In Japanese: “Toriki” - A slightly more advanced technique to propagate Bonsai is air-layering.
The principal of layering is to force a tree or branch to form new roots at a certain point by interrupting the stream of nutrients from the existing root system. This means you can use air-layering for several purposes; reducing the length of a trunk, growing a better Nebari (root flare or surface roots) or selecting a branch to be grown as a separate tree.
Air-layering should always be done during the spring, when the tree already started growing after its winter rest.
How to air layer a Bonsai?
There are two main techniques to air-layer a tree; the tourniquet method and the ring method.
The tourniquet method involves tightly wrapping the trunk/branch with copper wire to block the stream of nutrients partially. When the trunk/branch grows thicker the stream of nutrients will decrease more and more, forcing it to grow new roots just above the wire. This method is used for rather slow growing trees that need more time to grow new roots; these will not survive the more aggressive ring method.
The ring method involves cutting away a ring of bark at the point on the trunk/branch where you would like new roots to grow. The portion above the ring will have to grow roots immediately in order to survive. The ring should be wide enough to prevent the tree from bridging the gap.
The tourniquet method
Wrap a piece of copper wire all the way around the trunk/branch right at the point where you like new roots to grow. The wire should cut about halfway into the bark; the thicker the trunk/branch the thicker the wire should be (see photo 1, below).
Dust some rooting hormone (available at Bonsai shops) around the ‘wound’ and now wrap a good quantity of sphagnum moss around the wound, covering it with some plastic (see photo 2 and 3, below).
The ring method
Use a sharp knife to cut two parallel slits around the circumference of the branch (keep enough space between both slits, at least once the diameter of the branch).
Now remove the ring of bark between these two cuts right till the ‘shiny’ hardwood (see photo 1 and 2, below).
Make sure the ring is wide enough so the tree will not be able to gap the wound; also make sure you have removed the bark all the way to the hard wood; the tree will not start growing roots unless it has no other choice.
Dust some rooting hormone (available at Bonsai shops) around the ‘wound’ and now wrap a good quantity of sphagnum moss around the wound, covering it entirely with plastic (see photo 3, below).
And then? Aftercare
The moss should be kept moist at all times. After about one to three months roots should be growing in the moss. When the bag is filled with new roots carefully cut the layer just underneath the new roots. Do not try to remove the moss or sort the roots; simply plant the entire bundle without disturbing it in akadama, fine gravel and potting compost mixed together in a ratio of ½ to ¼ to ¼. Keep the tree protected from low temperatures and wind; a greenhouse or cold frame can be very useful. Leave the tree untouched until the next spring, when it can be trained for the first time. Small quantities of fertilizer can be used during the first summer.