We regularly hear of special pruning techniques for bonsai but before rushing off to try it out you should ask: What is the aim of this technique? And, possibly more importantly: At what stage in the life of my bonsai should I use this technique?
As I see it, a bonsai goes through a number of phases in its life.
First phase is ‘Growth’.
Second phase I call ‘Development’.
Then as the tree is close to ‘finished’ it goes into ‘Maintenance’ phase
Each of these phases have different aims and require different techniques to bring about the desired outcomes.Let’s look at each of these stages in more detail and see what techniques are generally used to promote the desired outcomes.
The first stage of any plant’s life is ‘Growth’. We want the tree to attain the trunk size and thickness we have planned as quickly as possible. To achieve this it is usual to plant the tree in an oversized pot or in the ground. Feed and water it well so it will grow as quickly as possible. Look for fertilisers that have a relatively high N(itrogen) level compared with the P and K. New shoots are usually allowed to grow long before being pruned back, sometimes only once each year. Depending on the ultimate size of the tree we want to develop, and the conditions it is growing under, this stage can take from 1 year to 10 or more. During the growth phase branches may be selected and given an initial shape but it is difficult to build good branch ramification while the tree is in such rapid growth. Sometimes we do not even select any branches, leaving this for the second phase.
When it has nearly reached the desired size we move to a new phase with different objectives. Note that I have said ‘nearly reached desired size’. The tree will continue to thicken somewhat slower during the following stages.
The development stage concentrates on selecting and growing the apex and branches of the tree and developing ramification – lots of smaller sub branches and twigs so that the bonsai looks like a believable tree. No longer do we allow shoots free growth. That would make the apex and branches too thick and awkward. Free growth is replaced with regular pruning. Keep up the regular feeding and water. We still want the tree to grow a lot but we now want to manage that growth differently. Most growers know that when a shoot is pruned 2 or more of the buds below the cut end will start to grow so that what was just one linear shoot is now branched. Pruning develops ramification. The more you can get the tree to grow the more you can prune and the quicker the ramification will develop. At this stage shoots are allowed to extend to 4-6 leaves (or pairs of leaves) then pinched back to leave the first few leaves at the base of the shoot. Shoots that are to become the main stem of a branch or trunk are allowed to grow longer than those that will become side shoots. This way the longer, main shoot grows thicker and more dominant while the side shoots stay more delicate and divide more with more regular pinching. If the tree is healthy and well looked after, the buds at the base of the leaves will shoot and begin to elongate. Every time the shoots get long enough they are pinched back to force more buds to start to grow. In this way 1 becomes 2, 2 becomes 4, 4 becomes 8, and so on and ramification and branch density develops rapidly.
It is worth pointing out here that the initial flush of growth sometimes produces very strong shoots with long internodes (the stem between leaves/buds). As buds only grow from the leaf nodes long internodes would produce a sparse branching pattern so shoots with long internodes can be cut off near the base to force a new shoot to grow. These new shoots are usually less vigorous and have nodes closer together which will develop a better branch pattern and closer ramification.
During development we also selectively remove shoots to direct growth in the desired direction and pattern. In this way we develop branches that have sub branches alternately on either side, preferably from outside of bends instead of all growing from one side. Selective pruning also builds up a trunk and branching with movement instead of dead straight sticks projecting out of the trunk!
Wiring can be used to position branches and to create movement in the main trunk or branches. Be aware that rapid growth means that wires can mark the tree very quickly. Rapid growth also means that wired shoots will be set in position in as little as a few weeks.
Selective defoliation can be used to slow the growth of some areas of the tree and strengthen weaker areas. Grafting might be used to add branches and roots where they are desired if nature and pruning have not provided them.
When our tree has reached the desired thickness, height and spread and has good ramification on branches and apex it is time to change techniques again in the Maintenance phase. Now we do not want excessive growth or thickness because these would destroy the fine taper and delicate tracery of branching we have built up so we reduce feeding and watering a little and start pinching earlier and more regularly.
Feeding must be continued to keep the tree healthy and actively growing but without the huge flushes of growth we wanted in the preceding stages. Look for fertilisers that have a lower proportion of N(itrogen) during this stage.
Pinch shoots when they have grown only 2 or 3 nodes or even earlier. An old maintenance pruning technique for Japanese maple is to open out the developing leaves and pinch the embryonic stem growing between them as soon as it can be grasped with a pair of tweezers.
Thinning also becomes an important technique during maintenance. Constant pinching during the growing season creates a tangle of twigs and shoots which need to be thinned each winter to allow more light and air to reach inner leaves and shoots.
You will note that some trees will go through growth and development phases several times as the owners restyle the tree.
You will probably now understand that applying techniques aimed at maintaining a finished tree to one you are trying to grow or develop will slow the tree’s development and it could take many years longer to get where you want to be.
There is probably a further final phase but we won’t dwell on such an unhappy subject here.