Yes, you can use a sealant on the interior of wooden urban planter boxes to significantly extend their lifespan. This creates a crucial moisture barrier between the damp soil and the wood, slowing rot and decay. However, the choice of sealant is critical for the health of your plants.
For a safe and effective barrier, use a 100% waterproof sealant like a pond liner or a food-grade, non-toxic sealant specifically formulated for planters. Avoid standard wood sealants, tar, or creosote, as these can leach harmful chemicals into the soil and damage plant roots.
The best practice is to line the entire interior—sides and bottom—ensuring a continuous coat with no gaps. Apply it to clean, dry wood and allow it to cure completely before adding soil. Remember to still provide adequate drainage holes at the bottom of the planter, above the sealed layer, to prevent waterlogging.
While sealing the interior protects the wood, leaving the exterior unsealed or using a breathable stain allows moisture to escape from the planter walls. Combining an interior sealant with a protective exterior finish offers the most comprehensive defense, making your wooden planters durable fixtures in your urban garden for years to come.