For decades, a common gardening mantra has been to place a layer of gravel or stones at the bottom of planters to improve drainage. However, for your urban planter boxes, this practice is not only unnecessary but can be counterproductive. Modern horticultural science shows that adding a coarse layer like gravel creates a "perched water table." Water does not easily move from the fine-textured potting soil down into the much coarser gravel layer. Instead, it saturates the soil above the rocks before finally draining, keeping the lower soil zone wetter for longer. This is precisely where many plant roots reside, increasing the risk of root rot.
The key to healthy plants in containers is not a drainage layer, but using a high-quality, well-draining potting mix specifically formulated for containers. Ensure your planter boxes have sufficient drainage holes at the very bottom. If you are concerned about soil washing out, covering the holes with a single piece of broken pottery, a mesh screen, or a coffee filter is far more effective than gravel. This allows free water exit while keeping the soil in. For deep planters, you can fill the bottom third with lightweight, inert materials like recycled plastic bottles or packing peanuts to save on soil cost without harming drainage, but gravel is not recommended. Focus on good soil, adequate holes, and proper watering for thriving urban gardens.