For years, a common piece of gardening advice has been to place a layer of gravel or stones at the bottom of a planter box to improve drainage. However, for urban container gardening, this traditional practice is now widely discouraged by horticultural science. The belief is that gravel creates a space for excess water to pool away from plant roots. In reality, this creates a phenomenon called a "perched water table." Water does not easily move from the finer soil layer down into the coarser gravel layer; instead, it saturates the soil just above the rocks before finally draining. This keeps the lower portion of your soil soggy, right where many roots reside, increasing the risk of root rot.
For urban planters, which are often shallow and exposed to variable conditions, optimal drainage is critical. The best practice is to use a high-quality, well-draining potting mix designed for containers and ensure your planter has sufficient drainage holes. If you are concerned about soil washing out, cover the holes with a single piece of broken pottery, a mesh screen, or a coffee filter. This allows water to escape while keeping soil in. Focus on selecting the right soil volume for your plant's root system and watering appropriately—this is far more effective for plant health than a gravel base. Skip the gravel, and your urban garden will have a much better foundation for success.