by Terry Erasmus August 09, 2017 2 Comments

This is going to be a short post, but as it was a public holiday yesterday I took advantage of the uninterrupted opportunity to repot two bonsai which were showing signs of waking up. I thought I would share a few comments which I think you might find useful, about the work I was able to complete.

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 ultimate. Rather consider what the plant is doing and determine from that what action is needed. Also, deciduous trees are generally repotting every 2 or so years but conifers may only be repotted every 5 or so years. 

Japanese elm bonsai repotting

Image caption. Buds swelling on the tree is for me the determining factor of 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. These would include:

  • The tree is pushing itself out of the container. In other words the root mass in the container has developed to such a point that the container is no longer large enough to maintain it.
  • The growing medium is not draining properly. In time growing media will break down or the root ball simply reaches a point that it becomes so dense that drainage is impeded. This is a dangerous situation for the roots as they are deprived of oxygen and perhaps moisture may also be impeded from penetrating the ball completely.
  • If you suspect that your growing medium is not "right;" perhaps your tree is showing signs of root rot or perhaps growth is stunted. A change of growing medium might be in order.

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 growing 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 trim about 1/3rd of the roots of the tree away. However experience is invaluable here as in time you will get to know how much you can removed and how much is needed.

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. This of course 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 to fatten as the healing tissue 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 drainage layer 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 saw 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 with a bent tip, 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.

This is probably the first of several posts related to repotting, now that repotting season has just begun. If you want to be the first when these posts are live then be sure to subscribe.

Terry Erasmus
Terry Erasmus


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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