Repotting and Rootpruning Bonsai - Part II

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Page 2 of 3:

Removing the Old Soil and Combing Out

combing out bonsai roots

After the removal of the rootball from the pot, it is now necessary to comb it out. This not only removes much of the old compost but also disentangles the more vigorous, longer roots that can then be trimmed back.

bonsai rootball

Remove as much old soil from around the edges of the rootball as possible, using either a wooden stick or by hand, chopsticks are useful for this job. Metal root hooks are still used by some enthusiasts, but in my opinion it is too easy to damage roots this way; roots end up being torn rather than cleanly cut, as they should be.

repotting a bonsai

Attention can then be paid to the base of the rootball, disentangling and combing out the roots growing downwards. The roots of a bonsai are often described as being a mirror-image of the bonsai itself and in the same way that the upper branches of the tree require heavier pruning, the roots growing from the base of the rootball require heavier pruning.

bonsai root pruning

After the old soil mass has been removed and the new root growth has been disentangled, excess roots can be trimmed back with a sharp pair of scissors or shears. The amount of roots that should be removed depends on a number of variables including the vigour of the tree, the density of the rootball and according to individual tree species. However, as a general guideline, aim to remove around 1/3 of the overall rootmass.

bonsai root pruning

The remaining root system should be carefully examined for any root problems that may exist. Remove any dead, decayed or injured roots. Any dead or rotted roots will be black, slimy and their outer bark will slip easily from the root itself; severely rotted roots will be entirely hollow and crumble away. (It should be noted that Larix/Larch species naturally have roots that during their dormant period resemble a severe case of root rot, care must be taken not to remove a healthy Larch rootsystem!).

Try to encourage the development of the rootball each time the tree is repotted. The trunk should have roots spreading radially from around its base; roots that grow upwards or recurve from the base (nebari) are considered ugly. Strong, thick, downward growing roots should be removed so that the rootball is flat and fit into the pot. Downward growing roots left unpruned can start to lift the tree out of the pot and will encourage strong apical growth in the crown (apex) of the bonsai.

Any other thick or straight roots should also be pruned back to a point where there are fine roots branching out. Thick or straight roots tend to rob the vigour of smaller finer growth. At all times when pruning back such roots, it is important that they are inspected carefully to ensure that their removal will not reduce the root-mass to a level where it cannot support the tree.

bonsai root prune

Note that the roots of this tree are already very dense and compact, and this allows them to be cut back hard, as shown in these images. It is safe to remove around 1/3 of the rootmass of all broadleaf and deciduous tree species. The base of the rootball is pruned back harder than the edges. This ensures that the bonsai sits low enough in its pot and also balances the vigour of the top branches of the tree.

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