All potted plants will eventually outgrow their containers. While houseplants need to be "up-potted", that is, placed in larger and larger containers, we maintain the miniaturization of a bonsai tree by keeping the roots confined to the small container.
On average, repotting will be necessary every 3-5 years, but the tree should be removed from its container and its root system inspected once a year. If the roots form a circular ball around the perimeter of the pot, it is time to trim the roots and repot.
When repotting remember to:
(a) use only bonsai soil
(b) remove air pockets by working the soil down through the roots
(c) do not remove more that 20% of the root system
(d) repot during the appropriate repotting season
(e) water well and keep out of the sun for a week or two.
Generally bonsai need to be repotted and root pruned every 1-5 years. Fast growing and younger bonsai are closer to 1 year whereas older bonsai are closer to 5 years. You should carefully lift the bonsai out of its pot yearly to inspect the roots to see if it has become pot or root bound. This means that the root system has nowhere else to go. You will notice the root ball encircling the pot and possibly also coming out the drainage holes. That is a good indication you need to root prune.
Pruning the roots allows your bonsai to develop finer feeder roots. This allows them to more efficiently absorb nutrients from the soil. If you don’t root prune it will be harder for water to soak into the soil. Additionally, when the roots can’t grow anymore the bonsai will begin to show signs of stress and may eventually die. Trees in the wild are used to making new feeder roots every spring. Similarly, if you root prune your bonsai you should do it in the spring. This is when the tree is ready to regrow the roots you cut back.
1) Root rake or root hook
2) Sharp dedicated shears
3) Chopstick or pencil
Once you gently remove the bonsai from the pot you scrub the walls of the pot. Next, start to rake out the roots using a bonsai root rake or chopstick (I like the root rake). This should untangle the roots and free them from the compacted soil. Now, take your scissors that are only used for root trimming and cut all the thicker roots back leaving the finer ones. The goal is to create some space between the remaining roots and the pot wall. Try not to cut more than 20% of the roots away.
Now that the roots are pruned put some fresh soil in the middle of the pot and sit the bonsai on top of that. Try and work the soil between the roots using the chopstick to remove any gaps or spaces. If you plan on wiring the roots (not covered in this article) now is the time to do it. This will help secure the bonsai in place so it doesn’t fall over when being moved. Fill the remainder of the pot with soil.
Last, water your tree very well and place it somewhere pleasant where the roots can heal. It won’t like being moved around by wind or rain. Also, it is important that the soil doesn’t stay very wet while the roots are healing. Do not fertilize your bonsai for awhile after either.