Repotting Workshop for the Bonsai Garden at Lake Merritt
By George Haas

On January 21, 2025, from 9:30 a.m. to 2:45 p.m., at the Bonsai Garden at Lake Merritt (BGLM), a bonsai repotting workshop was led by professional artist and instructor Adam Toth for BGLM volunteers, also referred to associate curators who work on the collection trees.
The repotting workshop was open to all skill level bonsai practitioners. This was an opportunity for BGLM volunteers to learn the latest bonsai repotting techniques from Adam who studied as an apprentice in Japan.

Similar workshops will be scheduled for working on collection trees during the year.

All bonsai require repotting from time to time. General rules for repotting bonsai trees are one to two years for small, three to five years for medium and five to seven years for large. However, repotting intervals may vary due to the following factors:

Age of the tree
Size of the tree and pot
Breakdown of the soil or compacted soil
Root growth or root bound
Water flow decreases

Repotting may require a variety of hand tools:

o Wire cutters
o Pilers
o Large root scissors
o Root cutters
o Rubber mallet
o Tweezers
o Chopsticks
o Steel pick
o Single prong root hook
o Triple prong root hook
o Japanese large sickle saw
o Japanese small sickle saw
o Repotting blade
o Soil tamper
o Whisk broom
o Plastic drainage mesh
o Aluminum wire

Adam started by pairing volunteers to work as partners on the repotting of collection trees. For this workshop session, a total of ten BGLM volunteers participated.

Various trees were selected from the collection to be repotted, both conifers and deciduous. The paired volunteers worked as teams to repot the trees, all of which were due based on the aforementioned repotting factors.

Most trees were repotted into to the same pots. In some instances, the teams selected different pots which may have been larger or deeper. Pot selections were due to evaluations of the tree’s health or root mass and condition since last repotting.

Generally, the procedures for repotting bonsai trees are the same. Start with removing the tie down wires cutting from the bottom of the pot. Use a Japanese sickle saw or blade to remove soil from the inside edge of pot. Adam showed how to hold and angle the saw so that soil is moved to the inside, using a scrapping motion and increasing the depth of the saw with each pass.

Removing the bonsai tree should be removed from the pot with ease. Although, a rubber mallet can be used to tap the rim of the pot, if needed. It works well with one holding the bonsai tree and pot and the other removing the entire tree.

The pot is then thoroughly washed and cleaned with a brush and water. The pot should be prepared to receive the bonsai tree by installing plastic drainage mesh and tie down wires. A single layer of pumice or soil mix is placed in the bottom of the pot with a mound built in the center. The pot is then set aside, and work is begun on the root ball.

Adam demonstrated starting at the bottom of the root ball by removing up to a third of the roots and old soil. A Japanese sickle saw is useful for this work. The loosened roots are then cut with a sharp root scissors, making sure the bottom surface is flat. Nest, the roots at the surface and sides are worked by raking with a prong root hook. Combing the roots away from the trunk and trimming the sides with root scissors.

Note that on confiers, the root ball may be dense and compacted so that water will not penetrate it, as needed. In this case, use a steel pick to make a number of holes in the root ball so that water will be able to penetrate its core.

The bonsai tree is ready to be placed into the pot. Take the time to adjust the front and sides of the bonsai tree and make it centered in the pot or as desired. Press the root ball and move it side to side to settle the root ball within the mound of soil previously placed in the pot. Add some more soil to the sides of the root ball and then secure the tie down wires to ensure there is no movement of the root ball.

Add more soil to the pot and begin to remove air pockets by using chopsticks. Adam showed the technique for using chopsticks to remove air pockets. It requires sliding the chopstick down the inside edge all the way to the bottom of the pot. Then raise the chopstick and observe the soil level to move lower to fill the air pockets. Level the soil slightly below the edge of the pot rim and proceed to press down on the surface using a soil tamper.

When the repotting is complete, it is critical to thoroughly water the bonsai tree. Light sprays of water will flush out any fines making the cloudy water turn clear.