I’ve tried a lot of different methods, mostly with success. Plastic plant pot cut then tied round a tree with compost and grit in it, moss wrapped round in a polythene wrap - sealed top and bottom, moss with added vermiculite and done using the same method. They were all successful but the open tops were more of a hassle than sealed ones with the watering. The only method I didn’t have any success with was with a cage of potting mesh placed around the tree (resting on the soil surface) filled with akadama. It was a hot summer and impossible to keep moist.
When sealing the top it shouldn’t be so tight the tree gets strangled. If sealed normally then water can enter when it rains and with a couple of holes punched in the bottom, excess can drain. I don’t even check mine until it’s time to separate.
The wire tourniquet method on deciduous is one commonly applied to beech, I’ve never used it on any other deciduous or broadleaf evergreen.
The distance has always been 1.5 ever since I first started in the late nineties.
Sorry it took a while to post the pictures, I carried out the separation mid march here in the U.K. Very pleased with the root production, both patients doing well 
Congrats on the successful air layer. Those look like some very healthy roots indeed. The follow up on this post is much appreciated.
Hi Michael. You mentioned on this thread from the post of my first air layer that you had done crab apple. I am hoping to do 2 from a Chinese crab apple this year. Anything different/ species specific I should know about?
That crab apple layer didn’t work. The tree made a nice callus and swelled where the roots should have appeared, but not a single root. The objective was to shrink the tree to fit in the muro for the winter, so I cut off the layered section at the end of fall. I did try sticking the cut off part in the ground, but it dried up in the following spring.
I was planning on trying cuttings from various crabs this spring, but didn’t get the time to do it.
Thanks, that’s a shame.