Some people have expressed concern about using treated wood in close proximity to plants. Specifically, individuals are worried about chemicals leaching from the wood and being absorbed by the plant. The type of treated wood generating this uneasiness is material pressure impregnated with CCA (chromated copper arsenate). CCA is by far the most commonly used inorganic arsenical preservative. Proper treatment procedures developed and monitored by the American Wood Preservers Bureau should allow this material to be used without any fear of leaching of chemicals. These chemicals are fixed in the wood. Research has shown that very little if any movement of chemicals occurs after treatment when the wood is placed in contact with the soil.
Three cautions should, however, be observed when using CCA- treated wood for plant containers or raised beds. First, make sure the treated wood is free of any chemical salt deposits on the surface. If the wood is not clean when purchased, scrub any salt deposits from the surface before using. Second, the wood should be dry. The fixation reaction is dependent on time and temperature. For example, the fixation reaction requires only 1 to 2 days at 90 F, 5 to 6 days at 70 F, 2.5 to 3 weeks at 55 F, and at least 2 months at 35 F. Since one cannot tell if the chemicals have been set in wood that appears to be wet, drying may need to be done prior to assembly. Wet, treated wood can be dried by stacking the pieces outside during the spring, summer, or fall with spacers between the different pieces. The time required may be only a few days during warm summer months but a longer drying period is required during cooler spring or fall days. Make sure the wood looks and feels dry before using. The other option is to purchase CCA-treated wood that has been certified by the American Wood Preservers Bureau as "Foundation" grade. This material is required to be redried after treatment with the waterborne wood preservative; therefore, the chemicals should certainly be fixed in the wood. The other commercial levels available for pressure impregnated wood are "Ground Contact" and "Above Ground Contact" preservative treatments. These levels do not require redrying after preservative treatment. Finally, avoid frequent or prolonged inhalation of sawdust from treated wood. Also, do not dispose of treated wood by burning.
Anyone who continues to have reservations about CCA-treated wood should consider using the heartwood of cedar or redwood for plant containers, raised beds, or other structures in the garden. This material may not last as long and is more expensive, but certainly should relieve concern about any potential environmental hazard.
This article originally appeared in the May 12, 1993 issue, pp. 1993 issue, pp. 68-69.