For generations, a layer of gravel or rocks at the bottom of a planter box was considered Gardening 101. The logic seemed sound: improve drainage and prevent soil from washing out. However, modern horticultural science has turned this common practice on its head. The short answer is no, you should not put gravel at the bottom of your urban planter boxes. In fact, doing so can be detrimental to your plants.
The reasoning is based on soil physics and the concept of a "perched water table." Different materials hold water at different tensions. When a layer of coarse material like gravel sits beneath finer potting soil, it creates a sharp textural boundary. Water does not easily move from the fine soil into the large air spaces of the gravel. Instead, it accumulates in the soil layer directly above the rocks until it becomes saturated, creating a perpetually wet, soggy zone exactly where your plant's roots reside. This environment promotes root rot and fungal diseases, the very problems you were trying to avoid.
So, what should you do instead? The key to healthy container gardening is not a drainage layer, but exclusive use of a high-quality, well-draining potting mix throughout the entire planter. Ensure your container has adequate drainage holes at the very bottom—this is non-negotiable. For deep planters, you can place a piece of breathable landscape fabric over the holes to keep soil in while letting water flow freely out. For exceptionally large planters, using a lightweight filler like upside-down plastic pots or recycled water bottles (with caps on) in the bottom third can save soil and weight without harming drainage, as long as they are topped with your potting mix.
By skipping the gravel and focusing on the right soil and proper holes, you create an environment where water drains evenly, air reaches the roots, and your urban plants can truly thrive.