It's a common gardening tip: add a layer of gravel or rocks to the bottom of your planter boxes for better drainage. However, this widespread practice is actually a myth that can harm your plants. The science of soil and water shows that water does not easily move from a fine-textured material like potting soil into a coarse material like gravel. Instead, it creates a "perched water table," where water accumulates in the soil layer just above the rocks, keeping the lower roots soggy and increasing the risk of root rot.
For healthy urban planter boxes, skip the gravel. The single most effective drainage feature is one or more adequate holes at the very bottom of the container. Use a high-quality, well-draining potting mix designed for containers. For extra insurance in deep planters, consider using a piece of breathable landscape fabric to cover the drainage holes, preventing soil loss while allowing free water exit. This method ensures oxygen reaches the roots and excess water truly drains away, creating a healthier environment for your plants to thrive.