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.
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 can be fitted into the pot. Downward growing roots left without pruning will start to lift the tree out of the pot.
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 inspect carefully to ensure that their removal will not reduce the root-mass to a level where it cannot support the tree.
Thick roots should be cut on a slant with the cut facing downward; this prevents water from accumulating on the cut-surface. Cut the roots cleanly with a sharp knife to help prevent rotting and accelerate healing. Thick cuts can be either sealed with cut-paste or preferably dusted with hormone-rooting powder as it normally also contains a fungicide that will help prevent infection to the root. To accelerate healing and rooting of a thick root that has been cut; it can be worth dressing the cut surface with a thin layer of long-stranded sphagnum moss.
After the tree has been rootpruned, it is then necessary to place the tree back into a bonsai pot. If the pot has not already been prepared, it should be done so now.
Cover the base of the pot with a layer of soil creating a small mound where the tree is to be positioned so that when finally planted, the tree sits just above the height of the rim. Place the tree into the pot and ensure that the correct front of the tree is facing forward. Tie it in firmly with the anchorage wires so that the tree is unable to be rocked about by the wind in the coming weeks whilst new roots are growing.
Add some soil and work it around the root mass carefully so that no air pockets remain. Make sure that the soil is not compressed and take care not to damage the roots. Continue to add soil until the pot is filled just below the rim. When the soil is fully worked in, water the tree very thoroughly to ensure that the soil is fully wetted and any remaining air pockets are removed. Watering will settle the soil and it may be necessary to apply more soil to the surface. Re-water until it is certain that the soil has settled fully throughout the pot.
Finally, the tree should be held in position by the anchor wires, not by the soil.
Most trees will show no reaction to repotting and continue on the through Spring without any problems. Some extra care should be taken however in the six weeks after repotting to ensure the health of the tree;
Avoid exposure to severe frosts, the tree should be regarded as less hardy than normal for six weeks after repotting.
Do not place the tree where it is exposed to strong winds or sun. This is particularly necessary with evergreens as the increase in loss of moisture through the leaves as a result of the wind and sun will increase the stress on the newly pruned roots. It is possible under windy or hot conditions for evergreens to lose foliage if the reduced rootball is not able to replace evaporated moisture. If foliage does start to dry out on evergreens, provide a shady position out of the wind and mist the foliage regularly.