Using a plastic liner inside a wooden planter box is a common practice to extend its lifespan, but it must be done correctly. The primary benefit is creating a barrier between the moist soil and the wood, significantly slowing down rot and decay. This is especially useful in urban settings where planters are exposed to frequent watering.
However, a critical step is ensuring proper drainage. Never let the liner completely seal the bottom. Instead, place the liner inside and then carefully puncture several holes in its bottom, aligning with the drainage holes in the wooden box. This allows excess water to escape, preventing root rot and soil sogginess. Use a durable, non-toxic plastic like polyethylene.
Alternatively, consider lining only the sides and leaving the bottom bare for more natural drainage, or use a specialized permeable landscape fabric. While a liner helps, starting with a naturally rot-resistant wood like cedar or redwood, and applying a safe sealant to the interior, offers the best long-term protection for your urban garden planter.