Many urban gardeners believe adding a layer of gravel to the bottom of planter boxes improves drainage, but this common practice actually creates more problems than it solves. When you place coarse materials like gravel beneath potting soil, you create what horticulturists call a "perched water table" - a saturated zone where water accumulates at the interface between the two different materials.
The physics behind this phenomenon involves water's natural tendency to resist moving from fine-textured soil to coarser material below. Instead of draining freely, water pools in the soil layer just above the gravel, keeping roots in constant moisture that can lead to root rot and oxygen deprivation.
For optimal urban planter drainage, use a high-quality potting mix throughout the entire container. Modern potting mixes contain materials like perlite, vermiculite, or coconut coir that create consistent pore spaces for both drainage and moisture retention. Ensure your planter boxes have adequate drainage holes at the bottom rather than relying on gravel layers.
If you're concerned about soil washing out through drainage holes, place a single piece of landscape fabric or coffee filter over the holes before adding soil. This simple solution prevents soil loss while allowing proper water drainage, creating healthier growing conditions for your urban garden plants.