When selecting wood to make raised beds often people will recommend avoiding pressure treated wood siting the preservatives may be harmful. In this month’s installment of our testing garden assumptions series I thought I would put this to the test.
Dead wood if left to the elements will decompose. If the wood has been used in a raised bed it is probably not favourable to have the bed decay quickly.
There are two solutions to this. The use of a hard wood or cedar will slow the decay process but in most areas these woods cost a lot.
Alternatively more common lumber can be treated and preserved in order to slow the decay process.
In recent decades the three most common treatment processes include Creosote, Chromated copper arsenate (CCA) and Alkaline Copper Quaternary (ACQ).
Both Creosote and CCA were abandoned in 2003 voluntarily by industry in North America for the sale to the general public. This was done as there were concerns over the hazard of hydrocarbons and heavy metals used to preserve the wood.  
ACQ treatment remains available in most lumber yards.
Our hypothesis today is that pressure treated wood may be harmful if used to build a garden.
ACQ treated lumber is most commonly used residentially. The method uses Copper with and an companying biocide to help resist the decay process.
Lets take a look at both biocides and copper to see if they are potentially harmful in a garden.
Biocides are any chemical or microorganism that deters harmful organisms. Biocides are used in medicine, agriculture and industry such as forestry.
Most commonly in pressure treated wood the biocide is DDAC Didecyldimethylammonium chloride.  It is also commonly used as a disinfectant for surgeries and restaurants.
When tested for leaching in an extreme environment researchers showed leaching results into the environment between 0.19 and 0.22% well below the industry allowable standard of 4% These extreme conditions were designed to facilitate leaching and are highly unlikely to be replicated in your back yard garden.