Repotting a bonsai tree is not all that hard but it is stressful on the tree and you have to know when and how to do it.
Everything you read and hear from people will tell you that spring is the best time to repot bonsai. Now, this is true but spring is also three months long and a lot happens during that time. Don’t let the calendar tell you when to do something to your bonsai. Let the bonsai tell you. The tree knows when it wants to be repotted.
In the fall a tree will store energy in the roots for the upcoming spring. As temperatures rise at the end of winter and the beginning of spring the energy that is stored in the roots moves up into the tree. This energy goes into sap flow and opening up the buds on the branches.
As this energy moves up into the tree new buds may appear on the trunk and branches and the buds start to swell. As the buds swell they start getting longer and they appear to be just about ready to open. It is at this point that it is safe to repot a bonsai. Most of the stored energy is up in the tree and the roots are very active at this time. Any damaged roots from repotting or any roots that you prune will heal quickly.
Harry Harrington has a great article on his website that describes this in detail with pictures. I will include it here but it is also in the Goodreads section.
My personal tips:
- Spring is the best time for repotting bonsai.
- Don’t let the calendar dictate when you do something to your bonsai. Let the bonsai tell you when to do something.
- Never remove all of the soil from a pine bonsai. Pines grow mycorrhizal fungus in the soil around the roots. This fungus is essential for the life of the bonsai.
- Add some of the old soil back in with your new soil to help inoculate the mycorrhizal fungus.
- A good method for pine bonsai is to bare root half of the tree each time it is repotted. Repotting black pine in nursery soil (Bonsai Tonight)
- Protect freshly repotted bonsai from frost and freezing temperatures.
Michael Hagedorn has some very helpful tips over at his website. Tips For Repotting
I hope these articles and video will help make you understand the process.
- When to Repot Your Bonsai (Harry Harrington)
- Transplanting (Andy Smith)
- Graham’s Guide to Repotting Bonsai (Graham Potter)
- Repotting black pine in nursery soil (Bonsai Tonight)
A two part article on repotting Pine bonsai.
- Part One: How to repot a young Japanese black pine (Bonsai Tonight)
- Part Two: How to repot a young Japanese black pine (Bonsai Tonight)
A three part article on repotting bonsai.