Is treated lumber safe to use for a planter box for vegetables?

Let’s dig into this question straight from the soil—because as someone who has spent years building garden beds and nurturing tomatoes, peppers, and herbs, I’ve had to face this dilemma myself. The short, honest answer is: it depends on when and how the lumber was treated, and what you’re growing. But here’s the deeper truth that I’ve learned through trial, error, and a lot of research.

Older treated lumber (pre-2003) was preserved with chromated copper arsenate (CCA), which contains arsenic—a known toxin that can leach into soil and be absorbed by plant roots, especially in acidic conditions. If you’re using old pressure-treated wood from the 1990s or earlier, please don’t use it for edible crops. I’ve pulled up planks from aging decks and repurposed them for flower beds, not where I grow my carrots.

Modern treated lumber, typically labeled as ACQ (alkaline copper quat) or CA (copper azole), replaces arsenic with copper and other organic compounds. Copper is a micronutrient, but in high concentrations, it can be toxic to plants and soil microbes. Here’s what I’ve observed in my own garden: when I built a small raised bed with ACQ wood and lined it with heavy-duty plastic, I had no issues with root vegetables. But when I skipped the liner—thinking, “It’s just copper, right?”—the soil in direct contact with the wood after two rainy seasons showed slightly elevated copper levels. My leafy greens looked fine, but I wouldn’t gamble with deeply rooted veggies like potatoes or beets.

My hard-earned advice? If you’re building a planter box for vegetables, play it safe:

- Always line the interior with a thick, food-grade plastic barrier (6-mil or heavier) to separate soil from wood.

- Avoid using treated lumber for acidic soil vegetables like tomatoes, peppers, or strawberries—acidity increases leaching.

- Choose naturally rot-resistant woods like cedar, redwood, or black locust instead. They’re more expensive but completely inert.

- If you must use treated lumber, use it only for the outer frame and use untreated boards for the interior.

In my community garden, we switched to cinder blocks and cedar for our vegetable beds. The peace of mind is worth the extra cost. So, is treated lumber safe? Yes—with precautions. But why take the risk when your family’s food is on the line? I always err on the side of caution. Your carrots will thank you.