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This series of four articles first appeared as part of Chapter 12 of my first book Bonsai Inspirations 1 and this excerpt contains what I believe to be essential information on how to design, build and improve the branches and branch structure of a deciduous or broadleaf bonsai.
Note that extensive information of how to build the branch structures of coniferous bonsai, such as Juniper and Pine, are contained within Bonsai Inspirations 2
DEVELOPING A NEW TRUNKLINE AND BRANCH STRUCTURE ON DECIDUOUS TREES
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The main principles to understand when developing new trunklines (new ‘leaders’) or branches on bonsai are as follows:
The shoots (be they potential branches or leaders) with the most growth will thicken the fastest.
The shoots with the least growth will thicken slowest.
To encourage growth in a shoot, do not prune that shoot.
To encourage maximum growth in a shoot, prune all other shoots on the tree.
To stop a shoot growing, pinch back that shoot.
To minimise growth, do not let the new growth extend (by pinching the new growth as it opens)
Using these 6 basic principles, the bonsai enthusiast can control which parts of a tree grow and eventually become thicker and which parts remain as they are.
Developing a branch structure and new leader on a bonsai using these principles can sometimes be something of a chinese puzzle. However, if one is focussed on which areas of the tree (the leader or the branches or a particular branch) require the most growth and which areas of the tree require the least, it is a solvable puzzle!
Using the Elm airlayer featured in Chapter 12 of Bonsai Inspirations 1, I will now illustrate how these 6 principles are applied to a tree stump in order that it can be turned into a bonsai.
This process, or parts of this process, can be applied to all deciduous trees whether they be completely raw material, a fully formed trunk that requires a new branch structure or even a fully developed bonsai.
At the beginning of the first year of developing the stump, (assuming it is healthy and vigorous) it will produce new buds all over the trunk. These buds will then begin to open into new shoots as temperatures warm in the Spring.
A bare trunk stripped of all shoots and branches in the Autumn will produce considerably more buds from the trunk during the following Spring. Any existing branches that are left after Autumn pruning will reduce the number of buds that appear on the trunk itself.
The first step for the enthusiast to take, is to decide which new shoots to keep for development into new branches and trunk leaders, and which to remove. The sooner that unwanted shoots or preferably buds, are removed, the more ‘energy’ the stump is able to divert into growth and development of the shoots that are wanted.
The exact shoots or buds to keep will depend on the future design you have for the tree and if you are not 100% certain whether a particular branch will be required in the future, keep it for the time being. In the future it will always considerably easier to remove a bud or shoot you are undecided upon, than it to regrow it again.
At this point it is important to understand or remind yourself about apical dominance. Every tree species will have a natural growth habit that will produce different rates of growth in shoots depending on their position on the trunk. At the uppermost part (a) of the tree (be it the top of the trunk and branches at the very top of the tree), growth will be strongest. This means that there will be more buds appearing in this area, the shoots that they produce will be more vigorous and will have a greater growth rate. Conversely, in the lowest areas of the trunk (c), bud production and shoot growth will be weakest. The areas inbetween (area b) will tend to have bud production and shoot growth rates inbetween areas (a) and (c).
This principle is pretty straightforward. However, it should be noted that different tree species naturally have different degrees of apical dominance. For our purposes as bonsai enthusiasts, there is no exact science in explaining these differences. Various tree species are just variously described as being ‘ a little apically dominant’ or ‘very apically dominant’. It is only with experience that one can establish what these descriptions actually mean in practice! There are also a much smaller number of tree species that can be described as being ‘basally dominant’. In these cases it simply means that the base of the tree is the most dominant part of the tree and the top of the tree is the weakest and the strengths and weaknesses of areas (a) to (c), are reversed. So, bearing in mind the growth tendency of the majority of deciduous trees, if the tree stump in this example were to be allowed to grow freely without our intervention, the leader at the top of the trunk would grow the most and thicken the most.
In this case that is what we hope to achieve, maximum growth in the new trunkleader so that it will thicken as rapidly as possible, creating better taper to the top of the stump. However, without any pruning, the new shoots in the middle part of the tree (area b) would grow faster than those shoots in the lowest areas of the trunk (c). This would produce branches in area b that were thicker than the branches in area c. This is the opposite of the desired effect in bonsai where the lowest branches are also the thickest.
A second important effect of leaving the remaining shoots unpruned is that some of the tree’s resources or energy would be wasted growing these middle branches. Pruning the growth of the middle branches will divert the tree’s resources to the trunk leader to ensure maximum growth.
And so, the correct way to deal with this example tree is allow the new leader to grow freely. To allow each of the middle branches to extend and open 3 or 4 leaves and then pinch out the rest of the new shoot so it will not extend any further leaving the tree’s resources to be diverted elsewhere. The shoots in the lowest parts of the tree need to be thicker than the shoots in area (b) and should therefore be allowed to extend further than the shoots in area (b), and are left to grow until around 8 or 9 leaves before being pinched out.
The result of this pruning or pinching out will be something akin to the diagram on previous page.
When the new leader has begun to grow strongly it is time to consider carving the top of the trunk to create better taper to the trunk.
The preferred time to do this on deciduous trees is during the growing season, preferably after the first flush of growth has hardened off.
At this time of year the wound resulting from chopping the trunk heals relatively quickly and callus formation (to cover the exposed cambium and to begin the years-long process of covering the large wound) is at it’s most rapid.
Carrying out this work during the dormant season or during early Spring is unlikely to harm the tree itself but healing of the resulting wound will be slow or non-existant if carried out in the Winter.
An imaginary line can be made from the base of the new trunkleader sloping either sideways or backwards (so that the resulting wound is not visible from the front of the tree) using a sharp knife and the excess bark and wood can be removed.
Actual physical removal of the excess wood can be difficult on thick trunks and requires the use of power tools as well as saws. However, where the top of the trunk has a smaller diameter, knob cutters are often sufficient and the excess wood can be removed bit by bit.