For decades, a common piece of gardening advice has been to add a layer of gravel or rocks to the bottom of planters to "improve drainage." However, for urban planter boxes and most containers, this is a persistent myth that can actually harm your plants. The reason lies in basic soil science and the concept of a "perched water table.
When you place a coarse material like gravel beneath finer potting soil, you create a sharp texture transition. Water naturally moves downward until it hits this boundary. Because the soil particles are much finer and hold water more tightly than the large air pockets in the gravel, the water hesitates to cross. It saturates the soil layer above the gravel, creating a perpetually wet zone exactly where your plant's roots reside. This can lead to root rot, oxygen deprivation, and ultimately, unhealthy or dead plants.
Instead of gravel, focus on what truly matters: using a high-quality, well-draining potting mix designed for containers. Ensure your planter box has sufficient drainage holes—multiple holes are better than one. For deep planters, you can use "filler" materials like crushed water bottles or recycled plastic pots at the very bottom to save soil and reduce weight, but always separate them from the soil with a durable landscape fabric to prevent soil wash-down. The key is to maintain a uniform soil texture throughout the root zone, allowing water to drain freely and consistently, giving your urban garden the best foundation for success.