How to Prune your Bonsai in Spring

Naturally trees tend to be large and thick and inevitably have disorderly shapes. In order to remove the disappointing and unaesthetic parts of a bonsai, you need to prune its branches, buds, and leaves.  It is not enough to cut off  branches or to merely reduce a tree's size. You also need the smaller branches and leaves to develop fully. If you prune part of a branch, then new buds will grow out of the lateral bud of what remains and they develop into new branches. In this way branch pruning adds new branches and leaves to the tree.

Pruning is the key to make from a regular tree a fine work of art. The goal of pruning is to cut off the tree parts that disturb the plant's shape so as to reduce the tree's overall size and to improve its appearance.
Bonsai are grown from ordinary, normal, not dwarf, species so their small size is basically the result of pruning, both branches and roots. 

Root pruning should be done following the same protocol as branch pruning because it affects the basic health of the tree. The same way, pruning branches and leaves will affect root growth. So, whatever you do above ground level will also affect what happens to the tree below the ground and vice versa.
The first step in how to prune a bonsai tree is that you always want to make sure you are pruning the tree in the appropriate season. Although there are some types of trees that can be pruned and trained throughout the year, many trees must be worked with during a specific time. The best period for pruning is when it brings minimal damage to the tree. Generally speaking, for deciduous trees it
is just after their leaves fall, and for conifer trees it is in the end of the tree's dormancy. For certain species of trees, if you prune in winter, then in the following spring or later the branches may develop overlong internodes and the large leaves will grow very dense.
Pruning the branches and leaves on the outer side of the bonsai tree stimulates the growth of those on the inner side of the tree, where the branches, buds, and leaves tend to wither due to insufficient sunlight and poor ventilation. Moreover, pruning the exterior branches and leaves makes the interior ones more visible. This brings out a sharper view of the entire bonsai. It also facilitates branch wiring.

There are many things we need to discuss properly, in order for you to understand the basic concepts of pruning a bonsai. I have read many books and several webpages about different pruning techniques and they all try to explain everything in only one or two pages of a book or in a couple of paragraphs in a website and it is impossible to do that. There is too much information to share and too many details to go over, that it may be even overwhelming for you and for me to go through everything in one post, so today we are only going to focus on Branch Pruning. (it will be more than enough time to write a post about Root pruning and Trimming techniques)
When pruning, you want to focus the majority of your attention on the trunk, which will add to the illusion of mature size and age, more than any other feature on the tree.

We have two ways to prune your bonsai.
It can be either a Heavy or Styling method and a Light or Decorative one.

Styling Pruning

It is the process of creating from an ordinary piece of nursery stock into a pre-bonsai. It is a very stressful for the tree so it should only be done when the tree is dormant, at the end of winter, beginning of Spring. If it is done too late in spring, the stress could kill the tree.
It depends on the tree, so the process could include decreasing the height of the tree by cutting off the trunk just below the final visualized height of the tree (that is the top of our prospective bonsai). You can imagine your bonsai or you can sketch the idea or you can even use some modern gardening software to design your bonsai style.

But for all trees, it will include removing undesired branches. This could include removing up to eighty percent of the tree’s foliage areas in a deciduous tree, but for evergreen trees, fewer leaves and branches means less for the root system to supply with water and nutrients and it could mean certain death, so those trees should be shrunk in stages. Remember that Heavy branch pruning will slow the growth of the root system. 

Styling pruning on your rookies may include cutting back the trunk, sometimes as far as the trunk base (drastic pruning). This is usually done to make your tree more conical. For some deciduous trees, this can be done by simply removing the unwanted trunk, cleaning up the cut, and waiting for new buds to form to choose a new leader.

It is better to leave around one inch (2.5cm) or more of the unwanted trunk  above the desired new leader to allow for dieback. If you are going to jin the branch, leave the stub long. If not, leave the stub short. There is no need to protect the stub from dying out (because that is actually what you want to happen). This will allow the new leader to become strong enough to survive after the cut is cleaned up. The stub should be cut off cleanly horizontally. This is done to allow the use of the best leader from the new buds that will erupt. If it is done in an angle, the growth will start from the tip of the pruned branch  and we have to repeat the process.  


Decorative Pruning

It includes the shrinking of secondary branches, the removal of some unwanted branch or the restructuring of the apex (the top of the bonsai) and it includes pinching back and leaf removal (defoliation). It does not change the big picture of your bonsai because you don't change its main structure but it will improve the look of an established tree that already when through an earlier heavy pruning. This type of pruning it is not that stressful for the tree, so this can be done at the beginning-mid spring. Pinching back can be done (in 90% of the species) throughout the season and leaf pruning (trimming) can be done during the summer.

Which tools should we use?

You may wonder which tool to use, well, for a large trunk, a pruning saw can be a great asset. Remember that the Japanese saws cut on the pull stroke, and leave more wood than you think you will want, so that the cut can be reduced and shaped. Once you have the main part of the trunk removed, use concave or spherical knob cutters to reduce the cut. 
Then, clean up the edges with a grafting knife or razor blade and seal the cut. You can buy a good grafting knife and learning how to sharpen it because you will not want to damage the living tissue. Trim in a rounded curve that will heal naturally. Do not make this a concave cut, but hollow it out just a little; as it is too large to heal properly and can hold water and promote rot. 

Finally protect the cut with a good gummy type wound sealer. But be careful, some pastes may actually be a good grafting sealer but can prevent the cut from cicatrizing properly. So I recommend to use bonsai made sealers, or believe me or not, regular school glue works great.

Remember that the Styles of bonsai are not rigid laws. They are only recommendations, based on observations about what makes certain trees beautiful.

Even though every bonsai has a front, it is important to note that the famous trees have both front and back branches as well as branches at the side. Thise gives a perfect balance to any tree and that is what you want to have in yours. A common mistake is to cut a piece of nursery stock until one has just a few branches left, but keep in mind that is better to leave more branches than you may ultimately use, because they can always be pruned later on but they can not be put back.

Once the main design of the tree has been established with a Hard Pruning, Decorative pruning combined with wiring will continue to advance the style of the tree until it is “complete.” (but do not get too anxious!! because there is never a complete bonsai, even the 200 years one are still wired and pruned to reshape their appearance each year). The branches should mirror the trunk. As we select branches at the outside curve of the trunk, we also select sub-branches on the outside curve of the branches. If the trunk has drastic angles or gentle curves, the branches should do the same. With most deciduous trees, this is accomplished more with pruning than wiring, although some wiring may be necessary. Be sure you leave extra branches on any weak area of the trunk to help promote growth in that particular area.
To prune the lateral branches, you should know the budding habits of your tree. This is where specific species knowledge is necessary. Usually, you will want to prune back to a bud that is growing in the direction you want the new growth to go. You will want to remove any growth downward or straight up; those that cross each other, and those growing back sharply toward the trunk and on trees that grow with opposite buds, you will want to prune to just two buds at each branching.  

You need to provide your tree with adequate time to recover after going through a severe pruning. With proper care and scheduled pruning, your tree will grow and remain healthy for years. After each pruning, new branches and leaves will grow out of the lateral buds and so will more smaller branches and leaves emerge.