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How often? It depends on the tree species, age and size of container how often a Bonsai needs to be repotted. Fast growing trees will have to be repotted every two years (sometimes even every year), while older, more mature trees need to be repotted every 3 to 5 years. Do not repot on a routine, instead check on your trees every early spring by carefully removing the tree from its pot. A Bonsai needs to be repotted when the roots circle around the root system, when the roots are still contained in soil wait another year before checking again.
When? Repotting work normally needs to be done during the early spring; when the tree is still in dormancy. This way the somewhat damaging effect of repotting on a tree is reduced to a minimum, as the tree does not yet have to sustain a full grown foliage. Repotting in early spring will also ensure that damage done to the root system will be repaired soon, when the tree starts growing.
Soil mixture. Choosing the right soil mixture is crucial for the health of your trees, it should be draining enough to prevent the roots from rotting, while absorbing enough water to supply the tree with water. Although some tree species need special soil mixtures, the following mixture is suitable for most trees:
Mix akadama, fine gravel and potting compost together in a ratio of ½ to ¼ to ¼. Akadama can be bought at (online) Bonsai shops, while you can find fine gravel and potting compost at local garden centers.
When you do not have time to water your trees regularly, choose a more water absorbing mixture (use more potting compost), while you should choose a more draining mixture(use more akadama and gravel) when living in a wet climate.
Choice of Bonsai pot. Choosing a pot that fits your Bonsai, both in size as in style, is crucial for the composition as a whole. Click here for more information about choosing the right Bonsai pot.
How to repot Bonsai trees? Step-by-step plan to repot your Bonsai
1. Carefully take the tree out of its pot, when it is stuck use a root-knife (see photo 1).
2. Inspect whether the tree needs to be repotted or not, as described above in the ‘how often’ section.
3. By using a root-hook or some chopsticks, carefully remove the soil and disentangle the roots somewhat (see photo 2). When repotting pine trees never remove all the soil; this would remove the mycorrhizal fungus which is essential for the tree’s survival.
4. Prune back long roots; this will help your tree to grow a more compact root-system suitable for placement in small Bonsai pots. Also remove rotting or vertical growing roots. Prune up to ¼ of the total root mass.
5. Put plastic mesh on the drainage holes, attached with some copper wire, and cover the base of the pot with a layer of grit to create a good drainage.
6. Now use the prepared soil mixture as the second layer and place the tree, slightly out of the middle of the pot, on top of it (see photo 3).
7. Fill the pot using the soil mixture up to about 1 cm (0.4’’) below the rim; make sure the soil fills the pot completely and no air pockets between the roots are left (see photo 4).
8. Water the tree thoroughly, this will settle the soil and fill up any remaining air pockets.
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