According to Miles McEvoy, who works in organic certification with the Washington State Department of Agriculture, no pressure-treated wood is allowed in soils used to grow organic food. If you want to meet this high standard, choose a different material. Studies have shown that arsenic from wood treated with CCA leaches into the soil and that copper, although much less toxic, leaches from ACQ and copper boron azole (CBA, a variant of CA-B).
Sally Brown, a research assistant professor of soils at the University of Washington, knows her way around both food and metals. Starting out as a chef and then a food broker between farmers and restaurants, she became fascinated with soils and went on to earn a PhD in agronomy. Brown’s current research includes identifying the mechanisms by which organic residuals reduce the availability of soil metals to plants. She has some hard-earned opinions.
Brown says that if you already have the older, arsenic-treated wood in your garden, don’t panic. Plants will not take up arsenic unless the soils are deficient in phosphorus. That is not a problem for gardeners who use compost generously. As for the new copper-based wood treatments, Brown believes the actual risk is minimal. First of all, if plants take up too much copper, they will die before a gardener can eat them. In addition, if homegrown vegetables make up a small percentage of the diet, exposure to any metal taken up is insignificant. Do not use copper near ponds and streams because it is toxic to aquatic life.
The perceived risk is another story. Gardening can be just as much an act of faith as it is science, and the opinions of all the scientists in the world may not convince you to let pressure-treated wood contact your soil. You can isolate pressure-treated wood by lining the inside of a bed with heavy plastic to prevent leaching of chemicals from the wood into the soil. Top your bed with boards of untreated lumber to make a flat top to sit on and to avoid contact with the wood, particularly if you have CCA-treated wood in your garden. Even though the new pressure-treated woods are considered safe, Wolmanized Outdoor, according to its Web site, does not recommend using pressure-treated wood where the preservatives may become a component of food. Its recommendation is to use an impervious liner between the wood and the soil.
Consider using alternatives, such as decay-resistant wood like redwood or red cedar. Discuss with your lumber dealer whether its wood is heartwood, which lasts longer then sapwood from the same tree. If you are concerned about sustainable harvesting of wood, contact the Forest Stewardship Council (www.fsc.org). You may also want to consider another point: Pressure-treated wood does, in fact, last longer then untreated wood, so using it might mean fewer trees would be cut.