Laidback Gardener Tip of the Day

Is Treated Wood Safe in the Vegetable Garden?

novembre 4There 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 more risky 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 is their copper content which 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. Experts on soils are essentially unanimous: they see no risk for humans in using copper-treated wood products in a vegetable garden, the amount of copper released being minimal and copper being considered essentially non-toxic unless present at extremely high levels. Despite this, accreditation agencies for organic products will not allow the use of treated wood in certified organic vegetable production. Oddly, they do allow the use of copper-based pesticides, including Bordeaux mixture, which release more copper into the environment than copper-treated wood ever could. So go figure! So, should you use copper-treated wood in a raised bed vegetable garden or not? I can’t pretend to be an expert in this field: I’m a home gardener, not an expert on chemicals! But I think enough information is out there for you to decide. Do you opt for the convenience and reasonable price of copper-treated wood relying on scientific studies or are you willing to pay top price for redwood or recycled plastic lumber so as to be as “organic” possible? That’s your decision. 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. If you’re still unsure, one compromise would be to use copper-treated wood but to line the inside of the bed with heavy-duty plastic to copper leaches into the soil inside the bed. Would I use copper-treated wood in a vegetable garden? Yes. Do I? No. I simply use raised beds with sloped sides. Not because I’m afraid of copper-treated wood, but just because that’s the way I’ve always gardened!

Garden writer and blogger, author of 65 gardening books, lecturer and communicator, the Laidback Gardener, Larry Hodgson, passed away in October 2022. Known for his great generosity, his thoroughness and his sense of humor, he reached several generations of amateur and professional gardeners over his 40-year career. Thanks to his son, Mathieu Hodgson, and a team of contributors, will continue its mission of demystifying gardening and making it more accessible to all.

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