Repot every three years for young trees and every five years for mature bonsai trees. Use a rust, brown, gray or deep blue pot for pines. Japanese imported pots are also nice, though expensive, but larger bonsai tree nurseries have a very good variety of stock at all times.
The other area of bonsai that needs to be addressed by the beginner is repotting; a very straightforward technique if carried out correctly and at the right time. Most trees need to be repotted annually or at very least bi-annually in Spring as the years new growth starts to appear. Trees that are not repotted will eventually lose their health and vigour.
Many beginners’ trees can also be found to be planted in very poor soils; they will need repotting into better quality soil.

Why Repotting is Essential to Bonsai

As plants of all kinds grow, their root systems become larger and more extensive in order that they can supply their ever-expanding canopy of foliage with the necessary quantities of water and nutrients. Trees and shrubs grown in the ground can have root systems that extend beyond the shadow of their own foliage canopy in a search for water and nutrients.
On the other hand, containerised plants are limited by the size of their pot as to how far they can extend. They need to be constantly supplied with food and water on which to survive. Their root systems however, continue to grow in tandem with their leaves and branches above the surface of the soil.
After a period of time that varie between different plants and plant species, the root systems of all pot-grown plants fill their containers, and become 'pot-bound'. Under these conditions, new fine feeder roots that are so essential to the uptake of water and nutrients in a plant have little room to grow, the soil structure deteriorates and the plant starts to suffer.
With an ordinary pot-plant the solution is to pot the plant into a larger container which allows room for new, fresh compost around the rootball. With a bonsai, the aim of repotting is the same, to allow fresh compost in and around the root system so that it can continue to form fine feeder roots and so that fresh soil can be introduced around the root system. However, with Bonsai, the container, and more specifically, the size of the container is not only part of the design but its size is specially selected to suit the tree. For this reason, Bonsai are root-pruned.
A side effect of root-pruning is that it increases the density of the root ball. From every root that is trimmed, a number of new roots will emerge from the root-tip that was removed. As the rootball is repeatedly pruned over the years, the rootsystem becomes denser and denser. Within a well-developed rootball, dozens of fine feeder roots can occupy the same volume of soil that one unpruned root may ordinarily use. So though the size of the rootball is regularly reduced, the actual volume of root within a certain amount of soil increases, and sufficient to support the canopy of the tree.
Root pruning does not dwarf or stunt the tree in any way. The tree may lose a little vigour for around 6 weeks after rootpruning, as it regenerates its root system (this is more noticeable with evergreen tropicals such as Figs), but after this short period of adjustment, the tree becomes more vigorous than before as new feeder roots are able to develop in the new soil.

How Often Should Bonsai Be Repotted?

Bonsai requires repotting when they develop into pot bound. A Bonsai is considered to be rootbound when its roots entirely fill the pot and there are long roots circling the rootball or inside of the pot. In some cases, roots will also be seen to be growing out of the drainage holes at the base of the pot.
The time a tree takes to become rootbound varies from one year to five. A number of factors affect the amount of time a tree takes to become rootbound in its pot; different species of tree have different levels of vigour. Fast growing species and individual plants (Figs and Larch for instance) tend to require more frequent repotting and rootpruning. Other factors that contribute to regular root pruning include size of container, development of rootball (a dense established rootball will not require as frequent repotting), and the age of the tree, older trees are not as vigorous and require less frequent repotting.
The rootball should be checked for its condition annually in Spring; gently ease the tree out of its pot and examine the rootball. If the roots of the tree are still contained within the soil, the tree can be returned to the pot and repotting can be left for another year.

