Repotting Bonsai Trees
Bonsai are generally kept in small containers. This has a number of benefits for the tree and the bonsai grower. The small nature of the container, generally a ceramic pot (though wood, plastic and metallic containers are all used) is the ease with which the tree can be moved or otherwise manipulated. Secondly, the restriction of the roots also helps to reduce the size of the leaves. Furthermore, restricting the roots also slows the growth of the tree, meaning that the bonsai artist can work with the tree more easily.
As the tree is kept in this small space with limited soil medium, the roots grow to fill all of the space in time. Only the tips of roots provide sustainance to the tree. the rest of the mass of the long roots serve to stabilise the tree, stopping trees in their natural environment from falling over. Partly, repotting is done to ensure that the roots have the space to carry on growing, giving the roots new space to get into. Root pruning is also done for another reason however.
When you prune the branch of a bonsai or any other tree, you encourage it to back-bud. One shoot, when cut back, sprount two or more side shoots. The process, repeated frequently is how we encourage pads of foliage, mirroring branches in mature trees in the wild. By pruning roots, the same back budding occurrs under the soil where is cannot be seem. This help to increase the number of fine feeder roots which the tree has into a smaller area, meaning that we can further reduce the amount of pot space which the tree needs.
Repotting, Potting up or Potting down
Frequently, mature bonsai are taken out of their pots, the roots pruned and the tree returned to the same pot.
Potting up is more ofter done with imature bonsai. A common misconseption amoung novice bonsai growers is that the starter bonsai / sapling / stick which they have bought from one of the less scrupulous bonsai sellers will naturally turn into a bonsai in time. Some people are also told that smaller bonsai will grow into larger bonsai in time just by potting up. This myth is also frequently peddled by sellers of bonsai seeds, which of course do not exist. Potting up can be used with you want to increase the amount of foliage which the tree has, as more foliage with need correspondingly more roots to ensure water supply. If you want to send less time watering, pot your bonsai into slightly larger pots than are required. The increased volume of soil will hold more water, giving you more leeway in when you water.
Potting down is part of the process involved in turning field grown, yamadori or nursery material into bonsai. Frequently the size of the root mass must be reduced gradually over the course of many years as the density of the fine, fiberous roots builds up.