Using plastic to line a wooden planter box is a common idea to prevent rot, but it often causes more harm than good. While a plastic sheet creates a physical barrier against moisture, it traps water against the wood, creating a perpetually damp environment that accelerates rot from the inside. More critically, it severely impedes drainage, leading to waterlogged soil, suffocated roots, and ultimately, dead plants.
A far superior method is to use a permeable landscape fabric liner. This material allows excess water to drain freely while keeping soil contained. For the best results, ensure your planter has ample drainage holes at the bottom. You can further protect the wood by applying a non-toxic sealant or stain to the interior before adding the liner and soil.
Ultimately, the goal is moisture management, not a complete moisture barrier. By choosing breathable materials and ensuring proper drainage, you significantly extend the life of your wooden planter and create a healthier environment for your plants.