Some plants naturally respond well to pruning, regardless of how intense, whilst other plants can find it hard to recover, especially when pruned at the wrong time of the year.
When should I Prune?
To prune correctly you must find out the type of plant your bonsai is and research when the best times are to prune old and new season growth. Generally, new growth is pruned during the growing season to maintain the shape of the bonsai, whilst pruning of hard wood (old season growth) is done in mid-autumn.
Forms of Pruning
Finger Pruning
One of the main forms of pruning for bonsai, especially evergreen coniferous bonsai such as junipers and cedars is 'finger pruning'. This involves pinching back new growth, which does not come within the general shape of the bonsai or is at the top of the bonsai helping to encourage bushy foliage and a more tree-like looking bonsai. To do this, take the growth between your thumb and forefinger whilst holding the branch with your other hand and remove with a twisting movement. This is better than trimming the growth with scissors; this leaves an unnatural look and leaves the foliage an unsightly brown. However for deciduous trees, scissor tip pruning is best. When trimming outward or 'overenthusiastic' growth, trim shoots back to just after the next series of leaves, but don't cut the foliage as such.
Branch Pruning
Pruning of branches is performed in spring. Much, but not all of the new growth is removed. Branches are selected early on as the only branches to be allowed to prosper, while the excess branches are mercilessly pruned off.
Leaf Pruning
Leaf pruning (also known as defoliation) in bonsai is used for several deciduous and tropical plants to reduce leaf size, remove unsightly leaves and speed-up growth by causing two seasons' growth in one. This is done in mid-summer, by cutting 60-90% of the leaves off the tree, only leaving a few to ensure that the tree keeps its energy. Remove leaves with fine scissors, cutting them from directly behind the leaf. In the next few weeks make sure that you keep the plant in a hospitable position and climate and supply it adequate water. Remember however, that this form of pruning is only applicable to certain types of plants.
Root Pruning
Bonsai trees are intentionally allowed to become root-bound in their containers, and the roots too, are pruned. But root-bound plants won't thrive forever in that condition and, indeed, bonsai trees must be re-potted every two or three years to furnish the roots with fresh soil.
Last but not the least, the main factor in maintaining bonsai is the removal of all but the most important parts of the plant. Bonsai is all about the reduction of everything just to the essential elements and ultimate refinement.
