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In the article Field Growing it was established that for a thick trunked bonsai, the tree must first be allowed to grow freely in the ground or a pot to help thicken the trunk.
Having thickened the trunk by allowing free growth for a number of years, it is unlikely that the trunk have much taper and will often lack any movement.
There are a number of ways of introducing taper and movement to a field grown trunk; this article describes one method that is known to be widely used in Japanese growing fields to 'build' myogi or informal upright trunks for bonsai. This process can equally be applied to a collected or nursery tree that is currently too tall for use as a bonsai and needs to be reduced in height.
By studying the following images, it is hoped that the reader will understand and use some of the techniques described to introduce taper and movement in the trunks and new branches for their trees.
This method is intended for deciduous trees only but some of its principles can be applied to coniferous species.
Image 1 shows the lower portion of the trunk of a tree growing in the ground. its girth is adequate for use as a bonsai but there is little movement or taper.
Ideally, the finished bonsai will be approximately 6 times the height of the trunk diameter. The diameter of this trunk is 3" so the ideal height of the tree when it is finished will be 18". If a taller bonsai is required, this tree will need growing on for further years before this process begins. Once the trunk has been chopped, it will barely thicken until the new section above it has all but reached the same girth and at which point, taper is all but lost. Refer to this article for an example
So the projected final height of this tree will be 18". The first branch should be at approximately a 1/3 of the overall height. This means that the first branch should be 6" from the base of the tree.
Image 2. Late Winter/early Spring. The trunk is chopped with a straight cut at a height of around 12", approximately 2/3 the height of the finished tree. If additional movement is required on a very straight lower trunk, the chop can be made at 6" or 1/3 of the height of the tree.
A straight cut reduces moisture loss and potential dieback; until a new shoot has appeared and been chosen as the new leader, there is no point in making a diagonal cut as is sometimes advised. As with all cuts, the chop should be sealed with cut paste.
Image 3. Autumn. The tree drops its leaves and reveals the effects of a growing season left to grow freely. The heavy chopping has resulted in strong budding from all over the trunk.