Whether to use a landscape fabric liner in a wooden urban planter box is a common dilemma for gardeners. The answer isn't a simple yes or no; it depends on your specific goals and conditions.
The primary argument for using landscape fabric is weed suppression and soil containment. It acts as a barrier, preventing soil from washing out through drainage holes while blocking most weed seeds from below. This can significantly reduce maintenance. For wooden boxes, especially those in direct contact with soil or pavement, a liner can also slow the interior wood's decay by separating it from constantly moist soil.
However, significant drawbacks exist. The fabric can impede proper drainage over time as soil particles and roots clog its pores, leading to waterlogged soil—a primary cause of root rot. It also disrupts the natural ecosystem of the soil, potentially hindering beneficial organisms like earthworms. Furthermore, as plant roots grow, they can become entangled in the fabric, making repotting or plant removal difficult.
A best-practice compromise is often recommended. Instead of lining the entire interior, place a single layer of fabric only over the drainage holes at the bottom before adding your drainage layer (like pebbles or broken pottery). This keeps soil in while allowing water to flow freely. For weed control, a thick layer of mulch on the soil surface is more effective and healthier for the soil than a bottom barrier.
Ultimately, for most urban wooden planters, a full landscape fabric liner is not strictly necessary and can create more problems than it solves. Focusing on creating a well-draining soil mix, using a bottom layer of coarse material, and employing top mulch will yield healthier plants and a longer-lasting planter box.