Repotting is done to keep the bonsai tree healthy,With a normal container plant, repotting is done before roots overcrowd the pot (become potbound) and are repotted into a larger pot, but with bonsai they are usually repotted into the same or similar pot because it is part of the plant and pot aesthetics. It is for this reason that repotting and root pruning are very often done at the same time.
Young bonsai trees and trees in early training should be repotted every year and older bonsai can be left for 2 to 5 years depending on the size of the tree and its pot.
Hardy bonsai can be repotted in late winter or early spring but if the buds are swelling don’t do any root pruning. Pines can be repotted in mid to late spring .
Tropical bonsai and sub-tropical bonsai can be done nearly any time if the conditions are even all year round or done in early spring to be on the safe side.
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