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.

Ordinarily, the fresh wood of hardwood species can take many years to rot if simply left to the elements. To encourage the wood to rot, it must be exposed to continual moisture for at least six months.

I have found that by anchoring fresh sphagnum moss to the area that is to be rotted, the wood is kept continually moist and rotting is greatly encouraged. It is then simply a case of ensuring that the pad of sphagnum moss is dampened every time the tree is watered. The addition of some peat or other moisture retentive material is a useful addition or alternative.

This tree has a very large wound at the top of its trunk where it had been chopped by workmen building a fence, 10 years prior to its collection. The wound had started to rot in places and this had spread downwards into the trunk. At over 4" wide, the likelihood of the opening fully healing is very unlikely so instead, the wound is being opened up to create a hollow trunk.

Areas of very hard wood have been drilled to allow deeper penetration of moisture; this also allows lengths of wire to be inserted into the trunk to anchor the moss. On a regular basis the moss is removed and any newly softened wood removed to keep a strict check on the progress of the rot.

Using this technique has little danger to this particular tree (see additional notes below) the previously damaged cambium has fully sealed many years ago and the rot is not able to penetrate into live wood; it only breaks down wood that is already dead. Once the necessary amount of wood has been successfully removed from the interior of the trunk, the remainder will be fully sealed and protected from any future unwanted rotting.

Important Additional Notes

>Encouraging wood to rot is an unpredictable technique. If a very small specific area needs to be broken down or a certain pattern or ridge on the deadwood area is to be retained, use an alternative method of carving.

>Use only on hardwood species and never on softwood species, particularly those that are ordinarily prone to problems with rot such as Fig, Bougainvillea, Elms, Fuchsia and most tropical species. I have successfully carried out this technique on hardwood species such as Crataegus, Quercus, Taxus and Cedrus. The spread of rot on softwood trees can be too fast and difficult to halt.

>Only use on species of tree that you are familiar with, already understanding the nature of its wood and with which you know you can halt the continued spread of rot.

> Never use on any part of a tree that will be in permanent contact with moisture (particularly soil) in the future. Exposed/deadwood areas of a tree in permanent contact with the soil are already very vulnerable to rotting. Use only on areas that can be successfully allowed to dry in the future.

>Check all areas regularly to check on the progress of the rot.