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Note: Since writing this article I have also published a related guide to creating artificial wounds (uro) to hide large wounds. The article should be read alongside this one to offer the enthusiast an alternative method.
Finding rotten areas of wood on a bonsai can often be an enthusiasts' worst nightmare. Rotten wood found within the base of the trunk is weak and often rapidly disintegrates, eating away at the physical and visual foundation of the tree. The resulting large unsightly holes can severely weaken the structure of the trunk and, if only one side of the base of the trunk is affected, can create reverse taper.
Despite these potential problems with rot, this article discusses some of the merits of promoting certain areas of a tree to rot.
This is a technique that allows the enthusiast to create more natural looking features on a tree by harnessing qualities created by the natural breakdown of wood due to continual contact with moisture. It is especially useful for masking large scars on trees that are either difficult or unlikely to heal completely.
This Hawthorn (Crataegus monogyna) has had two large branches (relative to the diameter of the trunk) removed. Hawthorns are very slow to heal so this wound could take many years to heal fully.
As can be seen from these images, the branches have been pruned back to stubs and then encouraged to rot. Once the wood has started to rot, the soft fibres have been carefully removed leaving behind the harder fibres of wood that have yet to begin to break down.
In this example, this yamadori Hawthorn has a naturally created root jin. On collection of the tree, the protruding root was found to be dead and had rotted up to a point very close to the trunk; all the soft, pulpy wood was carefully removed and the remaining stable areas sealed with woodhardener to create a very natural looking jin. If the root had been removed entirely, the resulting scar could have taken many years to heal and it is unlikely that the colour of the healed bark would have ever matched that of the surrounding area. (Note that the root pictured is just over an inch in diameter)
An additional advantage to this technique is that tree species with naturally hard wood can be difficult to carve effectively or sensitively, even with power tools. By encouraging the wood to rot and soften first, manipulation of the wood is far easier.