Bonsai tree care is a unique artform and it deviates from other kinds of houseplant care in some fundamental ways.
While most houseplant parents want their plants to keep growing bigger and re-pot them regularly so their root systems can continue to expand, bonsai artists will often do just the opposite.
Bonsai trees are intentionally prevented from growing taller and once they have “finished,” or reached an artist’s desired size. One of the techniques used to keep bonsai trees small is root reduction through pruning.
Remove your bonsai from its pot and comb out its roots with a root hook. The first time you prune your bonsai’s roots, you will need to cut its tap root severely. Keep younger roots and cut off older, thicker ones. Use disinfected tools and don’t cut off more than two thirds of the roots at a time.
The basics of trimming bonsai roots
Bonsai roots need to be trimmed to stunt the growth of the tree and to keep roots from continuing to grow outwards in search of more nutrients, which would help the tree grow larger if it were living in its natural habitat.
Bonsai are traditionally grown in very shallow pots, which means that their roots quickly come to take up their entire container. This condition is known as being rootbound.
This leaves very little room for soil or water in the pot and means that bonsai sometimes need to be watered multiple times per day.
If a bonsai is left rootbound for too long, it will start to die. Prune your roots at the right time to avoid having to try to bring a bonsai tree back to life.
To keep a bonsai happy in the same shallow pot and help it retain its small size, its roots need to be pruned before it is re-potted back into either a mid-sized “training” pot, or its “final” pot.
The first step when pruning your bonsai’s roots, is to remove it from its pot. Next, you will want to use a fit-for-purpose tool, a root hook, to disentangle its root ball.
Use your fingers to remove as much of the potting mix left in your bonsai’s roots as you can. That said, you do not need to get rid of all of the soil.
Do not be tempted to push remaining soil or bark bits out of the roots using a strong stream of water. Using water to wash the roots may damage the finer, younger roots, which are what you want to be taking special care to preserve.
Once you have combed out its roots, you will need to see how much of the root system you can safely cut off. You should never cut off more than two-thirds at a time.
The first time you prune your bonsai’s roots, whether you have grown your bonsai from a cutting, from a seed, or have purchased it in a nursery pot, you will need to cut down its tap root severely.
It is usually recommended that this be done with a small saw, and that you cut two thirds of the tap root off.
If you have purchased your bonsai in a bonsai pot, you will not need to cut the tap root down so severely, as this has likely already been done.
Make sure you are realistic about which pot you will be able to fit your bonsai into once you have trimmed its roots.
There is no point cramming your bonsai tree into a tiny, shallow pot before it is ready to live there healthily. You will damage the roots and will have to retrim and repot again in no time at all.
Disinfect your tools before using them to prune you bonsai to avoid infecting its roots with foreign bacteria.
For best results, you should use stainless steel bonsai root scissors. However, you can also use a pair of general pruning scissors to prune your bonsai’s roots. If you are cutting your tap root for the first time, you may need to use a small saw.
As long as you can make clean cuts and have disinfected your tools, your pruning should be a success.
Water your bonsai tree immediately after re-potting to allow its roots to get used to absorbing water in their new size.
When to trim bonsai roots
How often you need to trim your bonsai’s roots depends on several factors.
First and foremost, it depends what kind of tree species your bonsai is, as some trees need to be watered more than others.
Secondly, it depends on the container you are growing your bonsai in. The shallower the container is, the more frequently your bonsai’s roots will need to be pruned.
Thirdly, it depends on the environment your bonsai is growing in. The hotter it is, and the more sun exposure it gets, the faster it will grow and the more frequently you will need to re-pot it.
As a general rule, bonsai roots should not be trimmed during the growing season. The exception to this rule is if your bonsai is starting to struggle.
If it is exhibiting symptoms of chlorosis, for example if your bonsai’s leaves are turning yellow, or it is beginning to be pushed out of the walls of its pot by its roots, then you will want to make one to two cuts to the larger roots.
Wherever possible, however, you should keep any major pruning for the dormant season.
Cutting a plant’s roots reduces the amount of water and minerals it can absorb, and this causes it to experience stress.
You can minimize the stress your bonsai experiences by cutting its roots when it is not also facing the prospect of having to put energy into growing. Doing so allows it to direct all its resources towards adjusting to its new root system.
Once your bonsai is “finished”, and is growing in its final pot, you will need to prune its roots regularly to maintain its small size and ensure that it doesn’t become rootbound.
How to prune roots when transferring a bonsai tree from a nursery pot to a bonsai pot
It may not be possible to prune your bonsai tree’s roots down from the length they were in the nursery pot you purchased it to the length they need to be to fit into your final bonsai pot all in one go.
If you try to make the transition too quickly, you may damage the plant.
Instead, you will need to adopt a gradual approach. Trim your root ball down as much as you can without hurting your tree, and then re-pot it into a shallower training pot.
You should never remove more than two thirds of a bonsai’s root system at once.
How soon you can trim your bonsai’s roots down to the size they need to be for it to fit comfortably into its final pot will depend on the timeline of your bonsai. If you purchased it when it was five years old, it will take you less time to get it into its final, shallow container than if you seeded it yourself.