As the your bonsai begin to break dormancy, the energy moves from the roots to the buds, and the buds begin to swell. Once the energy has been allocated you can cut the roots without removing the energy from the tree in the process. Yesterday we repotted a Trident Maple (Acer buergeranum).
Trees do not allocate energy equally among all buds. The way that the energy is distributed depends on the amount of light that branches receive and the tree’s natural growth habit. Bonsai are just trees, and this holds true for them as well! Buds that have been allocated more energy break sooner, swell larger, and will put out longer growth. You can use this knowledge to your advantage when training your tree.
To start we cut the wires that were anchoring the tree into the pot, and carefully pulled the rootball out of the pot so that we could assess the health of the tree. The tree had been growing in pure turface and put out many new white, fleshy roots. These types of roots are designed to pull nutrients from the substrate into the tree. The more of these types of roots that you have, the more water, CO2, and nutrients the tree can absorb. This particular Trident Maple had many roots, to the point where they were wrapping around the pot. Maples usually grow their new roots after the leaves are out, so it was not clear whether the roots had formed over the winter or in the prior year.
We started by preparing the pot. Bonsai pots have large drainage holes in the bottom, much larger than the size of the particles. This is to provide ample drainage so that the tree does not have “wet feet”. We need to keep the pot’s ability to drain, while keeping the substrate from falling out the bottom of the pot. Here we are using black plastic mesh, cut down to a size slightly larger than the drainage holes.
To keep the mesh in place we create a set of wire loops that get threaded through the mesh, and locked in place by bending the wire. Between the wire and the weight of the soil, it’s enough to keep the holes locked in place.
All plants need a mixture of water, nutrients, and CO2. All three are taken up by the roots, but unless the water is moving (therefore oxygenated), the tree cannot absorb both its water and CO2 at the same time. Bonsai need a flush of water, followed by a flush of air. Too much time without water and the roots dry out and die. Too much time without air and the roots suffocate. It’s a balance, and one that is easier maintained by using larger particles which are porous. We will be potting this Trident Maple into lava rock mixed with a little bit of peanut shells.
We then started “combing” out the roots using a root hook. While we use the word “combing”, the hook is held very loosely between two fingers. The idea is to loosen the roots gradually, not to rip them off of the tree. A two-pronged rake can make the job go twice as fast, but remember to pull carefully, not rip!
When a bonsai’s roots wrap around the perimeter of the pot, they can be quite long! We want to remove enough of the roots so that new ones have room to grow, but not so much that the tree cannot support itself nutritionally. The rule of thumb is to only remove 1/3 to 1/2 of the tree’s roots. We cut them with a pair of sharp bonsai shears. A sharp, clean cut will encourage the roots to branch out where they were cut.
The tree is tested for planting debth. We want the soil line to approximately the same as it was before.
The depth is good so we run a length of wire through the mesh, from one drainage hole, across the outside bottom of the pot, and back up and out of the other drainage hole. This wire will anchor the bonsai in its pot, particularly important while the new roots are growing.
We place a small amount of the substrate on the bottom of the pot. This is where most of the water will be held naturally in the pot, and you don’t want the area to dry too fast or too slow. The bonsai is placed in the pot, the anchor wire is run up through the rootball, and we fill the rest of the pot with our potting medium. A chopstick is used to settle the particles down around the roots. This is done to reduce the size of the airpockets, and increase contact of the roots with the substrate.
We then run the anchor wire across the top of the rootball and twist the two ends together. In this case, we run the wire beneath the nebari as to hide it. A final quarter-twist of the anchor wire underneath the pot pulls the anchor wire nice and tight.
A little bit of Mycorrhizal innoculant is added to the surface of the substrate. Mycorrhiza are a beneficial fungi that form a symbiotic relationship with other plants, including bonsai. These fungi allow the tree to absorb water and nutrients more efficiently and can commonly be seen among the roots of bonsai, especially pines!
Finally we water the tree well but with a fine spray as to not wash the Mycorrhiza away. This Trident Maple should be good for about two years in its new lava rock!