Without doubt the most important way to train a bonsai is to prune it on a regular base. Essentially, there are two different techniques: maintenance-pruning, to maintain and refine the existing shape of a Bonsai and style-pruning, which involves more rigorous pruning to give a tree its basic shape or style.
Before handling both techniques in more detail it is useful to look at some background information on how trees grow. This will help us understand how to prune bonsai trees most efficiently.
Trees have a natural tendency to distribute growth to the top (and to a lesser extent outer parts of branches) which is called ‘apical dominance’. This natural mechanism encourages trees to grow higher in order to prevent it from being shaded out by competing trees. By distributing growth to the top and outer edges the tree’s inner and lower branches will eventually die, while top branches grow out of proportion; two effects not desirable for the design of Bonsai trees.
This basic background reveals the importance of pruning as a technique to counter apical dominance. Countering apical dominance is achieved by pruning the top and outer portions of a tree more thoroughly, forcing the tree to redistribute growth to the inner and lower parts.
The goal of maintenance pruning is to maintain and refine the shape of a tree. As explained above, trees will concentrate most growth on the top and outer parts of the tree; it is important to prune these growth areas regularly in order to encourage growth closer to the inner parts of the tree.
Maintenance pruning can be done throughout the growth season.
As mentioned previously, maintenance pruning is required to maintain a trees' shape. To do so, simply prune branches/shoots that have outgrown the intended canopy-size/shape using twig shears or a normal cutter (see photos 1 and 2, below). Do not be afraid to prune your Bonsai; it is important, especially in the outer and top areas, to prune regularly in order to force the tree to distribute growth more evenly and develop a dense foliage.
As opposed to deciduous trees, pine trees and conifers should be pinched by hand. Using scissors to prune conifers would lead to dead brown foliage at the cuttings (see photo 1, below). To prevent this from happening hold the tip of the shoot between your thumb and pointing finger and carefully pull it away; the shoot will snap at its weakest point and no brown ends will appear (see photo 2, below).
Another method of Bonsai pruning is defoliation, which involves removing leaves of deciduous trees during the summer to force the tree to grow new leaves. This technique ultimately leads to a reduction in leaf size and an increase in ramification. For more information check the defoliation page.
To give a tree its basic shape often involves pruning large branches. Deciding on which branches should stay and which ones should be removed can be difficult, not only because it is an irreversible action but also because it is part of deciding how the tree will look like. Before learning more about the techniques used for pruning Bonsai, you might want to take a look at the Bonsai progressions part of this website, where you will find examples of experienced Bonsai growers style-pruning nursery stock.
Overall, the early spring or late autumn is the right time to style-prune a tree (just before and after the growth season). In the tree species section you can check your particular tree, a Ficus Bonsai needs different timing from a Juniper bonsai for example.
Place the tree on a table at eye-level; first step is to remove all the dead wood from the tree. Now take some time to observe your tree and decide which branches do not fit the desired design and will need to be removed. A few guidelines are listed below, but deciding on the future design of your tree is a creative process, not necessarily bound by 'rules'. The bonsai styles page can help.
Pruning thick branches will result in creating ugly scars on the tree, but by using a special concave cutter you will reduce this effect significantly because of the indentation it makes when cutting off the branch (see photos 1 and 2, below).
A healthy tree should have no problem coping with pruning up to 1/3 of the trees foliage. Some theories prescribe to cut/remove an equal percentage of roots after a tree has been styled. Most experts however agree on performing only one big maintenance per time (or even once per year). This would mean that you style-prune one time and wait with repotting until the tree has fully recovered. More details about pruning roots can be found at the root flare page.
Finally, it is advisable to seal large cuttings with wound paste, available at most (online) Bonsai shops. The paste protects the wounds against infections and helps the tree to heal faster.
After a tree has been style-pruned place it in the shade and out of the wind. Fertilize as you would do normally and let the tree recover from the training for at least a few months.