How to Prune and Dwarf Bonsai Trees
The wiring of bonsai is primarily intended to train the tree into the desired shape, but it does also have a slight of dwarfing effect. This is because a branch that is bent out of its natural line has its flow of sap checked, and this, in turn, slows the rate of growth according to the severity of the bending.
It is on the various forms of pruning that bonsai mainly depend for their dwarfing, i.e., shoot pinching, leaf pinching, root pruning, and pruning during dormancy. Conifers should not be pruned too severely when they are fully grown, but deciduous trees and shrubs will stand quite hard pruning and produce replacement shoots. When one is pruning, the cut is made just above a node (leaf joint) take care not to damage the bud that lies between the shoot and the leaf axil. Equal care must be taken when pinching out shoots during the growing season.
It is most important to remember that a bonsai should not be starved to keep it small, but kept alive by carefully feeding manure and watering. A starved plant will be a sickly, and therefore unhealthy, plant.
Another point to remember here is that, although various forms of pruning can control the size of a plant, the size of the flowers and fruit will remain almost unchanged; the quinces and crab apples will look large sometimes when compared with the size of the plant. This can, however, be a rather interesting "discrepancy."
This pinching is started in the spring when the young shoots are about an inch long and helps to balance the shape of the tree. By removing part of the shoots leaves are also removed, and it is this that helps to dwarf the tree, as the leaves manufacture food from nutrients and water taken up from the soil by the roots. The very tip of the shoot is removed with the fingers, nails, or sharp, pointed scissors, taking care not to crush the remaining stem; in the case of conifers with needles, do not cut through the needles as this can be most ugly.
The tree, if active and healthy, will send out more shoots which may, in turn, may be pinched or removed entirely. The latter may well be necessary in many cases to prevent overcrowding. This secondary growth also very often produces smaller leaves than the first early growth, and these will be more in proportion to the size of the tree; it is most effective with trees such as beech, maples, oaks and other deciduous trees. Some very vigorous trees will keep producing shoots, and these must be pinched as soon as they are an inch long. Do not allow them to grow any longer, because the resulting scar will take longer to heal over.
Naturally, the shoots will not grow at exactly the same rate, so pinching cannot all grow at exactly the same rate. Pinching cannot all be done at the same time but must be spread over a period, pinching each shoot when it is about an inch long. Weak trees should not be pinched too hard and should receive a little extra attention with careful manuring to strengthen them.
When pinching flowering trees and shrubs, remember to pinch early as the resulting growths will carry the flower buds for the next season; e.g., forsythia, apricot, peach, Chaenomeles, etc.
This additional method of inducing dense dwarf growth is only used on the most vigorous deciduous trees; maples respond to it particularly well. If the tree is really strong, all the leaves may be removed, leaving a quarter inch of the leaf stalk which will wither and fall off, and the buds in the leaf axils will grow out. In this way, a false dormant or leafless season is created and two years growth is produced in one. If such drastic treatment is considered too harsh, half the leaves may be removed either by actually halving each leaf or by pinching off alternate leaves.
Leaf pinching is only done once in any growing season when the tree is in its most active state—mid-June to July usually—and when the base of the shoots have begun to harden. After leaf pinching, the bonsai needs to be protected from excessive rain, but exposed to sun; too much rain may cause the soil to become stale and airless, as no water is given off through the leaves. When the new growth appears feeding may start again, and shoot pinching must be carried out for the rest of the season as and when necessary. The new leaves will be smaller and more proportionate to the size of bonsai.
It is through the roots that a plant gets both water and plant food in solution. The larger the tree, the larger the root system it has; hence it follows that if the root system is reduced, the size of the tree may be kept in check because the food and water supply is reduced. When a tree is allowed to grow naturally, the root system will usually spread out to approximately the same distance as the branches; hence it would seem reasonable to keep the root system of a bonsai well pruned to correspond to the desired "spread" of the top growth.
When a wild plant or nursery plant is dug up, a certain amount of root pruning is automatically carried out, and this is one reason why a newly lifted plant is checked. Bonsai growers, however, do some intentional pruning whenever they report a specimen. The roots are trimmed to leave a space of about an inch between them and the side of the pot or container. The vigorous trees are repotted more frequently, and so undergo more root pruning.
Pruning During Dormancy
If pinching during the growing season has been carefully carried out, very little winter pruning will be necessary. Very weak, dead, unwanted of ugly shoots are removed, but severe pruning should be avoided as it promotes strong, sappy growth, which is undesirable in bonsai work.
Only the most essential winter pruning should be done on spring flowering shrubs, as flower buds are bound to be reduced. When possible, these plants should be pruned after flowering. Peaches, apricots, and cherries sometimes die as a result of winter pruning, so any necessary cutting on this group is best done in the summer.
When one is removing any branch with knife or secateurs, it is most important to make a good "clean" cut, leaving no "snag," which not only looks unsightly but may decay and possibly damage the trunk.