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5 min read

This is a fairly quick read but in the spirit of pending or even current repotting I thought I'd share a few comments which you might find useful for when you get to your repotting your own trees.

Japanese elm

When to repot

I don't mean to be different but when deciding to repot a bonsai tree, a guide in a book should not be the point of reference. Rather consider what the plant is doing and determine from that what action is needed. Deciduous trees are generally repotted every 2 if they are young but less frequent as they age. Conifers may only be repotted every 5 years. Don't get caught up in the heat of the moment and decide to repot "on the fly." 

Japanese elm bonsai repotting

Image caption. Buds swelling on the tree indicated the most optimal time when a tree should be repotted.

Although this post is not meant to be about repotting in general, let me just add that there are also signs to look out for which will indicate whether a tree needs to be repotted at all:

  1. When you water, is the water not entering the container any longer and is simply running over the sides? If so, then perhaps a repot is necessary but you might want to make a further check by simply removing the top layer of medium as it could simply have become clogged with fertilizer, moss or surface roots.
  2. Is the balance between water and oxygen upset? In other words does the container hold too much because your growing medium has slowly become more and more compacted leaving little space for oxygen to penetrate. If so, this is a dangerous condition and your tree should be repotted.
  3. Have you drastically changed the style of the tree and now need to change the planting angle? If so then yes a repot is justified.
  4. Lastly, is your tree pushing itself out of the container due to an excess of roots at the base on the container? If so then perhaps you merely need to lift it and slice off the bottom roots if the medium is still draining well. If not, and the tree is pushing out, then a full repot is necessary.

    Japanese elm repotting bonsai tree

    Image caption. The current condition of the root ball.

    I used the Professional Mix for this Japanese elm but I would now like to change over to a mix of Akadama and Pumice as I believe the growth will be superior. This will also afford me an opportunity to work on the surface roots.

    Japanese elm bonsai tree

    Image caption. The old media removed and the roots trimmed.

    Trimming roots

    Generally speaking you can safely trim about 1/3rd of the roots of the tree away from most trees. However experience is invaluable here as in time you will get to know how much you can remove and how much should be kept as particularly strong trees ie coming from field growing or those that have been allowed to develop freely in a larger container, can be handled far more aggressively with a lot more than 1/3rd of their roots being cut.

    An important tip I would like to share is that once the roots are trimmed the tree should be able to stand, on its own without support, at the angle it is to be potted at although of course this does not apply to leaning and cascade trees! This is a good way of telling whether you have pruned the roots correctly.

    Chinese maple nebari

    Image caption. Ever wondered how nebari like this is created?

    The technique for achieving roots like the above is a combination of root grafting but also trimming the underside of the roots. This repeated trimming with a root cutter on the underside of the roots forces them to fatten as the healing tissue (callous) adds to the width.

    Japanese elm bonsai tree nebari work

    Image caption. Previous root cuts healing over, and the callous spreading.

    As you can see in the preceding image, callous has formed over the cuts which I made the last time this tree was repotted. This is repeated over and over and each time the callous will increase. It is also possible to stimulate roots to form in this way, but this depends on the species.


    This is a tree which, if you have read some of my other blogs you would have seen before. It was originally grown from seed and was field grown for some years before it was placed into a container. For the last couple seasons I have grown the tree in Professional mix, with a bottom drainage/aeration layer of sifted coarse particles of Groperl. I must say that I was very happy with the results however I am expecting more from the new mix I wish to use on it. I plan now to repot into Akadama, Pumice in a 2:1 ratio with added charcoal.

    Hackberry bonsai tree repotting

    Image caption. The buds are swelling and in fact some are already partially open. Now is a good time to repot.

    Hackberry bonsai tree repotting

    Image caption. Very healthy roots encircling the root ball.

    After lifting the tree from the container I used a sickle to quickly cut the entire bottom half off the root ball. Then I cut around the edge of the root ball, removing about 1/3rd and thus reducing the diameter of the root ball considerably.

    Hackberry bonsai tree repotting

    Image caption. The roots all teased out.

    The final step was to tease out the roots using a pair of tweezers, for the surface roots. Then a root hook was used to comb the roots out. All the roots should then be laying in a position which essentially radiates from the trunk. This can take quite some time to do as you need to work gently and slowly to avoid breaking too many roots. Once completed though you can trim the roots which extend beyond the circumference of the root ball.

    Hackberry bonsai tree repotting

    Image caption. Testing the fit of the tree

    I chose an oval Yamafusa bonsai container in a baby blue glaze. Its fairly shallow but it results in the trunk appearing even more robust. The colour will complement the vibrant green leaves of this Hackberry too.

    Don't forget to wire your tree firmly into the container. This is very important to prevent the tree from being blown out and also to prevent any movement which may break the tender new roots which are issued just after repotting.


    So that's it for this post, hope you picked up something useful. Don't be caught unawares, the seasons are a little wonky and our trees are waking up at times when we expect them to be completely asleep still. If you are not adequately prepared with growing medium and other related consumables and tools you will find yourself being forced to take shortcuts which you may regret later. Make sure you have planned what containers you will be needing, how much growing media to have on hand and get your tools sharpened and ready for use.

    2 Responses

    Bill Rayner
    Bill Rayner

    August 22, 2017

    Thank you for your advice and teaching.


    August 10, 2017

    Hi Terry,like all the other talks yet another excellent presentation. I found the practical and theoretical explanation
    quite useful as I am about to repot my English Elm. I was not quite sure as when to repot , I have spotted all the signs though and feel confident with my actions to be taken.




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