There are hundreds of questions on bonsai forums, at clubs, and across the globe about pruning. What to prune, when to prune, why we prune, which species can I prune now, how often do I prune, etc. I want to help clarify these questions.
I’ll start with explaining why we prune. There are 3 main reasons to prune:
We clean to improve airflow and allow sunlight to penetrate into the interior and lower branches. This is also a precursory action to wiring and styling the tree. Trying to wire a tree without cleaning it first can be miserable, especially when doing it on a needle juniper. Cleaning consists pruning off weak interior growth with little to no ramification and clearing any foliage in the crotches between the trunks and branches. This includes leggy branches that are not developing in the desired direction.
Pruning is the way we control growth on our trees. This is how we tell the tree to direct more resources towards a specific branch or section of the tree. This helps to keep your branches a specific length or thickness and give them directional movement to draw interest. This is also the technique we use to gain width in certain branches and keep others from thickening too much. I will go into more detail later on how to do that.
Structural pruning is the way we improve taper, inverse swelling (taper), direct the line of the trunk and branching, and remove flawed or unnecessary branches. Being able to prune effectively is the foundation of successful bonsai practice and design.
Before we go pruning anything we need to step back and observe a few things about the tree we’re about to prune. We need to identify:
- What species is the tree?
- What is the tree’s current health?
- Did you repot the tree within the last few months?
- Why am I pruning?
- Is the ______(tree, branch, etc.) that I’m about to prune in refinement or development?
- What season are we in?
- What is the effect on the tree’s energy and health if we prune now?
1. Some species hold significant amounts of energy in certain parts of the tree. Junipers hold significant energy in the foliage. When someone interested in bonsai buys a juniper and then prunes a lot of the foliage off they frequently die, this is why. Pines hold their energy in the roots. Deciduous trees hold theirs throughout the tree. Other species have certain timing for an additional flush of growth (Japanese Black Pine) or timing to prune for flowers.
2. If the tree is not healthy, don’t do any work on it unless it involves solving the health issue.
3. If you recently repotted the tree you should avoid pruning so that the foliage mass can help restore the root system that you worked on.
4. The purpose of pruning can include any of the aforementioned reasons – improving structure, controlling and directing growth, or cleaning.
5. We need to understand the stages of bonsai. If you’re not sure what stage your tree is in do some research before.
6. What season we are in can be a great factor in determining if we can prune without damaging the health of our trees. With most coniferous, deciduous, and broadleaf evergreen species the seasons function in the same way. Winter is a period where the tree doesn’t metabolize freely and we often refer to it as dormancy. Spring is a period of heavy growth. There are several periods of using the trees resources to produce more growth, then gaining those resources back through photosynthesis. In Summer some trees can go into a Summer dormancy if the temperatures get above 90° regularly. If you are in a more mild climate your trees may not slow down growth completely. Fall is a time where the trees save energy and focus on thickening branches and roots to help with Winter hardiness.
7. An example of a bad time to prune would be in Spring right after your tree has pushed out a ton of new leaves. If you prune the branches off before the leaves have been able to form a cuticle (finish growing) and reaccumulate energy, then you’re weakening the tree by taking away its ability to gain energy back.
A generally safe bet is the prune in early Spring before buds pop open or additional candles / needles start growing.
I’d like to also state that you can operate outside of these general guidelines, and depending on how you care for your tree afterwards, you can prune at almost any time. However, I would not recommend pruning whenever you please without extensive knowledge and experience.