Before we learn how to repot a bonsai, we need to know a little about the soils that we use and why.
Balance of Water and Oxygen
When selecting soil components for the repotting process of our bonsai, we are looking for three qualities.
- The soil needs to hold Water and give it back to the tree when it is needed.
- We need soil that allows the free flow of Oxygen throughout our container.
- Our soil needs to allow Nutrition to bind to it or make it available when it is needed.
We need to maintain a balance of water and oxygen in our container. We do this by using a good soil mix like Akadama, Lava, Pumice. We use particles of the same size. We sift out all of the fines and dust. We use an aeration layer that is a bit bigger than our normal mix.
Popular Soil Components
The most popular soil components are Akadama, Lava, and Pumice. Equal parts of these components (1-1-1) are a good starting point depending on what you are trying to accomplish. This is the soil mix I use and all of the big professionals. I buy my premixed soil from Sean Campbell at Desert Bonsai. You can find him on Facebook here. Desert Bonsai. If you want you can mention to him that you found him via Sumo Bonsai Supply.
Now, let us take a quick look at the three components.
Akadama is a natural raw volcanic clay. Akadama is a unique soil component because it has a tubular structure. This structure allows the particle to expand and shrink as water enters and is given back to the tree. The tubular structure also allows roots to enter the particle. As the root grows, it swells inside the particle of Akadama. Soon enough the particle will break or divide. This division of the particle will allow a branch of this root to now grow into the tubes of that particle. Then that particle brakes or divides and the same action happens again and again. This is how Akadama helps create a well-ramified root system.
Lava is a volcanic particle that allows for the free flow of oxygen in our container.
Pumice is a very porous volcanic particle that does not break down in our container. Pumice holds water very well but it also gives it back to the tree when it needs it.
Why Would You Repot?
We don’t repot just to repot. We don’t repot on a schedule. Sometimes, depending on the tree and conditions, we could go up to 6 years without repotting. I am just using 6 years as an example. Here are the reasons we do repot our bonsai.
- Loss of Percolation
- We need to maintain a balance of water and oxygen in our pots. With the roots constantly growing they will eventually take up all of the space in the pot. This limits the flow of water and oxygen.
- Soil Decomposition
- Eventually, the soil in the pot will break down due to the tree acting on it by the roots growing. This will clog up the spaces between soil particles. This also interrupts the balance of water and oxygen in the pot.
- Aesthetic/Stylistic Change
- Sometimes you need to change the container that your tree is growing in. One example would be if you are moving the tree from a nursery container to a bonsai pot. Another reason would be moving the tree from a grow box to a bonsai container.
Where do roots grow?
Roots don’t just grow in the pot all willy nilly. Roots like to grow in an area of the pot where there are heat and oxygen. Roots will search for this. They will grow out to the side of the pot and then down to the bottom of the pot. These locations have the best airflow and have the best water drainage.
The area right under the base of the tree is called the shin (sheen). This area is also in the middle of the pot. This area tends to stay either to wet or to dry so it is important that we maintain that balance of water and oxygen. Roots will grow here, but they will grow better if we can give them the conditions that they like. It is important to keep this area healthy.
- Side of the pot
- Bottom of the pot
- Shin – Heart (area directly under the tree base)
- No oxygen
What is the Shin (sheen)?
The shin is the heart of the tree. It is the area right under the base of the tree. It dictates if the tree has a high tolerance for disease and insect attacks. Keep the shin healthy by maintaining a balance of water and oxygen.
How to repot a bonsai
- Start with the area of greatest limitations.
- This limitation is what is stopping you from getting the tree into a bonsai container.
- The bottom portion of a nursery container
- The weird long root of a collected tree
- Always maintain an area that you don’t touch and that you do not bare-root. (conifers)
- Clean area of roots around the edges, bottom and top to allow it to fit into an appropriate size container.
- Place it into your prepared container and fill it with bonsai soil.
- Next, repot in 3-4 years you will leave the edges alone and work on the shin.
Nursery Container to Bonsai Pot:
- Remove your tree from the pot.
- Use sickles or Kamas to loosen the soil and roots around the edge of the pot.
- If the tree is in heavy clay soil, go ahead and remove that soil by teasing it away.
- If you encounter roots it is ok to leave the soil there.
- Clean the top layer of the soil off to find the nebari (root structure)
- Most always you will find better roots under the soil.
- Stop when you find no more flare.
- Move to the bottom of the root mass.
- Area of most limitation on most nursery trees
- Establish the angle of the tree and remove soil to reflect that angle change. Do not just go straight across and make a flat bottom, then jam soil into the pot to reflect the change. Remove soil from the root mass to reflect the change in angle.
