There's a lot of confusion surrounding how to care for Japanese Black Pine bonsai. This confusion likely stems from the reality that these trees require different work to be done at different phases of their life. Additionally, black pine that are growing slowly require different care from those that are growing vigorously. It's important to carefully examine your tree and try to determine how vigorous it is and what type of work it needs, based on it’s age and condition.
Below is an outline of basic annual care cycles for caring for mature Japanese Black Pine bonsai.
Starting in November: Thin the prior year growth to two buds at each decandling site and pull old needles from the tree. Some old needles can remain either to keep the tree full for show or to balance weaker branches with stronger ones. Aim to balance the entire tree by removing more needles from the stronger parts to reduce the vigor to match that of the weaker parts. Ignore tiny buds and very weak areas. You can either leave or remove small buds that are along the branches; consider if they are needed for further branch refinement or replacement of a leggy branch.
December through February: repot if needed, do not bare-root more than 50% of the rootball of a mature pine. Mature pines can go 3-5 years between repottings. If you live in a climate where frost or freezing temperatures occur protect repotted trees from colder temperatures until spring.
March-May: As the new candles begin to elongate you can twist off the top half of a few if they are much stronger than the rest of the candles. Hold the candle with one hand and twist off half the needles with the other. This is not decandling, this is another technique for equalizing growth. Fertilize heavily during spring growth to prepare for decandling in summer.
June-July: Decandle from June 1st through mid-July. The timing of decandling depends on heat, fertilizer routine, and the health of the tree. Only healthy, vigorous trees should be decandled. In my yard in San Francisco I decandle starting on June 1st which is what I recommend for the eastern half of San Francisco. If you live in the western half decandle in the last week of May. Experiment with your conditions and adapt the date of decandling after assessing whether the needles were the correct length the previous year. If your needles were too short decandle earlier or use more fertilizer, if they are too long decandle later or reduce fertilizer after decandling. Decandle larger pines first and smaller ones later.
After the candles are removed thin old needles to control the relative vigor of the buds. Count the needle pairs left on each branch; leave more needles on lower and weaker branches and fewer needles on strong branches. 8-10 pair on strong areas, 10-12 on middle, and 12-14 on weak areas.
If you're looking for more information on caring for mature Japanese Black Pine, watch these videos from our YouTube channel, "Black Pine Decandling: "Reading the Tree" and Assessing Health"! There are three videos focused specifically on established bonsai.