Before the new plant is placed in its container, and each time it is repotted in the years that follow, the root system must be reduced. Restricting the root system to a size that will sustain the plant without encouraging vigorous growth is critical to the bonsai technique. Such pruning maintains the vital support relationship between the above-ground and below-ground parts of the plant.
Using a long stick, probe the soil ball until the soil has fallen away from the outer roots and they are hanging free. Cut away these roots. Continue until the soil ball and root mass are reduced to a size that will fit into the container chosen. It is easiest to do this when the soil is dry and important not to disturb the soil directly beneath and around the trunk. Young, fibrous roots will develop within the soil ball (Figure 12-17). Each spring, the bonsai can be lifted from its container and a
figure 12-17. Reducing the root system of a bonsai plant (Delmar/Cengage Learning. Photo by Ed Reiley)
similar treatment given to the root system. This will prevent the plant from becoming potbound and will also permit wires to be removed or added.
The bonsai should be set in its container, off center and to the rear. The dry, pasteurized soil mix can then be added, taking care to avoid air pockets around the roots. Some of the plant’s older roots may protrude above the soil. That is acceptable, even desirable, since it adds to the illusion that the tree is old. At least a quarter-inch of space should be left below the top of the container for watering.