The short answer is: generally no, not as a drainage layer. Lining the bottom of an urban planter box with rocks is a very common idea, but in most closed containers it does not improve drainage the way people expect. Water does not move freely from fine potting soil into a coarse gravel layer and then out. Instead, the change in texture can create a “perched water table,” where water sits in the soil just above the gravel because capillary action holds it there. The result can be a shallower effective soil zone, more moisture near the roots, and a higher risk of root problems, especially in compact urban planters.
Rocks also take up valuable soil volume. In a small patio planter or balcony box, that lost space matters. Fine soil particles can wash down and clog the spaces between rocks over time, and the gravel layer can make the planter heavier without giving you better drainage. For most urban planter boxes, drainage is better managed with clear drainage holes, a slightly sloping bottom, a well-draining potting mix, and a screen or geotextile fabric over the holes to keep soil from washing out.
There are a few limited situations where a rock layer can be acceptable. If the planter box is a raised bed open to the ground, a thin gravel layer or coarse material around drainage outlets can help slow soil loss and keep water moving at the exit. If the box has a built-in drainage mat or a structural drainage system, follow that design rather than adding loose rocks. Rocks can also work as a decorative top dressing, but that is different from a bottom drainage layer.
If you have already lined your planter with rocks, do not panic. Check that water drains freely, watch soil moisture, and make sure the drainage holes are not blocked. In a small or poorly draining container, removing the rock layer and replacing it with a good potting mix plus filter fabric over the holes is usually the better long-term choice. The key point is that rocks are not a magic drainage fix; healthy container drainage comes from the right soil mix, unobstructed outlets, and good planter design.