With the large number of tree species commonly used for bonsai and the wide variation of climates in which readers will be wiring, it is impossible to state exactly when your tree should be wired. There are also pros and cons of wiring at any particular time of the year with any particular type of bonsai. Theoretically, most tree species can be wired at most times of the year though trees wired during the winter may need frost protection in certain climates.
The only time that wiring can negatively impact the health of your tree to a large degree is during winter, in areas where temperatures regularly drop below 15°F. In temperatures this cold, any fractures that have not healed will be exposed to the cold and possible future dieback of the branch.
In warmer climates, the best time to wire deciduous trees is just as the leaves fall in autumn. With the branches bare it is much easier to wire and adjust the branches with a complete view of the tree. The branches should heal all but the largest cracks or severe bends before the tree becomes completely dormant for the winter.
Deciduous trees can be wired in spring before the leaves open but great care must be taken that the new leaf or flower buds are not dislodged. Deciduous and broadleaf trees can be wired any time through the growing season but when in leaf it is more difficult to study the structure of the tree and wiring around the leaves is more difficult. Branches wired at this time; particularly new shoots, will heal very quickly. On fast growing species, keep checking every few days that the wire is not beginning to dig in.
The second best time to wire deciduous trees is at midsummer after defoliating the tree. Again with the branches bare, wiring is clear and easy; the branches should take to their new positions before the end of autumn.
Coniferous species can be wired at any time from spring through to autumn. Coniferous species will continue to heal over winter so they can still be wired in autumn. As the wire on Coniferous species needs to be on the branches for a longer period of time; often over winter, frost protection is needed if temperatures drop below 15°F.
Coniferous species need wiring annually and need at least one complete wiring of the entire tree for a successful design. They are best wired (particularly if heavy bends are to be made) from late midsummer through to early autumn.
By late midsummer, new growth will start to need wiring and will heal faster than at most other times of the year. Many species such as Pines will have also made most of their annual increase in branch thickness by August; wiring after this time will allow the wire to stay on the tree until the following year without cutting in and scarring the bark.
Coniferous species can be wired in the spring and this growth will set into position relatively quickly but will need reapplying as the years new growth appears. Any wire still on the tree by midsummer should be checked regularly to avoid wire scarring; particularly on Pines that swell suddenly around this time.
Tropical species can be wired at virtually any time of the year as they are protected from frost and have little or no dormant period. The wire can cut in fast due to the typically vigorous growth of tropical species and must be checked very often.
The wire should be removed after 6 months. Usually the branch should then stay in that position on its own. Wire should be carefully cut from the branches. Do not unwind wires as this could break the branch. Use your wire cutters for best results. If you try to unwind, you could conceivably, snap a branch.
If a branch should snap, the ends can be rejoined if they are not completely broken. Wind some garden tape around the break. If a branch snaps off, prune it back at the first side branch.
After spending so much time making your bonsai look the way you want it to, you must, of course, take care of it!