Choosing the correct liner for your wooden urban planter box is crucial for both the longevity of the box and the health of your plants. The primary purposes of a liner are to protect the wood from constant moisture and soil contact, which causes rot, and to help contain the soil while allowing for proper drainage.
For most situations, a breathable landscape fabric is an excellent choice. It acts as a protective barrier, allows water to drain freely, and prevents soil from washing out. Simply staple or tack it to the interior walls and bottom. Avoid using non-porous materials like solid plastic sheets as they can trap water against the wood, accelerating decay, and cause soggy soil conditions.
If you need extra waterproofing, consider a heavy-duty polyethylene liner. The key is to puncture numerous drainage holes in the bottom and several along the lower sides. Always ensure your planter box itself has ample drainage holes as well. For an added layer, you can use landscape fabric between the wood and the plastic liner.
Another great option is a coir (coconut fiber) liner. It's natural, biodegradable, and provides good drainage and aeration. It blends seamlessly into the garden aesthetic but will need replacement every few seasons.
Before adding any liner or soil, always use a high-quality wood sealant on the interior of your planter box for an extra layer of protection. Ultimately, the best liner combines protection for the wood with a system that ensures your plant roots are never sitting in waterlogged soil.