Will the chemicals in pressure-treated lumber used for raised beds contaminate vegetables planted in them? —Tom Grimaud, Pico Rivera, CA
This is still a relatively controversial topic. Some people say it’s risky growing vegetables in these beds. Others point to research that shows that in pressure-treated lumber beds, contaminants such as arsenic are found in minuscule levels that fall well within federal safety guidelines.
There is some leaching of chemicals into the soil, though, particularly when treated timbers begin to break down after ten or more years. Leaching is most prevalent nearest the timbers, so you could plant ornamentals close to the wood and plant vegetables farther back, at least 12 to 18 inches from the boards. You could also line the treated timbers with heavy gauge (6 mil) plastic sheeting.
The pressure-treated lumber produced with CCA (chromated copper arsenate) that raised so much concern is no longer sold for garden construction. There are now alternative pressure treatments such as ACQ (ammoniacal copper quat) and CBA (copper boron azole). Also, redwood and cedar are both naturally more decay-resistant than most lumber, though not as resistant as pressure-treated wood.