Choosing the right sealant for the inside of your wooden urban planter boxes is crucial for both the longevity of the wood and the health of your plants, especially if you're growing edibles. The interior faces constant moisture, soil acids, and microbial activity, requiring a robust yet safe barrier.
For planters containing edible plants like herbs, vegetables, or fruits, you must use a non-toxic, waterproof sealant specifically formulated for direct food contact or deemed safe for garden use. The top recommendation is a 100% pure tung oil. It penetrates deeply, cures to a hard, water-resistant finish, and is completely natural and non-toxic once fully cured. Another excellent option is raw linseed oil (not boiled, which contains additives), though it cures slower. For a ready-made solution, look for sealants labeled as "Garden Safe" or "Non-Toxic for Planters."
Avoid conventional exterior wood sealants, polyurethane, or varnishes for the interior. These often contain volatile organic compounds (VOCs), fungicides, or other chemicals that can leach into the soil and be absorbed by your plants' roots.
Application is key: Ensure the wood is clean, dry, and sanded smooth. Apply multiple thin coats of your chosen oil, allowing each coat to soak in and dry completely as per the manufacturer's instructions. This creates a protective, breathable barrier that locks out moisture while safeguarding your garden's ecosystem. Reapply annually or bi-annually for ongoing protection, ensuring your beautiful wooden planters remain healthy and functional for seasons to come.