Repotting Bonsais

Good quality compost is essential, since it has a direct effect on the health bonsai. Opinions on composition are divided, but as a general rule, a bonsai will thrive in compost made up of equal parts of good quality loam, peat and either sand or rotted turf. Some plants, such as rhododendrons and azaleas, need a lime free or acid growing medium. The experts generally agree that the ideal mixture differs for deciduous trees, conifers and flowering trees. It is important to stick to the compost mixture in which the bonsai plant developed, so you should ask the grower about this when you buy your bonsai. The compost should always be sieved to eliminate any risk of damaging the roots.

Why repot bonsai?

This is an important matter, which is frequently neglected. Unlike a normal indoor or balcony plant, the bonsai needs a container in complete harmony with its size and style. In Japanese, the word bonsai means tree in a tray. Ignore this at your peril: you may end up with an aesthetic catastrophe and a spoiled bonsai.

There is an extensive range of bonsai containers, usually offered by the growers themselves. Most of these trays are of Japanese or Chinese origin. Flat trays are best suited to trees with a spreading or trailing habit, with slightly deeper trays for upright trees and even deeper trays for tall, slender or cascading plants. Forest or grove arrangements are most attractive displayed in very flat trays or even sometimes simply on stone slabs decorated with some rocks. Most bonsai trays are made of stoneware, either glazed or unglazed. They are seldom decorated and the most usual colors are blue, pale green and brown.

Preparing the bonsai container

Bonsai trays have large drainage holes, through which any excess water from the frequent watering can drain, so that it does not stagnate and set up root rot. Do not cover these holes with a stone or piece of clay pot, as in normal plant pots. Use plastic mesh (which will not rot or rust), fixed in place with plastic coated wire hooked on to the outside of the tray. In this way, none of the finely sieved potting compost will leak out, even after generous watering. The plastic mesh also has the advantage of stopping unwanted visitors, such as the wood louse, from getting into the pot.

Removing bonsai from its pot

It becomes necessary to change the pot or tray when the plant becomes 'pot bound' by the growth of its roots. To remove the bonsai from its pot, stop watering until the compost is relatively dry, but at the same time make sure your bonsai does not suffer. Then gently lift the bonsai by its trunk: if the compost is dry, the plant should come out quite easily.

Check the soil

It is a good idea to check the soil for the sake of the tree's health. Check that the compost is not harboring any undesirable creatures, such as ants, woodlice or insect larvae. You will also be able to monitor the root development and decide when to repot.

Root pruning

This is thought to be vital to the art of bonsai, root pruning directly helping to dwarf the tree. However, root pruning will also help to rejuvenate the tree by bringing the feeding roots a little nearer to the trunk.

Start by removing most of the earth from the old root ball by gently scraping with a special bonsai rake. This will also untangle the roots. It should be done as gently as possible, avoiding damage to the roots (especially the larger ones) so far as practicable. Take a pair of wide handled scissors and snip the roots down to about half their length. Remove completely any roots which do not seem healthy or which were damaged when the tree's roots were combed. Your bonsai is now ready for repotting.

Root pruning is a testing time for the bonsai tree: this is why it is vital that it takes place at the start of spring, when the tree is at its most vigorous. Water generously after repotting, then keep fairly dry until the tree is re-established.

Repotting

It is important to choose a container of the right size. When you have prepared the tray, spread a drainage layer of gravel or pebbles in the bottom. The potting compost should be sifted several times, with the coarsest material placed on top of the drainage layer, followed by successive layers of compost with the finest on top. Then plant your bonsai and add the very finest soil. If dry enough, the fine soil will easily filter down through the roots. Firm lightly before watering.

You may find it difficult to keep some of the larger trees in place unassisted. Avoid using any kind of stake. Instead try keeping the tree's root ball in place with metal wire passed over the base and through the drainage holes. This is an efficient, invisible method. Do remember to remove the wire once the tree has taken root. The tray should be about three-quarters filled with compost, with a layer of very finely sifted soil to finish off the surface. Firm it down with the spatula at the other end of the bonsai rake or use tweezers. One can also cover the soil with a layer of moss which is decorative and serves to keep the soil moist after each watering.

Watering should be done slowly, continuing until excess water runs out of the drainage holes. This job can take a long time, since the compost used for repotting should have been very dry. To prevent moisture evaporating too quickly, put the bonsai where it is sheltered from wind and sun for several weeks. It should start to flourish in no time, overcoming the shock of being repotted. Never forget that repotting is always traumatic for the plant and act accordingly. It is also imperative to choose the right tray for transplanting bonsai. If you change your mind after repotting is completed, it could do your bonsai irreparable harm to subject it to a second repotting after such a short time.