- When you have removed this soil go ahead and cut some of the roots that are growing straight up. Remove from the upper areas any fine coarse roots. Preserve others that could be used as a feature and the fine roots.
- Now that you have Removed that area if limitation, move to the next.
- This area might be on the upper opposite side of the root mass as it might stick up and out of the new pot.
- Once the soil has been removed to reflect the angle change you can lay the tree down and work the root area flat so that it can fit into the new container while maintaining that angle. Cut the roots that will not fit into the new pot.
- Always maintain an area that is untouched by scissors or chopsticks. (tools)
- Using a Kama or Sickle clean out the soil from around the edge of the bonsai pot to free the roots that have grown out to the edge.
- Cut the tiedown wires on the bottom of the pot.
- Remove the tree from the pot.
- While removing the tree watch the root system.
- If the trunk moves separately from the roots you may have a spot where the roots are not separated from the pot.
- With the tree tipped on edge remove the bottom matted layer of roots.
- Using a sharp paint scraper you can remove that bottom layer easily.
- Being that we are not changing the angle you will want to maintain a flat layer.
- Now is your chance to remove some field soil if your root mass still has some in it.
- It can take 2 repots and sometimes 3 or 4 repots before all of that field soil is removed.
- Remove soil from the bottom of the root pad to make room for repotting.
- Trim the damaged roots.
- Set the tree upright and work the outside edges of the root matt.
- Remove about 1/2 – 3/4″ around the edge so that the new roots have room to grow.
- Trim the roots along the edge as you clean out the area.
- Remove the top layer of soil. This consists of the following.
- Broken down soil.
- Trim the damaged roots or the roots that are overly long.
Preparing New Container
Preparing New Container
- Mark the front of your pot so that you remember where it is.
- Cut drainage mesh to cover the holes that soil could get washed out of.
- You secure the mesh down with wire. You do not want the mesh to move.
- Set your tiedown wires.
- Insert these wires from the bottom of the pot.
- These wires will get tied into the root system to tiedown your bonsai into the container so it will not move.
- Place a single particle layer of larger soil in the bottom of the pot to create an aeration layer.
- The particle size of 1/4-1/2″.
- This helps promote airflow in the pot.
- Using the soil that we are going to use for our bonsai we want to place a single particle layer on top of that aeration layer.
- Particle size 1/16-1/4″.
- It is ok to see your larger particles of your aeration layer.
- Now create a cone of soil in the middle of the pot.
- It should look like a volcano.
- This is what you are going to settle your tree on.
- This will be the soil that is on the bottom of your tree.
- If the bottom of your root pad is relatively flat, you can use a smaller cone.
- With a root pad that has lots of holes in it and is uneven, you would use a bigger cone.
- Place your tree down on this cone.
- Maintain the angle that you want.
- Shift your tree back and forth to move that soil around under your tree.
- Tuck the roots into your pot.
- Use your tie-down wire to tie down your tree into the pot so it will not move.
- Fill the space between the root mass and the pot with soil.
- Use a chopstick to help fill the large gaps with soil.
- Use an in and out motion with the chopstick while holding your other hand over the soil.
- Using a thinner chopstick you want to change your angle to the inside of the tree.
- This will help fill in the small gaps in the root mass.
- Start in one area and move around the pot until you return to the spot you started.
- As you are chopsticking, you will want to continually feed more soil into the hole that you create.
- You know you are finished when no more soil will fall into a hole where the chopstick is inserted, and it feels solid.
- Contour your soil.
- Use a hand broom to sweep the soil from the edge of the pot.
- You want to leave 1/8 – 1/4 inch of space along the rim of the pot.
- Remove any roots that have come free during this repotting process.
- Top dress your soil. (top dressing creation)
- Using a mix of sphagnum moss and green moss, create a layer on top of your soil.
- Solidifies your soil.
- Topdressing helps maintain moisture at the top of your soil.
- This promotes fine roots.
- Mist the topdressing so that when you take it outside to water, it does not blow off from the wind or get washed away.
- Water your tree.
- Take your tree outside and water it well.
These two images are of a Ponderosa Pine bonsai durring its repot in the spring of 2019. You can read about the progression of this tree here. Ponderosa Pine Progression (Fat Bottom)
Repotting Bonsai videos
I encourage all of you to watch these videos on this page. They are all very informative and from excellent sources. Take your time when repotting. Keep your roots moist. Some more advanced techniques can be utilized in repotting bonsai, but I am not going to cover those here. If you are looking for repotting supplies, you can find some in our store.
Again if you are looking for bonsai soil, I buy my premixed soil from Sean Campbell at Desert Bonsai. You can find him on FaceBook here. Desert Bonsai.