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The following series of pictures illustrates the practise of root pruning and repotting bonsai. The tree in this series is a Fuchsia magellanica, a very fast growing tree that requires annual root-pruning.
Before Removing The Bonsai From Its Current Pot
Before the process of repotting begins, it is always worth preparing the materials that will be needed, as time spent looking for materials during the course of repotting prolongs the amount of time the roots are exposed to the air.
Ensure that the following materials are to hand; sufficient good quality bonsai soil, tools, plastic mesh and wire. If a new pot is to be used for the repotting, prepare the pot.
Preparing The Pot
If a new pot is to be used, this procedure can be carried out before the tree is lifted from its old pot. If the old pot is to be retained, it will need to be thoroughly cleaned with water and prepared after the tree has been removed.
These three images show how the pot should be prepared for the tree. Plastic mesh is used to cover the drainage holes of the pot. The mesh stops the soil medium from falling through the drainage holes over the course of the following year(s). 'Butterflies' are shaped from bonsai wire to hold the plastic mesh in position.
Lengths of wire or string are then threaded through the drainage holes in a loop (not shown in this picture) to be used for anchoring the tree into position so that it cannot become dislodged within the pot. Bonsai that are not firmly anchored into their bonsai pot have a habit of being blown out of it in strong winds!
Preparing New Soil For Repotting
There are a very large number of soil mixes that are suitable for bonsai. Indeed, it is a hotly debated subject within bonsai-circles as choosing the right soil-mix for your tree and climate is of the utmost importance.
It should be noted that it is essential that the soil that is used is free-draining and does not compact easily. Never use garden soils or ordinary potting compost as they are not adequate for bonsai cultivation. For a more detailed examination of Bonsai soil mixes see Bonsai Soil Mixes
Removing The Tree From Its Pot
Cut the tying-in/anchor wires from the bottom and tilt the tree out of the pot. If the tree is reluctant to come out, try tapping the sides of the pot with your hand to try to separate the soil from the edges of the pot. If this fails to work, run a sharp knife along the inner edges of the pot to release the rootball. Gently, lift the tree to inspect the rootball.
In this picture of the rootball of the Fuchsia, it is possible to see that there is no room for root growth and the root system has become pot-bound in only one growing season.
Removing The Old Soil And Combing Out
After removal of the rootball from the pot, it is now necessary to comb out the rootball. This not only removes much of the old compost but also disentangles more vigorous, longer roots that will need to be trimmed back.
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 roothooks 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 not cleanly cut as they should be.
The amount of root that should be removed depends on a number of variables including 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.
The remaining root system should be carefully examined for any root problems that may exist;
Remove any dead, decayed or injured roots to prevent or cure problems with root rot. 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!)
After excess circling roots have been removed; check the rootball for faults, particularly around the area of the nebari.
If the rootball has become excessively dense, make wedge shaped cuts into the remaining rootmass. This is only necessary on a well-developed, densely packed rootball and ensures that fresh soil is applied to the centre of the root-mass and that water is able to permeate.