Defoliation: cutting every leaf to make the second set smaller

Defoliation is the most misunderstood technique in deciduous bonsai. It is not a way to make a tree look better this month; it is a controlled stress that produces smaller leaves and finer branching next time, and it does real damage if used on the wrong tree.

What defoliation actually does

Removing the entire leaf surface forces the tree to draw on stored reserves to push a second set of leaves from the buds that were already present at the base of each stalk. The second flush is produced under pressure and under shorter days, so each leaf comes out smaller — typically 30 to 40 percent smaller on trident maple, and the effect carries over partially into the following spring. The second benefit is branching: because the tree is forced to break buds along the whole branch rather than only at the tips, the ramification increases at every node.

Full or partial, and how to choose

Full defoliation on a strong trident maple in the first week of June produces the smallest leaves but costs the tree the most. Partial defoliation — removing the largest leaves only, or removing all leaves from the outer two thirds of the canopy and leaving the interior — gives roughly three quarters of the leaf reduction at half the cost, and can be done in consecutive years. On this project the rule is: full defoliation once every two years on the strongest trees, partial defoliation annually on everything else that is strong enough.

How to cut, precisely

Cut the stalk about 2 mm above where it joins the shoot, leaving a short stub that seals itself within a day. Do not pull leaves off — pulling tears the axillary bud away with the stalk and leaves an open wound that will weep and can rot. Terminal buds at the tip of each shoot are left completely intact, because those are the buds that drive the second flush. On a 40 cm trident maple this is roughly 400 individual cuts and takes about 35 minutes.

The week after, and the traps

A defoliated tree has almost no transpiration, so it uses a fraction of its usual water. Continuing to water normally will waterlog the roots, and a defoliated tree in wet soil in June is the classic route to root rot. Watering is reduced to about a third of normal for seven days and the tree is kept in shade. A week later the new buds are visible; from that point the tree is moved back to morning sun over three days and fed at half strength every fortnight. The most common mistakes are defoliating a tree that was repotted in spring, and defoliating the same tree in two consecutive years.