Using treated wood for urban planter boxes intended for edible plants is a common concern among gardeners. The primary issue lies in the chemicals used in the treatment process, which can potentially leach into the soil and be absorbed by plants.
Historically, wood treated with chromated copper arsenate (CCA) posed significant risks due to arsenic content. However, its use for residential purposes has been largely phased out. Modern pressure-treated lumber often uses alkaline copper quaternary (ACQ) or copper azole, which are considered safer but still contain copper to deter rot and insects.
For edible plants, caution is advised. While the risk from modern treatments is lower, a conservative approach is recommended. To create a safe barrier, line the interior of the planter box with a heavy-duty, food-grade plastic liner, ensuring it has drainage holes. This prevents direct soil contact with the wood.
For absolute peace of mind, consider using naturally rot-resistant woods like cedar or redwood, or alternative materials such as composite lumber, stone, or metal. These options eliminate chemical concerns entirely, making them the safest choice for growing vegetables and herbs in your urban garden.