Spring is coming soon .. so this is text about sprint care..
This is the time of year for regrowth, renewal, repotting, and restyling.
Faster-growing young trees or plants which have been in the same container for two or more years may need to be repotted. Before this is done, consider if a different size, style or color container will better enhance the appearance of the bonsai. Perhaps just a slight repositioning in the existing pot is all that is necessary. Or a radical restyling may be in order to finally bring out the true spirit of a particular "problem child." (Of course, during the winter slowdown you've already had time to do this kind of planning... )
Ideally, pot / re-pot just as the new leaf buds are reaching their maximum size before they start to open. This ensures that there are sufficient internal reserves in the plant to keep it going during the one to two weeks before new feeder roots have developed enough to continue absorbing nutrients for the plant. If major root pruning was not done at this time, feeder roots will be active much sooner.
Try to work in the shade, out of the wind, on a day when the dew point is at least 30 degrees F. Keep a spray bottle near to moisten the root ball as needed.
Use sharp scissors when cutting roots and make clean cuts. The younger and more vigorous your tree is, the more frequently you might prune roots. Remove the tap and other large roots with concave cutters. Be sure not to try to cut rocks or anything other than roots! The top growth of your tree is, to an extent, a reflection of the bottom growth, and vice versa. By keeping the roots fibrous and well balanced, the top growth will follow suit.
Fertilizer primary components of Nitrogen - Phosphorus - Potassium (K) are necessary, respectively, for Leaves & Stems - Roots & Flowers - Overall Health and Hardiness. At least fifteen other elements in smaller quantities are also vital for the life of plants. Organic fertilizer is not necessarily "better," but it can provide helpful small amounts of more complex substances which the inorganic commercial blends don't include. It is wise to alternate between the two types.
In the repotting mixture you may want to include about a quarter-label-strength application of slow release solid fertilizer with micronutrients. This could be either as Osmocote® granules or Gro-Power® planting tablets.
Have the roots not grown very much since the last time you potted? Consider your choices: use a coarser soil mix this time; make sure your pot drains better; decrease the nitrogen and/or increase the phosphate content of the fertilizer for this tree; do not disturb the roots for a few years.
Leave the plant out of direct sun or wind for a week, and gradually reintroduce it. Initially cut back somewhat on the amount of water you give it. Use the transition period to also spruce up your personal display area. Even if you never have visitors over, your trees deserve to be shown off in the best possible way. After a few weeks, begin adding some balanced organic fertilizer. Junipers and pines will benefit from weekly foliar feedings of fish emulsion. Finally, cut off any flowers just as they finish blooming to ensure the next year's good showing.
Unless you specifically want to bulk up a trunk or branch, do not let the new side buds and branches grow too long. Cut back and limit the active buds to only a pair or two per branchlet. The goal is to have compact, ramifying growth. Elms and junipers may require weekly "haircuts." Clean and oil your tools after each session.