Introduction
What causes deadwood to decay?
Fungal decay of Bonsai deadwood
Oxidization of Bonsai deadwood
Mechanical decay of Bonsai deadwood
Different types of wood, different approaches
Learning from nature
How to preserve bonsai deadwood & jin
Dealing with rotten wood in bonsai
Products for preserving bonsai deadwood
    Lime Sulphur – Jin seal
    Natural deadwood preserver
    Tree Gum
    Anti-fungal wood preservers

Introduction
Deadwood (also commonly referred to a drift wood) is an integral part of the artistic and structural design of many bonsai trees. Indeed it often forms the backbone of a great design. This article will not discuss the merits of deadwood or the methods by which we create it. The preservation of deadwood is a poorly understood subject and many folk are using products or methods that at best leave wood looking ugly and at worst will often actually accelerate the woods demise. That’s what we are going to be exploring here.

Deadwood in a bonsai context takes many different forms that mimic trees growing in the wild. These are normally referred to by Japanese terms…

Jin – Deadwood that protrudes from the tree, ranging from small spikes to large twisted pieces of sculptural wood.

 
Natural jin on pine and juniper.

Shari – The deadwood that is revealed when bark is missing from the trunk of the tree, often in the form of a long thin scar along the main trunk or branch.

Natural shari on a collected juniper.

Uro – A deep hollow in a tree where in nature rot has removed parts of the trees heartwood. Particularly common and very attractive feature when seen on deciduous trees.

Uro or natural hollows in the trunk of various trees.

Like or loathe deadwood, at some point most of us will own bonsai that incorporate some of the above elements. Preserving deadwood from decay is often very important. If deadwood forms the most attractive element of a special bonsai watching it decay and fall apart can be a most discouraging experience.

After more than twenty years working with bonsai I have learned a great deal about how wood behaves within the context of a tree. There are many factors that have a bearing on the dead elements of a tree. All of these need to be considered and understood before we decide to start work on deadwood whether it’s already in existence or we are carving and creating it for the first time.

What causes deadwood to decay?

There are many elements that contribute to the decay of deadwood. Many factors will conspire together to destroy deadwood and, whilst it’s not possible to hold these back indefinitely, we can certainly preserve deadwood successfully for a very long time.

Within the context of bonsai there are three main elements that cause deadwood to decay.

Fungal attack

Fungal damage that needs to be attended to.

Fungal damage due to continual water logging.

Oxidization

Heavily oxidized wood prior to cleaning.

Mechanical decay

Cracks in box wood due to continual expansion and contraction in an exposed situation.

Fungal decay of Bonsai deadwood

Fungi are so numerous and unique they have their own kingdom for classification. It’s estimated that there are more than 1.5 million species of fungi of which about 5% have been classified formally. Fungi have no leaves and so unlike plants do not create their own food. In order to obtain energy for growth and reproduction fungi digest dead plant tissue. To a million fungi your bonsai trees deadwood represents lunch!

Fungi spread by air and water borne spores and so there is no way of preventing deadwood from becoming infected at some level. Once introduced to a new site fungi will quickly multiply, often competing with other resident species for a piece of the action. Competing fungi raise a wall around their territory and this is known as spalting and can often been seen as a dark black/brown line that is revealed when carving. Fungi then proceed to digest the wood and remove all of the goodness from it, leaving all but powder in their path.

The progress of fungi is made much easier by a warm damp climate and rotting will be much faster in a warm wet summer than a freezing winter. In a bonsai context deadwood in contact with the soil that soaks moisture up into itself from the soil and sits in a warm sunny spot all day is in big trouble. Especially considering we create so many infection pathways like regular watering and organic fertilizing as well as using well aerated warm soil mixes.

On a side track it’s good to know that these fungi will not infect the live parts of healthy trees. Just like fungi trees create a barrier between areas of dead tissue and live wood, again this can normally be seen as a dark line separating the different zones within a tree.

Fungal degradation of deadwood is by far the largest cause of our problem but fortunately it’s reasonable easy to control in the short to medium term.

Oxidization of Bonsai deadwood

Oxidization is simply the stripping out of electrons from a material and is most easily recognised by rusting of metal. Wood oxidises in much the same way where water and various gaseous substances very gradually degrade the surface of timber. Oxygen, water, carbon dioxide and sulphur dioxide (acid rain) all conspire to strip the surface of dead wood. Ironically it is this process that creates the beautiful ‘weathered’ silver white wood so valued by bonsai collectors the world over but, as always too much of a good thing is a bad thing.