When Should A Bonsai Be Repotted?
Bonsai are repotted while the tree is dormant, this reduces the stress on the tree as it does not have to supply water and nutrients to its leaves or in the case of evergreens, and the supply of nutrients from the roots is minimum. To repot and rootprune when a tree is supporting a full canopy of leaves and is actively in growth would result in loss of foliage through desiccation, as the roots are unable to supply sufficient water and nutrients to its leaves. Repotting during the growing season is highly stressful to almost all tree species and can often result in death and dramatic loss of vigour.
Therefore, Bonsai require repotting when they are dormant or when there is a minimal requirement for the roots to supply the rest of the tree with nutrients. Another factor that influences the exact timing of repotting a tree is the length of time that injured/trimmed roots will remain exposed to moisture and frosts. The rootsystem will not repair itself fully until the tree starts into growth during the Spring; this means a tree repotted early on in Winter may not repair its roots for a number of months, leaving the root tips open to infection, root rot and frost. By repotting closer to the time that the tree starts back into growth, the less time that the cut roots are left exposed to soil moisture and freezing conditions.
The best time to repot a tree is therefore right at the very end of its dormant period, just as it is about to start back into growth. The tree is still dormant, but, as the tree is about to start back into growth, the repair of trimmed roots is fairly immediate. The exact timing of this varies between tree species, and more importantly, varies according to local climate and individual plants. It is not realistic to expect to find out when to repot a particular species on a certain date as there is so much variation between different climates; as weather conditions subtly change year on year in the same geographical area, it is also not true that a tree is repotted on the same date each year.
Observe each tree to see when it is ready to be repotted. Deciduous species during the Winter, if inspected closely, will be seen to have small leaf buds that are held closely against the branch. As the tree starts to come out of dormancy, these leaf buds will start to extend, ready to open out and reveal their emerging leaves.
Most evergreen species, including coniferous trees will similarly display new buds starting to extend and emerge. It is harder to judge when to repot tropical evergreen species such as Ficus, Serissa and Sageretia that are grown indoors during the Winter in the UK and Northern Europe. Generally, these species will slow their growth rate during the shorter daylight hours of the Winter months, and it is during this semi-dormant period that they can be repotted.

Supplementary observations

It is possible to repot a tree earlier in its dormancy than indicated above, and enthusiasts with large collections will ease the pressure of repotting time by repotting some of their trees before they are ready to emerge from dormancy. It is also feasible to repot some coniferous species in autumn, as some trees will exhibit root growth before the onset of dormancy. I would not however recommend this to the beginner until he or she has the experience to judge whether their tree or trees will cope with the additional stress however slight.


Practise of root pruning and repotting bonsai


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 process 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.

Plastic mesh is used to cover the drainage holes of the pot. The mesh stops the soil medium from falling through the drainage holes whilst still allowing the free drainage that is required. 'Butterflies' are shaped from bonsai wire to hold the plastic mesh in position. Lengths of wire or string must also be threaded through the drainage holes in a circle to be used for tying the tree into position so that it cannot become dislodged within the pot. In deep pots, it is also worth putting a shallow drainage layer of grit at the bottom of the pot at this phase.

Preparing fresh Soil for Repotting

There are a very large number of soil mixes, as refered previously, which are suitable for bonsai. I would recommend Beginners buy pre-mixed Bonsai soils from a reputable bonsai dealer if they have only a few trees to repot.
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.

Removing the Tree from its Pot

Cut the tying-in 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.

Removing the aged 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 any long 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 pointed stick or piece of wood; chopsticks are useful for this job. Particularly tangled areas can also be gently loosened by hand very effectively. Metal roothooks are often used by enthusiasts but it is too easy to damage roots this way; roots end up being torn and not cleanly cut.

Particular care must be taken to protect the nebari.

After the old soil mass has been removed and the new root growth has been disentangled, excess circling roots can be trimmed back with a sharp pair of scissors or shears. Try 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 putrefy. 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. All signs of putrefy must be completely removed to prevent its spread. It should be noted that Larix/ Larch species naturally have roots that during their embryonic period resemble a severe case of root decompose; care must be taken not to remove a healthy Larch rootsystem!
Examine the rootball for any other signs of infection or infestation, if found, these should be dealt properly.
After excess circling roots have been detached; 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 rootmass and that water is able to filter through.
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 inspected carefully to ensure that their elimination will not decrease the rootmass to a level where it cannot sustain the tree. Try not to remove more than 1/3 of the rootmass in one repotting.
Thick roots should be cut diagonaly 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 preserved 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.

Repotting

After the tree has been rootpruned, it is then necessary to repot the tree. If the pot has not already been prepared, it should be done so now.
Cover the base of the pot (and drainage layer of grit if this is used) with a layer of soil creating a small mound where the tree is to be positioned so that when finally planted, it sits above the height of the rim. Place the tree in 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 grow out from the rootball.
Add some soil that should be worked around the root mass carefully so that there are no air pockets. 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 thoroughly to ensure that the soil is fully wetted and 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 within the pot.
If using an organic, peat or soil based compost, ensure that the soil is not compressed so much that air and water can not enter the soil-mixture. The tree should be held in position by the anchor wires, not by the soil.

Aftercare

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;
The requirement of water by the tree will be lessened and though the tree should never be allowed to dry out, ensure that it is not overwatered.
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.