When setting up your urban planter boxes, a common question arises: should you add a layer of rocks or gravel at the bottom before the soil? The traditional advice was to do this to improve drainage. However, modern gardening science shows this is a myth. Adding a coarse layer like rocks or gravel at the bottom can actually create a "perched water table." This means water does not easily move from the fine soil layer down into the coarse gravel layer, causing the soil to stay soggy above the rocks, which can lead to root rot.
Instead of rocks, the best practice is to use a high-quality, well-draining potting mix designed for containers. Ensure your planter boxes have sufficient drainage holes at the very bottom. If you are concerned about soil washing out, you can cover the holes with a single layer of landscape fabric or a mesh screen. This allows water to escape while keeping the soil in place. For very deep planters, you can fill the bottom third with lightweight, inorganic materials like perlite or recycled plastic bottles to save on soil and reduce weight, but this is for volume, not drainage. The key is to have a uniform, porous soil medium from top to bottom for healthy plant roots.