Preserving deadwood from oxidization or weathering is all but impossible long term but don’t despair we do have a solution.

Mechanical decay of Bonsai deadwood

Wood is a very porous material. In a bonsai context wood is continually expanding and contracting as it gets wet and then dries out again. Couple that with constantly varying temperatures that have the same effect and you produce a small but significant mechanical load upon the material. Given time this constant movement of the wood will cause cracks to appear. They allow water into the wood more easily and so the effect increases. Whilst wood is flexible it has a low elastic limit and so, depending on the type of wood involved, constant expansion and contraction begins to break down the wood.

Another aspect of mechanical decay is that caused by insects. There are a vast number of insects around the world that are attracted to dead wood for any number of reasons. For instance wasps often use soft rotten wood as a building material for their nests as do a number of ants. Other insects will bore into the wood to lay eggs. This has an effect on the wood and if left unchecked will physically weaken the material as well as allowing excess water into the wood.

Different types of wood, different approaches

Every tree on the planet produces it’s own unique type of wood. Some woods are very robust and will last a very long time without any special care, examples are trees like taxus, junipers and pines. Other trees are much less resilient including elm, maples and other broad leaf deciduous species.

    In general the older and more close grained the wood is the more resilient it will be. Also very resinous woods like pine species are very long lived. Obviously there are exceptions to these general principles.

The wood of deciduous species are particularly susceptible to fungal attack and have fairly low mechanical strength. These will require regular attention in order to maintain their wood over extended periods.

Learning from nature

The whole core of bonsai is to learn from nature by careful and considerate observation of nature which we seek to emulate in miniature. Considering what happens when wood dies in the wild will help us understand how we can try to replicate the same process.

As wood begins to age inside the tree it dies and provides a supporting framework for the increasingly large live elements of the tree. Many species create resin that impregnates this heartwood and will both strengthen it and help preserve the wood. That explains how a tree can be completely hollow and yet remain perfectly healthy, because it’s inner part is dead. Trees like taxus and juniper show a dark coloured heart wood, usually dark brown through red whilst the younger live wood is a pale yellow colour. The discolouration is caused by the high concentration of resin within the wood.

Heartwood of prunus species showing discolouration due to resin deposition

When a part of a tree dies or is damaged a series of events are triggered by the tree and also outward influences begin to act upon the wood. A tree can very easily compartmentalize itself and this explains why damage, decay and disease will often only affect a small part of a tree. Deep inside the tree dead tissue is surrounded by a barrier of resins that prevent the problem spreading into the live tissues. Unlike animals trees can literally cut off a damaged part of itself and continue without the part quite happily. This barrier can often be seen when we are carving bonsai, it shows as a dark black/brown line between live and dead wood.

Once a part of a tree has died the sap begins to dry out and the water contained within begins to escape, this causes shrinkage and the bark will begin to crack and lift. As the wood dries it becomes much harder as the vacuoles within begin to shrink, this is known as seasoning. Seasoned wood is more resilient to fungal attack that soft water laden wood. Often the outer part of the wood will begin to decay at the same time as the inner part seasons. Very quickly the outer wood becomes soft and falls apart. In time this leaves the harder material beneath. In the U.K this is a familiar pattern of oaks and pines and explains the hard spikes you will often see protruding from the trees trunk and branches.

How to preserve bonsai deadwood & jin

Before deciding how to approach the preserving of our bonsai’s deadwood we have to ascertain exactly what we need to achieve and what final colour we like. The approach to resinous wood typical to conifers will be different to that of softer wood typical to deciduous species. We will also have to consider whether the wood is green, seasoned, old and weathered or suffering decay. The type and condition of the wood will determine the technique we use, the work we have to do and the product we will need to apply.

As a general rule it is not necessary to preserve recently carved green wood for some time after initial carving. Because the wood will still be sappy and fairly wet most products will not bind to the surface or penetrate into the wood. Leaving newly exposed wood to weather for a year is normally a good idea. This allows it to dry thoroughly and for the surface to begin to oxidize nicely. It’s possible that some fungal ingress will take place but assuming the wood is sound and not permanently waterlogged this will be of little consequence.

