Before I go in to it though there are a number of different ways to apply an Air-layer. People have their preferred methods and I don't say they are right or wrong - if it works for them, then that's great. This is the way I do it because it works for me.
Air-layers can be put on the trunk or on a good thick branch. I've done this one on the trunk, so that's what I'll refer to.
Ok, first off gather the tools and materials you are going to need together. Photo 1 shows the things I used. On the left is a bag of Sphagnum Moss. Next to that is some string. Alongside is a roll of sticky tape - Gaffer or Duct tape is quite suitable. Next to that is a length of wire. In front of that lot is a jar of Rooting Hormone Gel then a sharp knife and a pair of scissors (for cutting the clear and black plastic sheet). Underneath is the clear and the black plastic sheet I've used.
Put some of the Sphagnum Moss into a bucket of water and let it soak. While the moss is soaking, select where you want to do your air-layer. The ideal place is right under a node.
Photo 2 shows the trunk of this Benjamina and every brown dot on it is a node. Another place to do it is right where a branch has been removed. When you've selected where you want the layer cut through the bark right round the trunk. Make another cut around the trunk about 3-4 cm below the first one and then make a cut from the top cut down to the bottom
one.
When you've done this, remove the bark between the two cuts by levering it off from the vertical cut. What you should be left with is like photo 3. It's important to cut right through the bark layers down to bare wood - as if you are 'ring-barking' a tree in the garden. If you leave any of the bark layers there is a high possibility that the tree will repair itself without sending out roots - yes, it can bridge a large gap like the one in the photo. It happened to me with this very tree. Photo 4 shows where the gap was bridged because I didn't cut deep enough or didn't make sure I'd taken off all the fibres from the layers.
Wrap the clear plastic round the trunk and using a strip of tape seal it along the join, then bring it all together at the bottom and wrap a bit of tape tightly round it to hold it in place as in Photo 5.
Put some rooting hormone gel or powder on the top edge of the bark (I prefer the gel), then squeeze some of the water out of the Spagnum Moss, but don't 'wring its neck' - just enough that it doesn't drip. Put this into the cup formed from the clear plastic until it completely covers your 'ring-bark'. It must cover the bark at the top of the section - Photo 6.
Bring the top of the clear plastic together and wrap a strip of tape tightly around it. Now wrap some wire round the top and the bottom where you've put your strips of tape and twitch them up with a pair of pliers as tight as you can without snapping them. The whole thing needs to be watertight - no drips or leaks - Photo 7.
If the moisture leaks out, The moss will dry out and any roots will die off. If you do find your layer drying out you can either open it again at the top and put some more water in and reseal it or you can use a syringe and make a small hole near the top to put some more in - don't forget to put a bit of tape over the little puncture hole.
Finally, put the black plastic round the whole thing and tie it top and bottom - Photo 8.
I use clear plastic covered with the black plastic for two reasons. 1. I can take the black plastic off and with the clear plastic underneath undisturbed I can see if there are any roots growing. 2. Another reason for the black plastic is that by my reasoning, roots normally grow in the dark underground so I'm sort of re-creating this by covering with the black
plastic - it will also help keep the layer warmer in the cooler climates. As any gardener knows, bottom heat is good for cuttings and seed propagation - and this is sort of a cutting (you want roots to grow where you've taken the bark off).
I don't remove any of the foliage above any layer I put on - that's where the layer gets its energy from to grow the roots. Now all you do is sit back and wait till the plastic ball is full of roots.