dogestoevsky]

Nice work! Good amount of roots and stable in grow box. Heel it in some mulch for the winter and it will be fine.