Pruning.

Doesn’t it hurt the tree? No more than pruning hurts your roses or that dusty old privet hedge that you unceremoniously hack back too many times a year. In fact, pruning an established bonsai helps keep the foliage mass in equilibrium with the roots, thus balancing the tree’s needs with its ability to supply them.
Bonsai Pictures and experienced gardeners alike will tell you that the best time to prune is autumn, but there is no evidence to support this. Pruning in autumn risks considerable die-back around the wound, from frost and water damage. The tree has to wait several months before it can begin to heal or regenerate, which must be bad news.
Traditional gardening practice is quite probably based more on the fact that in spring and summer there are more chores to do than in autumn, rather than on sound horticultural sense.

There are three reasons for pruning a bonsai:

To encourage new vigorous shoots for developing into branches.

To reduce long branches to a more suitable length for further development.

To thin out congested areas on established bonsai.

We’ll look at each one in turn.

Pruning for new growth.

When developing deciduous bonsai from scratch, it’s quite common the cut all the branches off flush with the trunk and use some of the new shoots that arise as a result to begin branch development.
Later, when the shoots have themselves become thick and sturdy, further drastic pruning is done to generate secondary branching.
The timing of this type of pruning can have significant impact on its success. For a good crop of new adventitious buds to appear and grow away strongly, three conditions must be fulfilled. First, the tree must have plenty of energy-giving sugars in its system. Second, there must be chemical signals coming from the roots (cytokinins) to tell the area of cambium around the wound to get moving on new bud production. Both these conditions are fulfilled at almost any time during the growing season.
The third condition is that there must be sufficient growing time left for the new shoots to become established and harden off before winter.
The ideal time for developmental pruning is midsummer – around the middle of June in the UK. Within two or three weeks a crown of tightly packed buds will appear between the bark and the wood at the edge of the wound. More buds will also appear from random points all over the tree.

Many of the new shoots will spontaneously abort before they become established. This is nothing to worry about it’s just the tree deciding for itself which it would prefer to retain as new branches. You should allow it to do this, then further reduce the number of shoots in autumn.
If you prune out of season during winter or early spring – there will be fewer if any buds formed around the wound, and only a handful of new shoots growing from elsewhere.

Shortening branches.

The principle behind reduction pruning is similar to that of development pruning above, but the timing is usually less crucial.
More often than not, there will be some dormant buds, or at least visible internodes, at or near the point where you want to prune back to. If this is the case, you can prune either in midsummer or in late winter or very early spring, just as the buds at the tips of the twigs start to swell.
If there are no clearly visible buds or internodes, prune in midsummer following the procedure outlined above If you’re only pruning one or two branches, you must reduce the amount of foliage on the parts of the tree above the Wounds and turn the pruning cuts to face the sun. This prevents the tree from channelling all its energy towards the upper part of the tree at the expense of the pruned branches. Trees are essentially lazy, and will always take the easy way out if you let them.

Important – Needle conifers (e.g. pines) and scale conifers (junjpers, cypresses) will. never produce buds on branches that have no foliage elsewhere.
Pruning away all the foliage on a branch will kill that branch.

Maintenance pruning.

This should always be done in late winter or very early spring, some weeks before the buds are due to open. Maintenance pruning in summer will interfere with the tree’s system and cause adventitious shoots to emerge where you don’t want them.
Some pruned areas may collapse and die back. It’s essential to prune the whole tree rather than isolated areas, to retain the balance of vigour. Prune rather more aggressively toward the top of the tree than on the lower branches. This will further help with the overall balance.
When doing maintenance pruning, remember that you have to allow for the coming season’s growth.
The newly pruned tree should look a little thin – as if it had just had a military haircut! Areas that are very dense must be thinned by cutting away congested outer areas so that selected inner shoots can grow to replace them. After several years of careful maintenance pruning, the density of the entire tree will be uniform, and pruning will become an almost mechanical operation – but pleasant, nonetheless.

How to prune.

The first rule is always to use clean, sharp tools.
Dull, dirty tools will crush the tissues rather than cut them; the wound will not heal well and budding around it will be poor.
Garden secateurs are all right for rough pruning, but they always damage some of the remaining tissue and their design means that a close cut is impossible. Proper bonsai cutters (angled or concave) allow you to cut flush with the remaining wood and even create a slightly hollow wound. Hollow wounds heal flat, so there will be no unsightly lumps of scar tissue on your finished tree.
If you don’t have bonsai cutters, arm yourself with a sharp modelling knife for whittling the pruned stub until it is flush with the remaining wood, and slightly hollow. Take special care not to crush or tear the bark around the wound, because this will mean certain die-back.

Sealing the wound.

Because the wounds on bonsai should heal perfectly, leaving little or no evidence, fresh wounds should be sealed to keep disease pathogens and frost out, and to keep moisture in. By far the most effective sealant is Japanese bonsai cut paste. It has the consistency of modelling clay, its colour blends well with the bark, and it contains fungicide to help keep the wound healthy. Cut paste remains soft, so as the wound heals it is gradually pushed off by the callus, or it can be easily removed by hand.
A reasonable substitute can be made by mixing plasticene with a little cooking oil to stop it hardening in the open air. Never on any account use bitumen-based wound paint or any other sealant that dries hard – this will disfigure the tree for years, and any attempt to remove it by hand will cause very noticeable damage to the bark.