The wiring and bending of branches that give bonsai its shape is unique to the art. Wiring is done after pruning when the tree has been thinned to essential branches.
Copper wire is usually used for shaping bonsai because it is flexible. The sizes of copper wire that are best for bonsai are: 10, 12, 14, 16, and 18. (No. 8 wire is heavy and should be used only for the trunk.) Wire as light as No. 16 should be used for very thin branches, and for tying rather than bending.
Wire evergreen trees only during their dormant period when the branches can be shaped without damaging growth. Wire deciduous trees only during their growing season.
The day before you wire a plant do not water it; this will make the branches more flexible. Once a branch has taken on its trained form, remove the wire, straighten out its twists, and flatten it with a mallet for reuse.
Wiring and shaping should begin at the lowest point of the tree, working upward. Do the following when wiring:
- Anchor the end of the wire at the base of the tree before winding it. Push the end of the wire deep into the soil.
- Wire from the trunk to the main branch. Use a foam pad under the wire to prevent damaging the bark. Keep the turns about 1/4-inch apart and spiral upward at a 45-degree angle. Do not wire too tightly, and do not damage the leaves or stems.
One length of wire can serve for two branches by anchoring the center of the wire at the trunk.
After wiring, the plant is shaped or bent by hand. The trunk and main branches are gradually bent in the planned direction. Never try to straighten a branch that has been bent; this may split the bark.
Branches sometimes snap, even when carefully wired and bent. If the branch is not completely broken, rejoin the broken ends, and wind some garden tape around the break. These fractures often heal quickly. If a branch snaps off, prune back cleanly at the first side branch.
Wire should be kept on the plant for not more than 1 year. Remove the wire before the bark becomes constricted; ridges will form if the wire is left on too long. When removing a wire, start at the outermost end of the branches, and take care not to harm leaves, twigs, or bark.