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.
Copper is an essential element for plant growth and human life however as with anything in high enough concentrations it can become toxic.   In order to see if copper is leaching out of treated lumber in contact with soil I sent soil samples to Maxxam analytics. The first set of samples had been in contact with treated wood for 3 years and 9 years respectively. In order to understand if there is any leaching going on at home I also sent a control sample of soil from the same bed however further away from any treated lumber.
All of the samples were test for the total metals. The results we got represent both the total number including bioavailable and unavailable.
If the hypothesis is correct we should see higher copper concentrations in the soil that was in contact with the treated lumber when compared to the control. If there are higher concentrations for them to be harmful they would also need to be present in toxic levels.
Researchers from the University of Florida and the University of Miami demonstrated in their paper that leaching of the DDAC biocides into the soil happen in low concentrations even when conditions are extremely favorable for leaching. We are unlikely to replicate the favorable conditions for leaching into soil in our home gardens.
So you are not likely to get exposed through contact with soil but through simply physical contact with skin. This exposure is similar to the exposure one would get walking on a deck or using a hand rail made of the same materials.
The copper analysis showed a slightly higher number in the copper when comparing the 3 years in contact with treated lumber to a control; however the treated sample was within the statistical variance when comparing the sample that had been in contact for 9 years.
This indicates the three year variance could be as a result of chance or minor leaching into the soil.
In no cases did the results exceed regulatory criteria. In Canada the Canadian Council of Ministers of the Environment sets a residential guideline of 100 mg/kg in soil 
The same research paper that looked at the leaching of biocides found that treated lumber did release low levels of copper but again did not exceed regulatory criteria.
For this hypothesis there is evidence to suggest the risk is low however not completely eliminated. As such I recommend if you are looking to use treated lumber as a building material for a raised bed you take some time and research the issues and make an informed decision. If you would like some initial direction I have cited the sources I used for this episode in the blog post that always accompanies my episodes. A link can be found in the description below or at www.albertaurbangarden.ca .
I will not remove the treated wood in my garden as I feel the risk is low. ACQ materials do not leach readily, exposure is limited to skin contact and they are safely used in hospitals and restaurants.
Although not as a direct response to any risks I further reduce exposure by not tilling the soil, and avoid planting root crops near the perimeter of my garden. I will keep an eye on the decay rates of the treated and untreated lumber in my garden to see how much extra time the treatment gives me.