There's a lot of confusion surrounding how to care for Japanese Black Pine (Pinus thunbergii) 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 the growing season, and the tree's age and condition.
Below is an outline of basic annual care cycles for developing and caring for young trees, semi-mature, and mature Japanese Black Pine bonsai.
Young Trees in Development
With trees in development the challenge is to balance vigor between the sacrifice branch and the finished branches. Sacrifice branches are the only way to significantly increase the size and/or taper of the trunk. Consider using a well-placed branch on older and under-developed material to increase girth and taper. It is possible to maintain mature branching while simultaneously increasing trunk girth on black pine. You can greatly increase the quality of many pines in development through 4-5 years of growth of a sacrifice branch done simultaneously with branch development. At the same time treat the small branches like you would on a mature tree. Decandle them if the growth is vigorous to maintain a compact structure.
November: Wire young branches. Leave small buds on the trunk unwired unless they are the size of a chopstick and 4-5 inches in length. Reduce old needles only to maintain finished branches if they are dense. If branching is sparse leave old needles. If any branches were decandled perform the normal bud selection as with a mature tree on those branches.
Reduction of the sacrifice branch can be done at this time – anytime a sacrifice branch is reduced it will greatly increase the vigor of the lower branching. Cut up to 75% of the sacrifice branch off at one time but not all of it. Consider leaving the first side branch on the sacrifice as a new sacrifice.
December-February: 2-4 year old seedling pines can be completely bare-rooted. 1-3 years after the seedling cutting technique is used remove all soil and carefully spread the roots out to form the start of the nebari on the tree. Do not wire the roots or use wire to secure the tree in the pot. Use guy wires from the wire wrapped around the trunk to the container to secure the tree.
For pines growing in colanders (air-pruning the roots) in year 4-5 do not double-pot them as suggested in Bonsai Today #20 article. Slip the trees out of smaller containers and repot without root work into larger ones. You can also stack a smaller colander on top of a larger one to allow the roots to "escape." This type of change can be done at any time during the year, not just during dormancy. Scrape topsoil that is clogged with organic fertilizer remnants off. Examine the surface roots for undesirable crossing roots and roots that wrap around the trunk, cut them off or gently reposition them. Refill the top with a thin layer of fresh soil. Significant root work on young pines that are growing vigorously will greatly reduce wood production on the trunk the following year, minimize root work to only correct drainage and improve the nebari.
March-May: Fertilize heavily. For trees in development more fertilizer will lead to faster growth and development. Remove female cones by gently twisting them off of the tips of the strong branches.
June-July: Consider the development of your tree. If the finished branches are weak or short do not decandle. Decandle to shorten nodes or selectively weaken branches. For small trees – shohin and kifu size – consider decandling behind the node to cause needle-buds which allow more compact growth. Sacrifice branches can be reduced at this time. Reduce no more than 75% of a large sacrifice. Removal of a strong sacrifice during this time can lead to excessive budding at the cut point which can cause reverse-taper. Plan ahead and reduce in stages.
Semi-mature trees are ones where the trunk is not being developed further but where branching is still sparse. With healthy Japanese Black Pine trees, the cycle is basically the same as with mature trees (below). Adjust decandling times to allow for slightly longer needles to maintain health of the tree. With trees that are weak do not pull old needles or decandle. A weak black pine can take 2 or 3 years to regain normal growth cycle, watch your tree carefully and do minimal work until the tree is healthy.
November: Assess strength. On strong trees prune long branches, thin shoots to two per terminal, select for equal strength, position and angle relative to the other bud. Consider design – if the branches need to be longer do not prune them. On weak trees thin only the strongest areas; leave all needles and buds in weaker areas.
December-February: Repotting is often the single-most important thing that will allow a pine to regain vigor and grow well. Do not bare-root an entire pine. If the tree is weak bare-root one-third or half of the rootball and feed heavily in the following season to get the tree healthy. One year later bare-root the other half or two-thirds of the tree to complete the repotting. Note: Your local repotting timing may vary - repotting is best done in early spring. Wait until most danger of freezing temperatures has passed, repot as the buds begin to swell, but before they elongate significantly.
March-May: Fertilize, fertilize, fertilize! Use organic and chemical fertilizer. Do not remove new growth.
June-July: Consider decandling. On a weak tree decandling will be counter-productive. For strong trees follow the instructions below. Decandling of some branches, like the top half of the tree can radically shift the relative health of branches. If lower branches are weak and top branches are vigorous decandle selectively to try to re-balance the tree.
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 healthy trees 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.