If you notice that your bonsai tray holds water longer than it should, or the roots of your tree seem crowded together, it might be time to repot your bonsai. Evergreens such as juniper may need repotting every three to five years, while fruiting and flowering plants, such as citrus trees or azaleas, should be repotted yearly and deciduous trees, such as Japanese maple, should be repotted every other year. Repotting involves root pruning and replacing the soil. Doing this regularly helps maintain the bonsai's health.
Unwrap any insulated electrical wire from the bottom of the bonsai tray that may be holding the bonsai tree in place.
Remove the moss covering from the surface of the soil. Set it aside to return to your tray once the tree has been repotted.
Support the trunk of the bonsai tree with one hand. Tilt the tray on its side and gently tap the bottom to free the root mass from the tray.
Loosen the soil around the roots gently so that the roots hang free of the tree. Prune away one-third of the roots on the edges of the root mass and half of the roots on the bottom of the root mass. Hold your bonsai upright over the tray while you prune. Hold your pruning shears parallel to the tray. Wipe out the tray and discard the soil and feeder roots.
Cover the bottom of the tray with galvanized wire screening with 36 wires per inch. You can either re-use the screen that was already in your tray, or replace it with fresh screening.
Dry and sift coarse sand, garden loam, clay subsoil and humus into three grades using three separate sieves. Your first sieve should contain 4 wires per inch. The second sieve should contain 8 wires per inch. Your third sieve should have 15 wires per inch. Discard any soil that passes through the last sieve. This soil is too fine for your tree and will compact around the roots.
Create a drainage layer by mixing 1 part sand, ½ part an equal blend of loam, clay and humus from the sieve with 4 wires per inch. Place this over the galvanized wire in the bottom of your bonsai tray.
Place the tree back onto the tray and wire it in place with insulated electrical wire. You can either re-use the old wire or replace it with new wire.
Create a potting mixture by mixing 1 part humus, 2 parts sand, 1 part clay and 2 parts loam from the sieve with 8 wires per inch. Place this soil around the roots of your plant. Work the soil in between the fine roots with a chopstick to eliminate air pockets.
Create a topsoil mixture by mixing 1 part loam, 1 part clay and 1 part humus from the sieve with 15 wires per square inch. Cover the roots with a thin layer of this topsoil.
Place the moss over the topsoil in the tray.