The safety of using pressure-treated wood for urban vegetable planter boxes is a common concern. The answer depends heavily on the type of treatment used. Historically, wood was treated with Chromated Copper Arsenate (CCA), which contained arsenic and chromium. Leaching of these chemicals into soil was a genuine risk, making old CCA lumber unsuitable for food gardens.
However, since 2003, residential pressure-treated wood has shifted to safer preservatives like Alkaline Copper Quaternary (ACQ) or Copper Azole. These are primarily copper-based and do not contain arsenic or chromium. While copper is essential for plants in trace amounts, excessive amounts can be problematic. Studies show that leaching from modern ACQ wood is minimal, and the copper largely binds to the soil near the wood's surface.
For ultimate peace of mind, you can line the interior of the planter box with a heavy-duty plastic barrier (like HDPE, marked with a #2 recycle symbol) to create a physical barrier between the soil and wood. Ensure the liner has drainage holes. Alternatively, choose naturally rot-resistant woods like cedar or redwood, or use composite lumber.
In summary, modern ACQ pressure-treated wood is generally considered low-risk for raised vegetable beds. Using a liner provides an extra layer of safety. Always avoid using any old, reclaimed wood of unknown origin for food gardens. Focus on building healthy, organic soil, and your urban harvest can be both abundant and safe.