Old seasoned wood should be treated annually, preferably in summer when the wood is reasonably dry. A quick clean with a fine bristle brush before application your chosen product is all that’s really necessary.

Soft partially decayed wood will have to be cleaned carefully prior to treatment. This will involve removing soft oxidized material with wire brushes either manually or in a power tool of some form. Once completed the wood should be dried as much as possible before treatment.
 

Dealing with rotten wood in bonsai

Much has been written on this subject leaving many folk baffled as to exactly what to do. Having spent more than twenty years working with bonsai deadwood, what follows is based entirely on my experience over that period.

Rotten wood is that which has been degraded by fungus, bacteria, oxidization and mechanical stresses. Wood infected by fungus may still have sufficient strength to be saved but material that has become soft and corky is simply beyond help and needs to be removed completely. Because badly decayed wood is very porous it will remain wet and provide a nice place for fungus and bacteria to proliferate and this will quickly spread to sound deadwood.

Many people are recommending the use of various types of chemical wood hardener in this context. However experience has shown that in the long term these products will actually accelerate the degradation of sound wood. The reason for this is fairly simple. As already stated fungus and bacterial thrive in a wet environment. Applying wood hardener, which is in effect a liquid plastic, will form a skin over the surface and stop the wood from breathing thus trapping moisture within the wood. Moisture will often soak up from deadwood in contact with the soil behind the treated surface. In the case of untreated wood the moisture will escape into the air fairly quickly keeping the wood drier for most of the time, particularly in summer when the temperature is higher. Wood with water trapped inside that gets warm in summer will degrade much faster. The ‘hardening’ effect of wood hardeners is going to be very short lived and will only skin over the surface and be of benefit for a short period.

If the wood in question is so soft that you need a wood hardener type of product to stabilise it my recommendation is that the material should be removed. Often this will compromise the design of a tree. However with a little creativity this can normally be overcome and, taking into consideration that not dealing with the problem properly could lead to much more severe decay, it may be the lesser of two evils.

When cleaning away very soft corky decayed wood use a stiff wire brush either manually or in a power tool. Best results will be achieved if you can take measures to dry out the wood before beginning work. Ultimately once the soft material is removed you will arrive at much harder stable wood and this can easily be preserved with a good quality product that will still allow the wood to breath naturally.

Severely rotten wood surrounded by sound material. The rotten material must be completely removed.

Products for preserving bonsai deadwood

Lime Sulphur – Jin seal

Lime sulphur imparts a silvery white colour.

Lime sulphur or Jin seal is simply a saturated solution of lime and sulphur. Originally used as a horticultural disinfectant, lime sulphur is very effective at preserving old seasoned wood and at the same time imparting a nice white colour that, in time, weathers to a very natural silver colour.

Applying lime sulphur is simple. All you have to do is very lightly damp the woods surface with a mist spray of water and then apply sparingly with a brush. Once finished you will need to allow the product to dry naturally in a protected spot for 24 hours. This ensures the sulphur fully penetrates the wood before rain has a chance to carry it away. Once weathered the wood will become whiter and look very natural.

Lime sulphur is best limited to use on coniferous wood like juniper, pine and yew. These types of tree exhibit white deadwood in the wild and so the colouring imparted by lime sulphur will look perfectly okay. Whilst the product is perfectly good at preserving other types of wood the colour looks out of place.

Treatment with lime sulphur should be repeated annually. This ensures the wood will last a VERY long time and also retain it’s clean appearance.

GREAT care should be taken when using lime sulphur. The solution is a very strong alkaline and can cause chemical burns. Also eye protection should be worn when the product is in use, even a minute particle in the eye can cause very severe pain for several hours. When first applied lime sulphur smells bad and so use in a very well ventilated place. Lime sulphur seems harmless to plants and so small areas of contact with live tissue and splashes to the soil are unlikely to cause problems but do try to keep this contact to a minimum. Always protect pots from splashes, unglazed pots in particular can be badly discoloured. Wash brushes in water.

Natural Deadwood Preserver

Wood previously treated with N.D.P retains its natural colour once dry.

More than twenty years battling with the British climate has enabled us to perfect this unique wood preserver. As already discussed there are a large number of factors to consider when trying to preserve deadwood in a bonsai context. We have designed this product to meet those special needs.

