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 an basic annual care cycle for developing and young 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.
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. 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.