Pruning Bonsai Trees
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There are a couple of reasons why you prune your bonsai tree. It’s both to keep the size of the tree small, and also a way of shaping and promoting denser growth in certain places. Bonsai is a fantastic hobby that combines creativity and your own expression of art with horticulture and the science of growing plants.
The two main areas of focus for pruning a bonsai are continuous pruning of smaller twigs, and basic training and shaping of the main structure of your tree. Both of these are necessary in order to reach the look of a small tree that looks like a fully grown one. Combine pruning techniques with repotting and wire-training your tree and you have all the basic techniques to successfully grow and shape a bonsai.
CONTINUOUS PRUNING
Continuous pruning is done in order to maintain the shape and size of your tree. If you have bought a ready-made bonsai tree that already has a basic shape with the trunk and branch structure you like, you will still need to regularly prune the twigs to keep the tree small. Bonsai is, after all, just a collection of techniques for growing a tree or shrub. If you stop pruning the plant it would eventually become a large tree or bush again.
On deciduous and evergreen trees, branches should be allowed to grow for a couple of nodes before pruning, leaving two to three nodes. This will force the plant to branch out on the nodes left on the twig. Repeat the process with each branch and eventually you’ll end up with great ramification on the branch and a dense pad of foliage.
A node is the spot where a new branch can start growing on a twig. It's just above where a leaf attaches, and you can often see a small bud between the leaf stalk and the twig on many trees when looking closely.
For trees that hav an opposite leaf arrangement (two leaves attach to the same spot on the twig opposite to each other) you only need to save one node, as it can result in two new branches from pruning. Examples are: Chinese privet and japanese maple
For trees that have an alternate leaf arrangement (only one leaf attaches to one point along the twig, in an alternating pattern) you need to save at least two nodes in order for it to branch out into two new twigs. Examples are: Chinese elm and ficus trees
BASIC TRAINING
When starting a bonsai from scratch from a “regular” nursery plant or from a tree or bush from your garden there are a number of different techniques you can use in order to shape a base structure of the trunk and main branches. It’s fun, but it’s also not that easy. As a beginner to bonsai you might want to start with a finished or partly finished tree instead. Starting a bonsai from scratch might take several growing seasons (years) before it’s ready to be planted in a smaller proper bonsai pot. During basic training of the tree it’s often grown in a much larger pot or even in the ground, and you often let the tree strategically grow much larger than the end vision.
By letting a branch or the trunk grow very long it’s a fast way of thickening the branches where you want to. These branches are meant to be sacrificed and removed as soon as they have thickened to the desired level. These are strategically grown at certain parts of the tree to thicken it.
When it comes to the hobby of bonsai, a big part of it is studying different techniques. There are many great written resources out there, and a lot of video content that illustrates detailed and niche techniques for many different species of trees in different styles of bonsai. Since the subject is seemingly endless we recommend reading a lot to get inspiration and take part of other grower's experiences with certain trees.
TOOLS & MATERIALS
There are a number of different tools you can find adapted to growing bonsai. There are also some alternatives you can find that aren’t specifically meant for growing bonsai but will work well either way. Some of the tools and materials used when pruning are:
Bonsai scissors or small pruning shears (preferably with narrow, straight blades)
Bonsai cutters or knob cutters
A small saw
Wound sealant paste
When pruning bonsai it’s recommended to have small bonsai scissors or little garden shears for pruning the finer twigs of the tree. Choose a tool that suits you and makes it easy to reach twigs deeper in the foliage. The blades should be rather sharp in order to cut the twigs clean.
When it comes to removing bigger branches it’s best to use proper bonsai cutters. Either with a straight blade or a knob cutter for removing whole branches from the trunk. These are very sharp and leave as little damage as possible on the surrounding bark. Straight blade bonsai cutters are more versatile, while knob cutters will take a clean chunk of the wood when cutting. This is perfect when removing a whole branch as it leaves some space for the bark to grow over the wound and end in a smooth trunk once grown over the wound.
A small saw can be handy when you need to remove branches or parts of the trunk that are too big even for the cutters. Choose a small saw with small serrations on the and a thin blade that will make a clean cut edge with as little frayed and harmed bark from the cut as possible.
Wound sealant paste is highly recommended by some growers while others think it's unnecessary. There are many different ones out on the market. Some contain active ingredients that are said to promote faster healing of the wound. Even without it, the tree will eventually heal the cut wound.
WHEN DO I PRUNE?
For tropical bonsai grown indoors all year around (like Ficus, Ehretia, Portulacaria etc.) it’s recommended to use grow lights during the darker months of the year. This will enable the plant to grow well all year around and in turn also mean that you need to prune the plant regularly regardless of what time of year it is.
For deciduous temperate bonsai you can prune it before the leaves break dormancy in spring, during late spring or early summer as the first growth starts to harden a little for very fast growing trees, or closer to fall a second time of the growing season.
Note that pines are a special case of tree that you only prune in spring during the short time their candles (the shoot of yearly new growth on each twig) start to expand. This is the only time of the year you can prune pines and it's often recommended to break the candle at half its length or a little more. You can also prune them with a tool. This should be done as it starts to grow in late spring or early summer, but before the needles are expanded fully.