Shaping Bonsai — Branch & Root Pruning, Trunk Thickening

First root pruning. Branch pruning is unnecessary for young trees. Early on, the priority is root, nebari and trunk development. After that, branch growth needs to be encouraged so the basic form of the tree can take shape; this growth can be assisted by wiring the trunk and branches. Branch pruning is the beginning of the "fine tuning" process of creating your bonsai. Because it is a moderately stressful event, avoid it during weather or temperature extremes. In summer the cuts can be damaged by strong heat and sun; in winter they are easily susceptible to freezing, and winter pruning risks overly dense foliage and overlong internodes the following spring.

Branches and leaves. If you are still thickening the trunk, branch pruning helps — the process of tate-kae involves selective pruning to increase the trunk's size. Pruning is typically done on the trunk right above the branch you want to model as your new trunk, and can also change the direction of trunk growth. Internodes are the spaces between new buds, and extra space between buds is not desired. The milder conditions of spring and autumn cause the least potential damage. The type of plant also matters: deciduous trees are ideally pruned in spring before leaves and new growth appear; coniferous trees should be pruned at the end of the dormant period, usually between autumn and early winter, before freezing begins.

Selecting branches. Decide which branches to remove to keep the tree balanced. Two branches at the same height are undesirable (one should go), as are vertical branches too thick to twist, branches with strange curves, and abnormally thick branches at the top. Styling is about how you want your tree to look, but these are the basic rules.

Care after leaf pruning. Reduce watering after pruning and discontinue fertilization until new budding occurs. If the leaf amount has been radically reduced, place the plant in a bright but shaded area for about ten days to assist recovery.

Trunk thickening. (1) Split the trunk — at the start of the growing season, take the tree out of the pot, wash soil from the roots, and split the trunk from the bottom up with a trunk splitter, chisel, rotary tool or saw; hold the split apart with wire or wood wedges and let the tree heal (an oversized pot speeds recovery). Only use on trees able to recover from such trauma. (2) Induce swelling — an ancient Japanese technique using a wire tourniquet at the very base to create a tapered trunk even on species not naturally tapered (e.g. Chinese Elms). It gives quick results on fast-growing species but leaves permanent scarring, so apply with moderation. (3) Sacrifice branch — allow selected branches to grow as thick and long as possible to thicken the trunk, then cut them off; repeat as needed. Works best grown in the ground or large pots. Branches at the top thicken the whole trunk; lower branches thicken the lower trunk.

Tachiagari and kokejun. Tachiagari development is accelerated by new limbs from the trunk, which force more water and nutrients through and make the trunk thicken; removing limbs removes that flow. Ensure adequate sunshine, water and fertilizer. Repeated pruning for kokejun encourages budding on the stump (works for plants like Chinese elms); buds are repeatedly pruned and regrown, with pruning in early summer and late autumn, and the trunk shaped with wire as it grows — a multi-year process taking up to three years. In tate-kae, prune the trunk just above a desired branch so that limb thickens and becomes part of the trunk; repeat to cultivate ideal kokejun. Pruning is scheduled mostly when the tree is dormant (as with conifers), though some trees may be pruned any time.