(Originally published on the Bonsai Society of San Francisco website by Eric Schrader.)
To create a well-designed bonsai requires finding a balance between two competing factors – human perception and the natural form of a mature tree. While a tree in nature exists indifferent to human approval or interest, a bonsai is a caricature of the natural tree, created in partnership with a human. A bonsai is shaped by our perceptions, and represents our vision of the natural tree - not the natural tree itself.
We've created the concept of visual flow in bonsai because it supports our perception of what an ideal tree should look like. Thus, the natural tendency of our inspection is to look at the base of the trunk and then follow it upward as it twists and turns and then pause in the mass of foliage at the apex. The eye then travels down the side of the apex and finds the key branch which completes the viewers journey by moving the eye away from the tree.
The role of the trunk
The direction of the visual flow starts with the lean of the trunk and continues with the changes in its direction. On a typical informal upright the trunk gently changes direction rather than twisting. On yamadori or exaggerated style trees the trunk can twist enough that the upper and lower sections of the trunk flow in opposite directions. On a good bonsai the trunk is the center of attention and the foliage acts like a picture frame. The trunk is normally the primary determining factor in the overall flow of the tree.
Figure 1: Tree “a” has poor flow – the apex goes left while the key branch goes right. Tree “b” has good flow to the right – the apex, trunk, and key branch all in agreement. Tree “c” has mild cross-flow – the trunk flows right but not strongly and the key branch and the apex flow to the left. The accent plant in a formal display with this tree would be place on the left.
Understanding the apex
The flow of the apex is controlled by the direction that the base of the apex takes and by the relative mass to the left and right of the woody part of the upper trunk and apex. The shape of the apex can also determine the direction of flow – generally the longer and more gently-sloped side of the apex is the direction that the eye naturally travels when viewing. The branching of a poorly-formed apex will rob the tree of good flow. The branching should be even, small, and generally slightly longer on the side that agrees with the overall flow of the trunk and key branch.
Understanding the key branch
The key branch is usually the largest directional branch on the left or right side of the plant; rarely (perhaps never) a back branch. Under-developed material will require that a key branch be developed to a larger size to improve the flow of the tree. Select a branch based on its position relative to the direction of the apex and the movement of the trunk. Typically the key branch is also the lowest branch, however this is not always true.
Figure 3: Tree “a” has good flow and a key branch that is not on the bottom. Tree “b” has good flow and the key branch is an extension of the apex. Tree “c” has poor flow, the key branch is the lowest branch on the left, and flows strongly opposite the flow of the apex.
Whether the foliage or the trunk are a more powerful part of the overall composition determines their influence over the overall flow. Trees with foliage and trunk flowing in the same direction and the trunk slanting strongly have an overall flow that is strong. Trees with foliage and trunk flowing in the same direction but with little or no inclination in the trunk have weak (which is not the same as poor!) or neutral flow. Trees with trunk and foliage that flow in opposite directions have cross-flow. Trees with foliage masses in the key branch and apex that flow in different directions, regardless of the trunk flow, typically have poor design and poor flow.
Figure 4. Tree “a” has neutral flow. Tree “b” has weak flow to the left, tree “c” has weak flow to the right. Tree “d” has poor flow because the large branch on the lower left needs to be reduced. Tree “e” has cross-flow – the apex and key branch flow to the left while the flow of the primary and secondary trunk is to the right. Tree “f” has strong flow to the right.
Enhancing visual flow
To enhance the visual flow of a tree or correct cases of poor flow, make a plan to shorten some branches and simultaneously elongate sections of the key branch and apex over time. In Figure 1a the right low branch could be eliminated while also growing out the branch on the left in a fashion similar to figure 1c. Figure 3c could be corrected by reducing the right side of the apex while growing out the left side, the result being similar to 2a. The flow of figure 4d could be corrected by simply shortening the lower left branch and reducing the mass of the foliage so that the secondary trunk acts as the key branch instead. To correct problems with the flow within the apex consider wiring and/or removing any large branches that are headed opposite to the direction of the overall flow.