Based on San Francisco Bay Area Climate by Mitsuo Umehara
February and March are the busiest months for the bonsai person. All of the heavy "work" is done during this time including styling, root pruning and potting of the collected material. And don't forget the last dormant spray at this time also.
Transplanting of deciduous trees should be done in the order of the bud's movement. Akebia first, then karin, quince, some maples, stewaria and beech. Transplanting at this time allows drastic root-pruning and severe elimination of old soil. This must be done with care so that the water-line (soft root surfaces) will not be damaged.
After care of transplanted trees: Protect from sudden frost and heavy rain. Select an area which gives you maximum sun exposure. With the warmth of the sun, the buds will start to move within 3 weeks. Then start fertilizing with a diluted liquid fertilizer.
Time to work on Satsuki azaleas. Remove dead leaves, cut back where it is needed and re-shape. Go through all the surface work first, marking the pots that need transplanting at the end of the month.
Deciduous trees not transplanted this year are ready for fertilizer. The first dosage should be diluted liquid fertilizer. Any hard wood cuttings should be done before the buds start to move.
If you have a maple group planting which is too tall and the crown area became too heavy, you may want to air-layer that portion. It must be done before the buds start to move. By June, enough roots should be developed to separate the air-layered portion. When separating the air layered portion, cut all the leaves off from the air layered portion to minimize the shock and to balance with the new roots, which is now on its own to feed the entire little tree. Let the new buds on the original tree(s) grow to make the new crown. Cut back and shape the rest of the group planting. You now have two bonsai. Japanese will say "One stone hits two birds."
All conifers, excluding cryptomeria and needle junipers.
All remaining deciduous trees
All flowering/fruit bearing trees, excluding crape myrtle, the citrus family and pomegranate, can be transplanted at this time.
Transplanting is done to:
Avoid problems of a root bound tree.
Improve soil conditions for the tree.
Upgrade to better pots.
The cardinal rules of transplanting are:
Timing - Selecting the perfect time to transplant is a supreme technique by itself.
Avoid heavy wiring either right before or after transplanting.
After heavy top pruning, do the same with the roots. The tree above the soil and under the soil must always be in balance.
Select the pot with great discretion. Study the harmony between plant and pot.
Make sure the tree was watered the day before so it is well hydrated, but you don't want the soil so wet that's like mud.
When loosening the soil around the root-ball, be very careful not to damage the root waterline (soft root system surfaces).
Always use sharp scissors to cut the roots. Never pull off roots.
Once you are satisfied with the position, height and angle, securely fasten the tree in the pot.
The soil should be sifted to remove "fines" (dust) before use.
No cavities or air pockets should remain in the soil after the repot. Use chop sticks in a circular motion in the soil to achieve this.