The process of repotting bonsai to be black pine can be done once in three or four years, because black pine’s root grows slowly. Do not repot before two years because its roots haven’t been old enough. If we repot after 4 years, the roots will be too dense in the pot, and this will retard its growth and fertilization.
Please repot this way. After pulling bonsai out of the pot, we may see the lump of root that has been converged with the soil. Throw 1/3 part around the lump by scraping the soil and shearing the roots that is too long. Then put it again into the pot with the medium described above, by scraping, so the tips of the old roots can directly penetrate into and converged with new medium. This will guarantee its life after repotting. Don’t even repot by cutting or sowing the lump of soil and root flat as if we’re slicing the cake and then plant it again. This way make the old root lump covered by the new medium, causing an undistributed heat in the lump and will finally make the root rotten.
According to my experience in planting black pine, we shouldn’t do pruning while repotting. Once my gardener didn’t follow my instructions. When I went out of town and didn’t supervise him, he was repotting and pruning at the same time. This caused some half-grown black pine trees died.
In Indonesia which has two seasons, the best time to repot black pine is between the end of dry season and the beginning of rainy season (around the beginning of September), but pay attention to black pine condition too. Repotting should be conducted when new bud has just come up before the leaves blossom. When the leaves have blossom, then the tree will be weak and the risk of repotting will be bigger.
Well, that’s my experience on treating black pine bonsai. Hope it’ll be useful for those who want to treat or collect black pine bonsai.