Once deadwood on a tree is exposed to the elements, 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 seriously affect the appearance of a bonsai. As the wood breaks down, more of the heartwood becomes exposed to moisture and the rotting area spreads. If left unhindered, the rot will continue to spread throughout the heartwood of the tree.
Note that, with a few exceptions where bacterial or fungal infections are involved, rotting wood is a natural process and does not threaten the live cambium, live areas or actual health of a tree.
Allowing wood on a bonsai to continue to rot can produce some very natural effects; in particular hollowing of deciduous tree-trunks and wounds. However, where deadwood exists on the external areas of the tree and its loss (through rotting) will create inverse taper in the trunk or the loss of an attractive deadwood-feature, the deadwood should be preserved.
Curing rotten wood on Bonsai
Unwanted rotting areas should be dealt with quickly; the longer a decaying area is left, the larger and more difficult to cure 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 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. (Note that applying Lime Sulphur to wood that has already been hardened with a preservative produces poor results as the Lime Suplhur is unable to penetrate into the wood).
Deciduous species can have lime sulphur applied but the effect of bleaching it 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. Very often it is better to leave the natural wood colour and patina.
Finally, the wood needs to be sealed against moisture so there is no potential of the rot re-appearing in the future. A wood hardener needs to be painted onto the entire area, this will penetrate into the wood and is hardened on contact with moisture, bonding and sealing the decayed wood, strengthening it and stopping any further deterioration.
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' and Ronseal Wood Hardeners, both of which seem 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.
For a full step-by-step guide to applying wood hardeners to bonsai, please see 'Preserving and Styling a Fuchsia Bonsai'