Below is an outline of the basic annual care cycle for developing semi-mature Japanese Black Pine bonsai.
Semi-mature trees are ones where the trunk is not being developed further but where branching is still sparse. 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.
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.
Want to see decandling a middle development japanese black pine in action? Watch this video from our YouTube channel, "Decandling Japanese Black Pine: Refining a Bonsai in Mid-Development"! Eric works on the same tree from this article!