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 can be moved or otherwise manipulated. Secondly, the restriction of the roots also helps to reduce the size of the foliage and promote tighter growth. Furthermore, restricting the roots slows the growth of the tree, meaning that the bonsai artist can work with the tree more easily.
As bonsai growers, we have to balance this restriction with making sure that the tree is healthy. Mature bonsai tend to look best when they are on the edge of being pot-bound.
As the tree is kept in this small space with limited soil, the roots grow to fill all of the space in time. Only the tips of roots provide sustenance to the tree. the rest of the mass of 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, sprouts 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 occurs under the soil where is cannot be seen. This helps 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. This is Repotting.
Potting up is more often done with immature bonsai. Potting up can be used when you want to increase the amount of foliage which the tree has, as more foliage will need correspondingly more roots to ensure water supply. If you want to spend 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 how often you have to water.
It is also a way to increase the vigour of the tree. When often take smaller, starter bonsai and put them into a slightly bigger pot. This gives them a little more space and helps the tree to develop more quickly. Don’t go too big though, a small tree looks lost in a big pot.
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 years as the density of the fine, fibrous roots builds up. You may start with the tree in a large plastic pot. When you root-prune, the aim will be to go down to a smaller, more shallow bonsai pot. Later one, you may move it into an even smaller bonsai pot.
When you see bonsai trees in amazingly shallow pots, this process of root-pruning and potting down into ever-more shallow pots has been completed over many years. It can only be rushed to a certain degree – you have to work with the tree to achieve this.
*Yamadori is a Japanese term for trees collected from the wild. Historically, many bonsai were collected from the mountains in Japan. Over here, the term is often applied to any trees dug up. Its often a source of amusement to bonsai enthusiasts when photos of saplings are posted online and labelled Yamadori. True Yamadori has age and character. The most highly prised being very old, craggy and having clung to life in a harsh environment.