Anne Spencer -- Reference: Bonsai, The Art and Technique, Dorothy S. Young 1985, Prentice-Hall, Inc.
The purpose of wiring a bonsai is to offer the bonsai grower control over positioning a trunk or branch to affect the final shape of the tree. It is not a replacement for pruning. It is a tool for your tool kit. It is a skill that requires practice.
One may wire anytime, but it is most easily and safely done just before growth in the spring. Don't do it when a plant is in flower, or has very tender, new foliage. Do it before repotting rather than just after you have worked on the roots.
Two kinds of wire are used, copper and aluminum. Copper wire is stronger than aluminum wire of equal thickness. The thickness of copper wire is specified by-gauge. The smaller the gauge number the thicker the wire. The thickest copper wire generally used for bonsai starts with #6 or #8 gauge and the thinnest is #22 or #24. Aluminum wire is measured not in gauges but in millimeters, starting with the thinnest 1.0 to the thick 6.0. The aluminum wire is copper coated to look better on bonsai.
Copper wire needs to be annealed to be pliable. An easy way to anneal it is to do it in a self-cleaning oven. Roll the wire up and put it on a rack in the over & set the oven to clean, for about 3 hours. After the cleaning and cooling process, carefully put newspaper under the wire before you take it out of the oven. Take it outside, shake off the black residue and store away for future use. Bob Sigourney of the Portland Bonsai Society has been doing annealing this way for 15 years. Annealed wire retains its pliability until it is bent for flexed, which causes it to harden. Copper wire may be repeatedly annealed and reused. Aluminum wire is softer and does not need annealing.
It is your choice which to use. Copper is a bit harder to use, but it holds well and doesn't stretch. Aluminum is softer and easier to use but does stretch it certain situations.
To choose a wire size, flex the branch to be wired and choose a wire just a little stronger. If that wire doesn't hold the branch, put on another wire beside the first to get more strength. Use a length of wire that is estimated at one and a third times the linear distance to be wired. Experience helps when choosing and using!
This is the wiring sequence that most growers find effective: Wire the trunk first, branches next, and twigs last. Wire a bonsai from the bottom upward and the interior outward. Apply the heavier wire before the lighter wire. Where more than one strand of wire will be needed on the same part of the bonsai, use the heaviest wire first, followed by successively thinner wire. Keep all additional wire parallel and close to the first wire. Coil the wire directly on the surface of the branch or trunk without pressing' leaves, needles, or small shoots beneath the spiral.
See the attached PDF from Bonsai Today magazine (issue #63) to see and read how your fingers must move and wind the wire onto the plant. If you are right handed, you will hold the wire with your right hand while holding the just wired section with your left fingers. Wrap the wire as tightly as possible without bruising or pressing into the bark. Loosely applied wire with space between the wire and trunk is unsatisfactory because it has little holding power. It needs to touch along its entire length without causing indentations. Try to keep the spirals evenly spaced, with the angle between 45 and 65 degrees. Heavy wire is more widely spaced than thin wire.
Predetermine where you will be bending the branch or trunk and then do it and stop. Bending a wired section first one way and then another bruises and may destroy the cambium in many places.
If a wired section is to be twisted either clockwise or counter-clockwise, wind the spirals in the same direction. This will strengthen the grip exerted by the wire as it is twisted.
Try not to cross wires, but it you have to, do it at the back of the tree so scaring won't affect the looks from the front. Think ahead, using a string or thin wire, to plot the path of a wire so you won't have to cross wires. One CAN wire a whole tree without crossing wires.
To bend a branch or trunk, start at the thickest section, work outward, and slowly inch along. Hold the trunk or branch tightly and bend it with a slow, steady pressure. Do not force the wood beyond the limit of its flexibility. This varies among species. The flexibility in most species may be increased by withholding water for a few days prior to wiring, or `exercising' the branch ahead of bending.
To bend a branch downward, place the wire over the top of the branch at its juncture with the trunk. If the branch is to be raised, bring the wire beneath the branch. On each branch, continue the wire outward beyond the section to be changed. If a wired section is to be twisted either clockwise or counter-clockwise, wind the spirals in the same direction. This will strengthen the grip exerted by the wire as it is twisted.
To remove the wire, either cut it off with Japanese wire cutters, or unwind it carefully by hand. Don't ever use any of your other tools to cut wire; it will ruin them. Use a wire cutter. After removing the wire, if the new position is not yet set, rewire the same trunk or branch along a course slightly above or below the previous wire.
Leave the wire on the plant until the wood `sets' and stays in the `new' place. This could take 3 weeks or several years, depending on the species and the time of year the wiring is done. Regularly check wired plants for tightness and marking. You can always remove the too-tight wire and put new wire on.
The more you practice putting on wire the more skilled you become and the easier it gets to use this technique to improve your bonsai. A slight change of the branch or trunk in one direction or another may create beautiful negative space in a young or old bonsai. Don't miss the opportunity by thinking wiring is too hard. Add this skill to your list of tools!
Bonsai wire is also used to wire screen into pots before planting the bonsai. It is a real irritant to have to start all over repotting when an unfastened screen accidentally gets moved off the hole, so always wire them in. Wire is used to tie the plant to the pot, so there is no way it will fall out after it is potted. It is wise to do this.
Download PFD: Bonsai, The Art and Technique