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Coating pressure-treated wood to make it safer

Q: Is there a way to seal the pressure-treated wood used to build my raised beds to keep the arsenic from leaching into the soil?

Cynthia Linden, Williamsburg, VA

A: Ruth Lively, former senior editor for Kitchen Gardener magazine, responds: Painting wood pressure-treated with chromated copper arsenate (CCA) may help prevent leaching of arsenic, but the Environmental Protection Agency stresses that the data on this is both limited and inconclusive. If you decide to paint over the wood, be sure to use an oil-based, semi-transparent stain that will penetrate the wood. Do not use a nonpenetrating or film-forming stain (including latex semi-transparent, latex opaque, and oil-based opaque stains) or pool paint, because these products can flake off, compromising the integrity of the chemical barrier. 

To treat existing beds, you’ll need to disassemble them, scrub the wood clean, let it dry, then apply the stain, reassemble the bed, and refill it. This chore will have to be repeated yearly or at minimum every other year. With so much work involved and no guarantee of its effectiveness, you may want to consider other options. 

A simpler way of creating a chemical barrier would be to attach a lining of high-mil clear plastic to the inside of the bed. Try to avoid tearing the liner when working the soil with tools.

Further Reading
Does Pressure-Treated Wood Belong in Your Garden?
Are Pressure Treated Woods Safe in Garden Beds?
Is Pressure-Treated Lumber Safe to Use for Raised Beds?

The other solution is to use something else besides CCA-treated lumber to build your beds. In my New England garden, I have beds made of 2-inch-thick white oak, 1-inch cedar decking, and 2-inch redwood. The white oak is finally showing signs of decay after eight years, while the cedar decking is beginning to decay in weak places after just four years. The 7-year-old redwood bed is still strong and beautiful. In my experience, the weak points are the 2x2 posts at each inside corner of my beds, which last only a few years. 

If you’re not a wood purist and like the idea of recycling, consider Trex, a manufactured lumber made from waste hardwood fiber and reclaimed plastic grocery bags. Trex has a dense, even texture with a weathered gray color. Although it’s been on the market less than 15 years, tests indicate that Trex has a probable life span of nearly three decades. There are a few negatives: Trex is heavy and expensive, and it doesn’t look much like wood. If you decide to make beds from some other kind of wood, using Trex for any inner corner posts would probably prolong the life of those beds.

From Fine Gardening 90, pp. 98