For decades, a common piece of gardening advice has been to add a layer of gravel or stones to the bottom of planter boxes to improve drainage. However, modern horticultural science shows this practice is not only unnecessary but can actually be detrimental to your plants' health.
The core issue lies in the physics of water movement. Water does not easily move from a fine-textured material like potting soil into a coarse-textured material like gravel. Instead, it tends to pool at the soil-gravel interface, creating a saturated zone just above the drainage holes. This "perched water table" can lead to waterlogged soil around the roots, increasing the risk of root rot—the very problem you were trying to avoid.
For urban planter boxes, the most effective strategy is to use a high-quality, well-draining potting mix throughout the entire container. Ensure your box has adequate drainage holes. To prevent soil from washing out, simply place a single piece of broken pottery or a small mesh screen over the holes. This allows free water exit while keeping the soil structure intact.
Focusing on a uniform soil medium promotes healthier root growth and more efficient water use. By skipping the gravel layer, you provide a better environment for your urban garden to thrive.