Air layering - not to be confused with simple layering - is used to shorten established plants, as well as to produce relatively mature new specimens. It is similar to growing from cuttings, except that root formation is accomplished prior to removal of part of the plant. Rooting thus becomes easier, though air layering may itself not work.
Air layering is certainly more energy-intensive than other propagation techniques, and should be used only if other methods are more difficult or less reliable, or where more mature specimens are desired.
Early summer is the ideal time, except where otherwise marked. Cut upward into the bare stem at a 45 degree angle, about 1 1/2-2" deep, and place a match-stick in the upper end of the cut to keep it open. Dust the cut surface with rooting hormone. Soak sphagnum moss in willow extract or vitamin B1 solution overnight, and work into and around the cut, to form a ball extending several inches either direction, held in place by clear plastic. Keep moist, and support the weakened stem if needed.
Alternatively, a 1/4" ring of bark may be removed from around all or most of the stem - a process referred to as ring-barking. This is a heavy injury for any plant, but may be necessary to root more difficult specimens; it also results in faster rooting. Leaving a small 'bridge' in place can still allow for some movement of water and nutrients, and lessens the amount of injury sustained. Cover the wounded area in moss, as before.
Less drastic than ring-barking, and much slower, tourniquet-style air layering involves looping copper wire around the trunk of a selected stem, just below a nodal junction. The wire is pulled tight until it begins to cut into the bark, then, the area is again wrapped in sphagnum moss.
When roots are seen, possibly many months later, cleanly cut the new plant off, not disturbing the root ball, and pot up. A light peat-rich medium will be easiest on the new plant. Trim remaining stump. Keep both warm and shaded for several weeks.
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