Whether you need a liner for your urban planter box depends primarily on the material of the planter itself. For long-term health of both your plants and the container, using a liner is generally recommended, but there are exceptions.
Planting directly into a wooden planter box will cause the wood to constantly be in contact with moist soil, leading to rapid rot and decay. The soil will also wash out through small gaps. A plastic or metal liner creates a crucial barrier, protecting the wood and containing the soil. Similarly, for metal planters, a liner prevents rust and reduces soil temperature fluctuations.
You can plant directly into planter boxes made of durable, non-porous materials like thick plastic, fiberglass, or glazed ceramic, as these are designed to withstand constant moisture. However, even these require excellent drainage holes at the bottom.
The primary benefits of using a liner include extended planter lifespan, improved drainage control, easier soil and root ball removal, and protection from soil stains or mineral leaching. To install one, simply place a pre-formed plastic insert or a heavy-duty pond liner inside your planter. Ensure you puncture drainage holes in the liner that align with the planter's own holes. Adding a layer of gravel or pebbles at the bottom before the soil can further enhance drainage.
In short, while direct planting is possible in certain inert containers, using a liner is a simple step that safeguards your investment and creates a healthier environment for your urban garden plants.