I thought I would shed some light on this topic. There are a few techniques when it comes to cutting branches on a bonsai tree. I will first try and explain the best I can the difference between trimming and pruning. After that I will share a few useful tips on pruning.
The first one is trimming. Its like when your beard or hair becomes to long. You simply trim the top off and if it grows out you repeat the process. The same with bonsai. You never stop cutting back on the new growth. By trimming your bonsai tree regularly you can redirect growth to areas that are weak, you can reduce leave size and get finer ramification. So how does trimming work? As a general rule you cut back to two sets of leaves after every six to eight sets of leave growth on a branch. I say general rule as this might differ depending on the tree you are working on.
Here you can see a Wild Olive tree (Olea europea subsp. africana) trimmed back into shape.
Before I get caught up in the detail let me explain pruning. Pruning is slightly more cutting. With trimming you are maintaining the shape of you tree. As with pruning you are shaping your tree. The two commonly used methods are directional pruning and winter pruning. Now I do not know how these techniques where discovered but I know why we use them. With winter pruning you prune new shoots after all the leaves have fallen (normally a deciduous tree in the middle of the dormancy period which would be winter ) before new shoots emerge. There are many opinions as to why this is done. How it has been explained to me and how I understand it is as follows: The tree is in a dormant state so cutting the tree, when the sap doesn’t flow or flows at a slower pace. The tree is not disturbed to much and is under less stress. Then spring comes and the tree’s sap starts to flow at the normal rate the wound should have healed over and there would be less sap leakage as the rate increases. All the energy of the increased sap flow can then be used by the tree to encourage more shoots and even back budding if you are lucky.
Here is a Celtis africana that has been winter pruned.
I mentioned directional pruning. This can be very helpful. If you prune to a bud that faces to the right the new branch/shoot will come out in that direction. This makes it easy to shape your tree. This is also a good technique to use if you are keen on starting a bonsai from seeds or cuttings. You can direct the growth to where you want it.
Below you can see a tree that I have pruned heavily at the bottom. I left the top as I want to grow in a new apex and for that I need all the growth to go to the top to thicken the branch. We call this a sacrifice branch as I will remove the top part of the branch after the branch has reached the preferred thickness.
A few useful tips
When pruning please keep in mind that you should not remove too much of the foliage on the tree. I normally try not to remove more than a third of the trees total foliage mass. Do not prune weak trees, as this may only damage the tree even further. If you see any leaves that are hanging/wilting move the tree to a more shaded area and let it recover for a few weeks before you prune. Use sharp tool to cut of branches. You do not what to tear of branches as these scares will dry out and leave brown marks on the tree. Do not leave long stumps, if you remove a branch completely as it will take long for the bark to callus over this. Rather use a specialized bonsai cutter for this. You should use a bonsai side cutter or a bonsai concave cutter.
Here are a few different cuts made on trees. Some of them have healed over completely others are new and starting to callus over.
Here are a few cuts that should be avoided. These will take a while to be rectified.
Before pruning a tree find out more about the tree and it’s growing habits so that you can use the correct technique on the tree. Ask for advice at your local bonsai club or nursery.