Pressure-Treated Lumber For Vegetable Gardens

CCA Lumber

• Since it has a long life outdoors, plenty of structures built with CCA-treated lumber are still intact. CCA stands for copper, chromium and arsenic, the three major chemical components of the treatment process. The process forced a liquid with those chemicals deep into the lumber fibers. Arsenic presented the greatest concern since the EPA lists it as a known human carcinogen and says the metal is acutely toxic.

Leaching

• While lumber treatment facilities were certain CCA would remain tight within wood fibers, concerns were raised that arsenic and the other metals could leak into soil and be taken up into plants. Some seepage of these components does occur near the wood, but Oregon State University says it occurs at low levels with little movement once the metals reach the soil. Organic matter within many garden soils raises acidity levels enough that CCA materials are bonded strongly with the soil, making it tough for plants to take them in.

Absorption

• Plants do not tolerate metals well and take in very little under most any conditions. With soils holding tightly to CCA chemicals, Oregon State University states that plants absorb only small quantities of copper, chromium and arsenic. Since most vegetables are annuals, there is little chance that higher concentrations could occur over time.

Precautions

• The EPA says that CCA-treated wood should "not be used when routine contact with food or animal feed can occur." The department lists "compost, mulch, or structures or containers for storing human food" in particular as areas to avoid. Oregon State University advises that a 12-inch border be maintained between vegetables and CCA-treated lumber in gardens. Root vegetables, such as turnips, carrots and radishes, should be scraped clean instead of washed to remove soil particles that may have arsenic on them.

Alternatives

• Since 2004, the treated lumber industry sells alkaline copper quat (ACQ) treated lumber as an arsenic-free alternative to CCA-treated wood. Naturally rot-resistant woods like redwood and cedar make attractive and safe alternatives to borders made of treated lumber. Non-wood borders of stone, brick or plastic are also safer choices.