Using a sealant on the inside of your wooden urban planter box is a common consideration for extending its life, but it requires careful thought. The primary goal is to prevent rot caused by constant soil moisture. While a waterproof sealant like a pond liner or a rubber-based coating can create an effective moisture barrier, it's crucial to ensure the product is non-toxic and safe for edible plants if you're growing vegetables or herbs. A sealed interior significantly slows the wood's decay.
However, this method has trade-offs. A completely sealed interior can trap moisture against the wood if any small gaps exist, potentially causing hidden rot. It also eliminates the wood's ability to breathe. Many gardeners prefer a more balanced approach: using a high-quality exterior-grade sealant or stain on all *outside* surfaces and the top rim, while lining the *inside* with a removable physical barrier like a heavy-duty plastic sheet or a separate plastic container with drainage holes. This protects the wood while allowing for easier maintenance and soil replacement.
Ultimately, for maximum longevity, focus on using naturally rot-resistant wood like cedar or redwood, ensure excellent bottom drainage, and consider a removable interior liner. If you do choose an interior sealant, select a plant-safe, flexible product designed for prolonged soil and water contact. Regular maintenance of the exterior finish remains the most important factor in your planter box's lifespan.