If you are asking whether to put gravel at the bottom of an urban planter box, the short answer is usually no. Gravel does not improve drainage in a container the way many gardeners expect. In fact, it can create a perched water table: water moves through the finer potting mix, reaches the coarse gravel, and struggles to move downward because the texture change disrupts capillary flow. The result can be a saturated zone right above the gravel, which is exactly where many roots sit. This raises the risk of root rot, especially in compact urban planters where airflow and evaporation are limited.
A better approach is to rely on drainage holes, a free-draining potting mix, and a geotextile or mesh screen over the holes to stop soil from washing out. Keep the planter slightly raised so water can escape, and avoid sealing the base. If the box has no drainage, add it if possible. If that is not practical, use the box as a decorative cover for a nursery pot with drainage, or convert it with a false bottom and overflow outlet. A thin layer of gravel can be useful on top of the soil to reduce splash and keep pets out, but it is not a substitute for drainage. In very shallow or lightweight rooftop planters, skip gravel unless it serves a specific structural or erosion-control purpose, because it adds weight without solving waterlogging. In short: design for drainage, not a gravel layer.