Once deadwood on a tree is exposed to the weather, it naturally starts to decay. Hardwoods can take a number of years to fully breakdown whereas softer woods (particularly those found on tropical species) can disintegrate within a year. 

Dead and rotting wood can also affect bonsai in ways that look not only unsightly but also threaten the well-being of the tree itself. Deadwood, and in particular dead softwood that is continually wet either through poor drainage of a hollow feature or more commonly, that is in contact with the soil, can be particularly troublesome. As the wood breaks down, the rotten area will spread into any adjacent areas of the trunk. If left unhindered, the rot will continue to spread.

Curing rotten wood on Bonsai

Rotten areas should be dealt with quickly; the longer a decaying area is left, the larger it will become. Firstly, gently remove all of the very soft, crumbly wood as this is very difficult to preserve. If this rotted wood travels into the soil of the pot, carefully pull back the soil as far as possible without exposing any live roots. It helps here if the soil is on the dry side. It is not necessary to remove all dead or rotten wood; just anything that is particularly fragile.

Next, the remaining wood needs to be treated to kill any remaining bacteria and/or fungi and prepared before the wood is sealed. On coniferous species it is usual to apply a mixture of Lime Sulphur; this will bleach the wood white after a number of applications.

Deciduous species can have lime sulphur applied to help preserve the wood and to treat the wood but the effect of bleaching white is often unsuitable for these species. It is possible however to achieve more suitable tones of grey by adding black Indian Ink or acrylic art paint. Brown tones can be achieved by adding coffee granules. If the natural wood colour and patina is to be retained, the wood can be treated with Armillatox; this will have an anti-bacterial/fungi effect.

Finally, the wood needs to be sealed against moisture in the future so there is no potential of the rot re-appearing. A wood hardener needs to be painted onto the entire area, this will penetrate into the wood and is hardened on contact with moisture, the Hardener bonds and seals the decayed wood together, strengthening it and stopping any further deterioration.

Wood Hardeners

 Wood Hardener is a clear resin purposely formulated to solve problems with, and cure, rotten and decayed wood. It is often recommended that the Minwax brand is used in the USA though I have been unable to source it here in the UK. I have however had much success with 'BONDA' Wood Hardener which seems to be an equivalent product. 

The Wood Hardener is applied until it is no longer absorbed into the wood. Try to avoid using too much as any hardener left on the surface will leave the wood shiny. However, this can be successfully rectified by gently sanding to remove the sheen once dry. 

I have had no problems with the hardener coming into contact with the live wood of any trees I have applied it to; indeed I have accidentally spilt small amounts onto foliage with no obvious detriment to the plant!

The hardener takes around 24 hours to fully harden during which time it is necessary to keep the area free from moisture.

It is difficult to change the colour of the wood after Wood Hardener has been applied; as the hardener seals the wood, any Lime Sulphur that is applied afterwards will not be able to penetrate and is not as long lasting when on the surface of the wood. Wood Hardener applied after Lime-Sulphuring will seal in the colour permanently.