Root pruning and repotting

Bonsai grow in the confined space of a relatively small pot. The roots of all plants keep growing to renew the active root tips that actually do the work of absorbing watera nd nutrients. After a couple of years these constantly growing roots have so filled the spaces in the potting mix that vital air and water cannot enter the mix and the tree will not stay healthy. It is very important to repot bonsai (and indeed, all potted plants) regularly to keep them healthy. Root pruning does not keep a bonsai small. In fact just the opposite. After rootpruning and repotting in fresh mix with new nutrients and room for new roots to grow we usually see an explosion of growth for the first year. The picture in this post will give a good idea of how I root prune and repot deciduous bonsai.

trident maple before repotting

This is a trident maple before repotting.

The thin shoots growing from the base are seedlings that have been grafted on to improve the nebari (rootage) of this tree. Look for more about this in a future post.

new roots in the base of the pot

There is a dense layer of new roots in the bottom of the pot. I need to remove these to give room for more roots to grow.


cut around the edges

First I cut around the edge of the rootball to remove the matted roots around the outside. Use root shears or an old saw for this job.
Then cut the layer of new roots from the base of the rootball.

cut the layer of matted roots from the base

rake the old mix out

Once the tangled roots are gone from the base and sides it is easier to rake out the old potting mix. A root rake like this, a steel hook or the point of the root shears are all suitable for this job. Rake from the base of the tree outwards. Raking this way will align the roots and improve the nebari in future. Quite a few small roots will break off but there will still be plenty left on a tree like this. Rake out under the rootball too.

When you have removed much of the old potting mix you can trim the remaining roots short. Cut strong roots quite short to force them to ramify. Now is the time to remove or realign any roots that cross over or grow at an awkward angle.

Cut back larger roots harder underneath
Underneath the rootball cut thick roots really short. This will make the tree rely more on the lateral roots which will improve the nebari and help the base of the tree to flare out wider.

cover holes with mesh

Use mesh to cover the holes in the pot to stop the potting mix falling through.

A layer of mix in the pot

Place a layer of potting mix in the base of the pot,
position the tree and top up with more mix.


Water it in well as soon as possible but use a gentle spray to avoid washing all the fresh mix away. I like to soak my pots in a tub of water to make sure the mix is properly wet then lift it out to drain well.
You may have noted that I have not tied this tree into its pot. This one has such a well developed root base that it is highly unlikely to fall over. I only ever tie in trees that still have an underdeveloped root systen and those which show a tendency to tip over.

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