Michigan State University Extension
1994-98 Landscape CAT Alerts - 41594003
03/31/99
The most commonly available treated wood is impregnated with a combination of chemicals known as CCA (chromated copper arsenate). This is sometimes known by its trade name, Wolmanized wood. The chemicals protect the wood from attack from agents such as fungi, which can shorten the useful lifetime of the wood. Since these chemicals can have toxic effects at high enough levels of exposure, there is concern about using CCA-treated wood around gardens where these chemicals might contaminate fruits and vegetables.
From what is presently known about this type of treated wood, the risk of such contamination is low. First, the chemicals are forced into the wood at very high pressure and remain effectively bound to the wood for very long periods of time. Only very small amounts of these chemicals leak out and get into the soil. Second, these elements (i.e., arsenic, chromium, copper) do not move very rapidly through soil, so migration is limited.
Movement from treated wood would not be expected to cause a significant increase in the normal background levels of these elements (commonly found in all soils) except within 6 inches of distance from this type of wood. Plant roots growing within 6 inches of treated wood could be expected to take up higher levels of copper and arsenic than would occur in similar soil located greater than 6 inches away. The significance of this increase uptake depends on several factors (e.g., plant species, volume of soil in close proximity to treated wood, soil fertility conditions, etc.) and is difficult to predict. Due to the general unavailability of chromium from soils, no increase in plant uptake of this element would be expected.
Overall, the potential for contamination of vegetables and fruits by CCA is very small and any residues would be at levels far below those known to cause toxic effects. However, if the gardener wishes to reduce this potential even further, barriers can be used. For example, if this wood is used for a raised bed, the inside of the wood can be lined with heavy duty plastic. Another possibility in this and other situations is to coat the wood with an appropriate sealer. In general, these stapes are not necessary but they may be used to provide an extra margin of safety.
Turfgrass Management