There is a long-standing controversy in the field of organic gardening: can pressure-treated wood be used as part of an organic vegetable garden? Here’s what I know:
Pressured-treated wood sold in the US and Canada used to contain chromated copper arsenate (CCA). In the long term, this product released arsenic into soil, more under certain conditions than others. The fear was that the arsenic could be absorbed by vegetables and other edible plants and transferred to humans. And everyone knows that arsenic is toxic. In addition, burning wood containing arsenic is much riskier than using it in the garden!
In North America and in much of Europe, CCA-treated woods were removed from the residential timber market (they are still allowed for use in some commercial applications) sometime at the beginning of the 21st century (in early 2004 in Canada and the US and in 2006 in Australia, notably). Since then, two other products containing copper have been widely used in pressure-treated wood: alkaline copper quaternary (ACQ) and copper azole (CBA).
They contain no arsenic: it’s their copper content that helps to preserve the wood. Any treated wood will eventually leach some of its chemicals into the surrounding soil (again, more so in some soils than others). However, copper is considerably less toxic than arsenic. The risk to humans and warm-blooded mammals from copper-treated wood is considered minimal, but fish and insects are very sensitive to it (copper-treated wood should never be used in ponds or streams). Interestingly, human beings absolutely need copper in order to to survive, but in very small quantities.
Soil experts disagree on using copper-treated wood products in a vegetable garden. Some see no serious risk for humans in using them in a vegetable garden, the amount of copper released being minimal and copper being considered essentially nontoxic unless present at extremely high levels. Others think it is safest to err on the side of caution and avoid the use of these woods near food. Health Canada discourages the use of these woods as an edging for an edible garden and accreditation agencies for organic products do not permit the use of wood treated with copper in certified organic vegetable production.
Curiously, the same accreditation agencies do allow the use of copper-based pesticides, including Bordeaux mixture, which release more copper into the environment than copper-treated wood ever could. Go figure!
What To Do?
So, should you use copper-treated wood in a raised bed vegetable garden or not? I think it would be wiser to use this product cautiously. If you choose to opt for the convenience and reasonable price of copper-treated wood, that’s your business, but I recommend using non-treated wood, either regular timber even if that means replacing it every 7 to 10 years or, if you are willing to pay top price, one of the rot-resistant woods, like cedar. Plastic lumber is another possibility.
Also, remember that, if it’s just a question of raised beds, you could use bricks, stones or cinder blocks to raise the bed above the surrounding soil. Or you could simply mound the soil and slope the sides, using no containing material whatsoever.
Some Other Possibilities
There are still a few other possibilities. Like painting untreated wooden boards with some sort of waterproofing agent—paint, stain, varnish, etc.—so it will last longer. Unfortunately, few of these products are considered acceptable in the field of organic gardening, at least not if you’re growing vegetables commercially and hope to get your organic certification one day. (The criteria for organic certification are so stringent in most countries that the average commercial organic vegetable grower could never hope to obtain it.)
Untreated wood can, however, be painted with linseed oil. This treatment will help waterproof it… and linseed oil is one of the few waterproofing products that is 100% acceptable in organic gardening circles.
Alternatively, you could build the frame of your vegetable garden with pressure-treated wood (ACQ or CBA), then cover the inner wall with a sheet of plastic so that no copper from the wood ever touches the soil it contains. Many organic gardeners use this combination and feel fully comfortable with the result.
And If Your Vegetable Bed Is Already Framed with Treated Wood?
If your vegetable bed was installed after 2004 using copper-treated wood and you’re a concerned organic gardener, what should you do? This is a situation where there will always be divergent opinions. Even among people who take organic gardening very seriously, many would see nothing wrong with it. Maybe this is one of those cases where toeing the organic line too closely is being more Catholic than the Pope!