Root pruning

Root pruning is the practice of cutting back a portion of a bonsai's root mass during repotting, removing thick, circling or downward-growing roots while keeping a healthy population of fine feeder roots intact. Because bonsai live in a limited volume of soil indefinitely, root pruning is what keeps the root system from becoming pot-bound and declining in health over time.

How much to remove depends heavily on species, season and the tree's overall vigor, with vigorous, well-established trees able to tolerate a larger reduction than a weak tree or one recently collected from the ground. Root pruning is usually paired with fresh, well-draining soil, since old, broken-down soil around the roots is often part of the reason a repot was needed in the first place.

Cutting too much off a stressed tree, or root pruning at the wrong time of year for that species, is one of the more damaging mistakes in bonsai care and can set a tree back for a full growing season. Sharp, clean root pruning shears matter here too, since crushed or torn root ends heal more slowly than a clean cut.