Wood can be very vulnerable to life forms that like to consume the wood. However, many of the chemicals most effective at protecting wood are harmful to human health. Pressure-treated wood once used in raised garden beds was eventually found to leak toxic chemicals into the nearby soil. Many of the less harmful wood treatments are not always as effective at protecting the wood. Fortunately, continued research leads to the development of new kinds of wood preservatives that are less harmful to humans and the environment.
Fungi is the most common wood destroyer and often infects the wood when the wood becomes too wet, according to the University of Kentucky. Moisture is especially a problem with gardens that are exposed to rain and other elements. Wood can also be damaged by termites, carpenter ants and powder post beetles. Fungi is usually killed using fungicide, while the wood-destroying insects are killed with specialized pesticides. For instance, termites are usually killed with growth inhibitors that are mixed in with wood bait.
Creosote is an oil that is applied to wood in order to kill wood-destroying organisms. Creosote is mostly safe for outdoor structures, though children who play in the garden might still be exposed to the creosote toxins. Wood with creosote is also more flammable than wood without creosote, according to Iowa State University.
Some preservatives simply make wood more water repellent. This does not specifically protect the wood from wood-destroying pathogens, but does reduce the chances that the wood will become infected by these pathogens. These repellents usually do not have any odor and can be painted over, according to Iowa State.
Some trees produce chemicals that protect them from wood-destroying fungi and other pathogens. However, trees that grow very fast sometimes do not produce enough chemicals to kill these pathogens, according to the University of Kentucky.
Some forms of wood treatment have caused problems with water supplies when the wood preservatives leached into the soil and found its way into the groundwater. This results in heavy metals that make the water dangerous to drink. This preservative seepage is remedied using methods such as microbial degradation and by allowing plants to absorb and release harmful materials into the air through their stomatas, according to Mississippi State University. Therefore, many of the wood preservatives are usually not as harmful in gardens because the plants can actually purify the leached preservatives. However, some leached wood treatments chemicals, such as arsenic found in the wood treatment chromated copper arsenate, can accumulate in soils and stunt plant growth, according to a 1977 study by C. Grant and A.J. Dobbs.
According to the University of Kentucky, the US Department of Agriculture Forest Service has developed ways to treat wood with borates in order to make the wood more resistant to pathogens and damaging insects.