Wiring a bonsai tree is done to hold branches in place and train them into new shapes. It is sometimes believed that wiring is used to keep the plant small, but this is not the case. By holding the branches and trunk temporarily in place, it is possible to achieve the various styles of bonsai that contain bends, like the Cascade or Informal Upright. Some species do not even require wiring in order to be trained. Just was with pruning, it is important to research the type of tree you are wiring in order to determine the best time of year to attempt it.
Two kinds of wiring can be used for training bonsai. Aluminum wire is easier to work with, and copper can also be used. Wire is measured by thickness, generally from 1mm to 8mm, for bonsai purposes it is recommended to use a thickness from 2mm (for the thinnest branches) up to 3.5mm for the thickest or the trunk. The size of the wire is also dependent on how much the branch needs to be trained, as well as how the particular species takes to training in general.
It is important that the bonsai be in good health before attempting to wire, as some branches make break with the pressure. Wiring is done from the bottom up, anchoring the wire first in the soil. It is a mistake to attempt to bend the branch first and then wire it, the wiring is done first and then it is much easier to bend the branch to the desired position. The wire should be at a 45 degree angle to the branch it is being wrapped around. Lots of small coils of wire around the branch are better than fewer, further apart coils. The wire should not be wrapped tightly as the bark of the tree will cut into the wire with growth.
After a branch is wired, it should be bent carefully while listening and watching for signs of breakage. Some species of plant are much more susceptible to breaking or splitting than others. If a branch does break, it is best that it be pruned back completely. Before wiring and bending the branches, the plant should be allowed to dry out and not watered for a day before. This will allow the branch to be more malleable. The tree should be checked every few weeks to make sure the bark does not begin to grow into the wire. If it does, it will leave cut marks that may well be permanent. When it comes time to remove the wire it is best to use wire cutters on each coil; although some bonsai enthusiasts will uncoil the wire whole, it is difficult to straighten out.