Choosing the correct liner for your wooden urban planter box is crucial for plant health and the longevity of the box itself. The primary purposes of a liner are to protect the wood from constant moisture and soil contact, which causes rot, and to help manage drainage effectively.
For most situations, a breathable fabric liner made from geotextile or landscape fabric is the top recommendation. It allows water to drain through while containing the soil, preventing wood rot and protecting plant roots. It's also flexible and easy to install.
If you need a completely waterproof solution for indoor use or to protect a deck, a plastic liner (like a pond liner or heavy-duty plastic sheeting) works. Crucially, you must drill ample drainage holes in both the liner and the planter box to prevent waterlogged soil. For a more rigid option, consider a rubberized or PVC sheet liner.
Avoid using non-porous materials like solid plastic without drainage holes, as this will drown your plants. Also, lining only the sides and leaving the bottom bare is not advisable, as rising damp will still damage the wood.
The best practice is to use a breathable fabric liner against the wood and add a 1-2 inch layer of gravel or pebbles at the bottom of the lined box before adding soil. This creates an additional drainage reservoir, keeping roots healthier and your wooden planter box thriving for seasons to come.