If you’re thinking about putting in raised beds for your garden, you might have wondered about the safety of treated lumber for the walls. You are not alone.
The strangely green chromated copper arsenate wood, which used to be on the market, was taken off (voluntarily, so you may see some still for sale), leaving us to ponder alternatives. Plastic, rock, and raw lumber are all good choices, but if you’re considering the new treated lumber, I’d like to share a bit of information with you.
The wood is treated with alkaline copper quaternary, which is free of arsenic but has higher amounts of copper in the treatment. Information provided by the EPA on ACQ says that this treatment should be stable in Colorado soils, because we are fortunate to have dry, highly alkaline soils. Items in our soil do not break down, they petrify.
ACQ is only compromised in chronically moist soils with high organic matter. In such a soil, the copper migrates from the wood and into the surrounding soil, particularly during the first seven months of exposure. The extent of this migration of copper is approximately six inches from the wood.
Let’s do some thinkin’ here. If copper leaches under chronically moist, high organic matter type soils, vegetable gardens might pose some risk; after all, we deliberately try for four to five-percent OM. And, of course, we keep moisture consistent. So, the copper has a better chance of leaching in this system.
Second, let’s look at the size of our raised beds – typically we make them four feet across so that short-armed people like myself can reach the center of the bed. If the copper leaches six inches into the bed, we eliminate a full foot of growing space. That is, if the plant cooperates and only extends its roots into the ‘good’ soil, and not the ‘bad’ soil. But we know plants won’t do this – it’s ridiculous to assume they’ll stretch their roots out, dip a toe into the bad soil and pull back without reaching further.
Copper is taken up by plants and stored in roots, so root veggies would be risky in this system. But you could line the bed with a heavy-duty, weather resistant plastic. Or you could go with untreated lumber that, though not permanent, lasts 10 years.