Having the proper tools is the most essential thing in most everything we do. The same holds true for air layering. Although it is true a knife will work, having a few simple tools speeds up the process and makes it safer with better results. The questions of when to air layer comes up and the best answer is after the tree has fruited and before you prune for shape and size. Since having a healthy tree includes proper size and shape management for cold protection and fruit production, why not use those branches that don't fit to propagate your trees for our plant raffle or booth at our annual tree sale?
First, pick a branch about the thickness of a magic marker. A branch that gets sun will air layer better. Pick only upright branches and stay away from side branches that hang as they will tend to always act as side branches even when off the tree and attempting to grow out. Pick a spot on the branch that will allow you to cut below the future root ball so the new feeder roots will be at the top of the container to be potted in. Just clamp on the branch (not too hard; they are very sharp), and one quick twist makes a clean straight cut through the bark to the wood. Make a second cut about one inch from the first. Next, a simple pair of pliers work to grab the bark between the cuts. Twist off the bark between the cuts. Scrape the wood with the pliers to cut cambium layer. This usually makes a difficult process of cutting with a knife safer and a lot faster and cleaner for much better results.
Next, prepare a rooting medium. Use sphagnum moss with water and I like to use a small amount of rooting hormone. Soak the moss and then ring it out so it is moist, not wet. Tear off a foot long piece of aluminum foil and lay the moss out in a 4" long by 2" wide layer on the foil held in your hand. Next, simply wrap the area of removed bark completely with the moss using the foil to hold it in place. Twist and squeeze the foil to make it as tight as possible around the branch and the moss. This helps keep ants out and a tight ball on the moss increases success of root ball. If your foil rips, simply put another piece or foil over the other and squeeze tight. A tight, moist, not wet ball of moss will give faster and better results. Now be patient and wait for roots to grow inside the foil.
You can usually tell after 4 to 6 weeks how it is doing by simply gently squeezing the foil and feeling for roots. If roots are present, it feels hard, not squishy. Once the roots have formed, cut off the branch 4" to 6" below the root ball. This allows some support for the air layer in the container it is growing in. Place your cut branches in a bucket of water. Do not remove the foil yet. This process keeps the roots wet and helps to chase out any bugs inside the root ball. Trim off some branches and shape the new tree a little.
To plant, remove foil carefully, these young roots are very brittle and tender, and place in a container; 1 gallon or 1 ½ gallon container is usually big enough. If it is too big of a container, the soil stays too wet. Water in the soil around the root ball. Do not push down the soil or pack it in, this will damage the roots. Remember you are working with feeder roots; they need to be near the surface of the soil; that's the reason for the long tag end that goes to the bottom of the pot as well as providing support.
Now place your new plant to be in a shady area or a mist house and keep moist the leaves, wood, and soil on at least a daily basis. You want well drained soil, not mud. If you do everything right, in a short time new growth will appear. Now start introducing your plant to sun and continue to water frequently until it takes full sun and is rooted out. Now you can plant your new tree in the ground or in a larger container to grow off. Good luck!