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In Southern California and similar climes, Black Pine bonsai are best root pruned and potted (or repotted) between Christmas and the first week of April. Once a Black Pine starts pushing long candles, it is too late to repot it, and the preferred time to repot a Black Pine is in December and January.
Soil mix should be twenty to twenty-five percent organic, and the rest aggregate. While some bonsai folks may tell you to use entirely inorganic matter for soil, this is a very bad idea with Black Pine bonsai. Japanese Black Pines have a symbiotic mycorrhizal fungus that grows on the roots, and this fungus helps the tree break down the nutrients in the soil. An organic component in the soil is essential to the health of the fungus, and therefore the health of the tree.

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