Natural deadwood preserver is a complex product designed primarily for use with broad leaved species used for bonsai. It’s unique formulation penetrates deep into the wood killing fungi and bacteria as well as nourishing the wood with natural oils, resins and acids. Best of all once dry it imparts no colour and so the wood ends up looking perfectly natural. The product also allows the wood to breath naturally and so ensures moisture is not trapped deep inside the wood.

Applying natural deadwood preserver couldn’t be simpler. Ensure the wood is thoroughly seasoned (no need to apply to recently carved sappy wood). In late spring allow the area to be treated to dry naturally for several days in the sunshine in an airy spot. Once dry through simply apply with a brush liberally until no more solution is absorbed. Repeat again at the end of summer before the onset of winter weather.

During application take precautions to prevent contact with skin and eyes. Use in a well ventilated space. Wash brushes in a concentrated detergent like washing up liquid. Splashes to foliage bark and roots are basically harmless but do try to avoid them. Natural deadwood preserver will not harm pots in any way.

Natural deadwood preserver can also be used for coniferous types of wood but the absorption rate and depth will be less than with deciduous wood. Once dry the woods natural colour will return.

Tree Gum

Tree Gum will not change the colour of existing wood as seen here. Olive wood treated with Tree Gum.

Tree Gum is another new product pioneered by and exclusive to Kaizen Bonsai. In nature deadwood, including the heartwood of trees, is preserved by the tree itself. Put simply, this is accomplished by the tree annually packing redundant sapwood with resin. In the event the heartwood is ever exposed by damage to the tree the wood will be very resistant to attack and so will give the tree time to adjust to the situation long before the structural integrity of the material is compromised. This explains why the heartwood of many trees is much darker than the live sapwood.

When it comes to bonsai and particularly old yamadori natural weathered deadwood is highly prized. This wood has often been exposed to the elements for many decades. During such extended periods of weathering the woods natural resins will be broken down by oxidation, UV exposure and the like. Tree Gum is a 100% naturally occurring tree resin diluted to a perfect consistency to penetrate deep into weathered wood. Once there it dries and fills the wood fibres with new resin. This protects from water ingress and also replenishes the natural resin, oils and acids that the tree would have originally made to combat decay. At the same time, and unlike artificial wood hardeners, it will allow the wood to breath.

Applying Tree Gum is very simple. Apply to old well weathered wood, not to recently carved material. Firstly ensure the wood is the colour you desire. Tree Gum imparts no colour once dry, however following application it will not be possible to change the colour of the wood for maybe up to a year. Colour existing wood with products like lime sulphur or wood stains and allow these to weather well before application. Having accomplished the desired colour allow the wood to dry naturally in the sunshine and wind for several days before application. Once dry apply Tree Gum with a large soft brush liberally and quickly to the wood working in one area until no more solution is absorbed. Try to work in a cool shady place. Direct sun will cause Tree Gum to dry before full penetration has occurred. Once the wood is thoroughly soaked the tree can be returned to it’s bench, there is no need to keep the wood protected from rain following application. Repeat application will not be required for at least two years unless the wood is in very poor condition. Any shiny spots that remain after application should be lightly buffed with a wire brush once fully dry.

During application take precautions to prevent contact with skin and eyes. Use in a well ventilated space. Wash brushes in methylated spirit. Splashes to foliage bark and roots are basically harmless but do try to avoid them. Tree Gum will leave marks on pots but these can be cleaned away using methylated spirit.

Anti-fungal wood preservers

Visit any DIY store and you will be confronted with a whole host of wood preservatives for everything from joists to fencing. Most of these are white spirit based solutions carrying various chemical compounds and fungicides. Bearing in mind that you will not know what exactly is in the product and the fact that it will not be designed for use in a horticultural environment use only with extreme caution. A commercial fungicide designed to kill tenacious wood rotting spores can easily kill a plant like bonsai. Our recommendation is to use products tried and tested for bonsai and horticultural use then you know your and your trees are safe!

Graham Potter
© KAIZEN Bonsai Ltd – 2010
The information contained within this article is provided as a guide only. It is the readers responsibility to ensure that all actions undertaken and products used are safe suitable and legal in their own area. We accept no responsibility or liability for any loss, injury or damage caused to persons or property resulting from information contained within this text.