Raised bed vegetable gardens are used for a number of reasons. Typically the soil is contained in these beds with treated lumber. The question often asked on the Extension Office Green Line is: "Are vegetables grown in treated lumber beds safe to eat?"

Richard Stehouwer, Assistant Professor of Agronomy at Penn State, has published an article on this subject. While his findings do not conclude that the use of CCA-treated wood is harmless, the risks of using it are extremely small.

The three elements used in CCA-treated lumber are copper, chromium and arsenic. All three of these metals are essential for human and animal nutrition. This means that humans must have small amounts of these elements present in their diet. If a person were to consume a very large dose of any of the three in a short period of time, acute toxicity and possibly death could occur. The question is, what affect will small amounts in vegetables consumed on a daily basis have on the human body.

Tests have shown that copper and chromium are readily bound up in clay soils and soils with high organic matter. Therefore, even with the treated wood in raised garden beds, chronic health effects of eating vegetables grown near CCA-treated wood are extremely unlikely, if not impossible. The human body can tolerate larger amounts of copper and chromium than plants can. This means that copper and chromium would kill plants before plant tissue concentrations could get high enough to cause a chronic toxic effect in humans from eating the plants.

Arsenic can have a chronic toxic effect on humans at much lower intake levels than copper or chromium. This leads to the question of whether or not vegetables grown in CCA-treated wood can accumulate levels of arsenic that would exceed safe dietary intake for people who eat the vegetables.

Studies have shown that soil in contact with the treated wood in raised garden beds had no increase in arsenic 1 to 12 inches away from the wood. Treated posts that had been in the ground for 30 years showed increases in arsenic within 6 inches of the posts but no change further away.

A manufacturer of CCA-treated wood has grown carrots, peppers, cucumbers and tomatoes in raised beds made with CCA-treated lumber. They found that the content of the 3 elements was in the same range as vegetables purchased at a grocery store or grown in raised beds made with untreated lumber.

The conclusions of the Penn State publication are that the health risk from gardens using CCA-treated lumber appears to be extremely small and there are steps gardeners can take to further reduce any such risks.

If establishing new raised beds, use untreated woods such as red cedar, white cedar, osage orange, white oak, locust or redwood which are naturally decay resistant.

If using CCA-treated lumber, line the walls with heavy plastic to prevent contact with garden soil.

Do not put sawdust from the building project into the soil or into your compost.

If your current raised beds have CCA-treated wood, plant vegetables, especially root crops at least 12 inches from the wood. Peel root crops grown in proximity to the treated wood.

For more detailed information, Penn State Publication, "Garden Use of Treated Lumber" is available at the Extension Office or can be found on the internet at www.cropsoil.psu.edu/extension/Facts/Treated_Lumber.pdf.

Pete Hale is a Master Gardener with the Penn State Master Gardener program.