The best time to pot or repot bonsai trees is usually after the winter season and just before the spring season or growing season occurs. This is because during the growing season, your bonsai will be able to heal and recover itself much more quickly. You do not want to interrupt this growing period and really you should only repot just before this period of time. However all this repotting advice given should take into consideration what type of plant your growing and also what climate you are living. Different seasons occur during different times of the year and it really depends on which part of the planet you're living in.
Here are a couple of things you should look out for which indicate that your tree needs to be repotted. If the soil is raised from the bowl and is slightly higher than usual, this indicates that your bonsai tree has a root clogging problem. This will require you to repot your tree because if you leave the root clogging and allow it to continue, your plants will slowly become very unhealthy and could eventually die. To prevent these problems, generally you want to repot your tree every 1 to 3 years depending on the species and age of your tree. Younger trees should be potted more frequently compared to the older ones..
The process of repotting your tree is relatively simple. You should first use a sharp object and insert this between the soil of your pot or bowl. Place the sharp object around the bowl slowly loosening up the soil and allowing it to be wedged out. Use this sharp object as a wedge or lever to move out the tree and then gently lift up it up and place your hand beneath the soil base. Avoid touching the trunk too tightly as this can be damaged if you are not careful. Once you have managed to remove the tree from the pot, gently place it on a flat surface so you can perform the next stage of repotting which will require you to perform root trimming.
To root trim first remove the soil and then cut a third of the way in horizontally and a third of the way in vertically, removing any roots which you come across. With the remaining area which has been cut you will now want to remove any other major roots which may have grown too large. Afterwards reapply some new healthy soil or compost and place the tree in a new larger pot or bowl. The tree should then be watered thoroughly and you should keep a careful eye out for your tree over the coming weeks to ensure its health.