To prevent your Bonsai from being pot-bound and ultimately starving to death, It's crucial to repot, or transplant regularly. A tree becomes pot-bound as it uses up the available nutrients in the soil and the roots grow to the shape of the pot. Due to the lack of space and nutrients, the trees stop growing, and if no action is taken, it will surely die. Repotting or transplanting your Bonsai will resupply the tree with the nutrients it needs to grow and flourish.
How often you should repot depends on the size of the pot and tree species of your Bonsai. Fast-growing trees need to be repotted every two years, sometimes sooner. Older, more mature trees need to be repotted every three to five years. Repotting is not something that should be routine. Check your Bonsai early in the spring by carefully removing the tree from its pot. If you see the roots circling around the root system, your Bonsai needs to be repotted. If the roots are still contained within soil, leave it and check again the following spring.
The best time to repot a Bonsai is early in the spring, while trees are still dormant. At this stage trees are not sustaining full-grown foliage, so the damaging effect of repotting will be minimized. Repotting in early spring will also ensure that damage done to the root system will be quickly repaired, as soon as the tree starts growing.
Choosing the proper soil mixture is crucial for the health of your trees. It should allow water to drain enough to prevent the roots from rotting, while also absorbing enough water to hydrate the tree. Some tree species need special soil mixtures, but the following mixture is suitable for most trees:
Akadama, pumice, and lava rock in a ratio of 2:1:1. If you know you won't have time to water your trees regularly, add more Akadama for a more water-absorbing mixture. If your tree is in a more humid climate, add more lava rock for a mixture that permits more draining
The most important thing to keep in mind when repotting is the timing. Repot your tree early in the spring, right before the growing season starts