Using treated wood for urban vegetable planter boxes is a common practice, but it does raise important safety considerations. The primary concern historically centered on older wood treatments, like Chromated Copper Arsenate (CCA), which contained arsenic and could potentially leach into soil. For growing edible plants, this posed a contamination risk. However, most residential treated lumber sold today uses newer preservatives like Alkaline Copper Quaternary (ACQ) or Copper Azole. These are generally considered safer as they do not contain arsenic or chromium.
For maximum safety when building planters for vegetables, experts recommend using a barrier. Lining the interior of the planter box with heavy-duty plastic (like a pond liner) creates a physical barrier between the treated wood and the soil. Ensure the liner has drainage holes to prevent waterlogging. Another key recommendation is to avoid using any treated wood for the parts of the planter that will have direct, constant contact with the soil where roots grow, especially for root vegetables. Using untreated, rot-resistant woods like cedar or redwood is the safest alternative, though they are more expensive and less durable.
Ultimately, the risk from modern ACQ-treated wood is considered low by many authorities for above-ground planters, but using a liner provides an extra layer of security for your peace of mind and the health of your plants. Always check the label on the lumber to know which treatment was used and follow current best practices for safe urban food gardening.