For thriving plants in urban planter boxes, effective drainage is non-negotiable. The common advice to simply put gravel at the bottom is a myth that can create a "perched water table," trapping moisture and harming roots. The best practice uses a layered system that *facilitates* water movement out of the soil zone.
Start with a base layer of coarse material. Instead of fine gravel, use medium-sized landscape rock, broken pottery shards, or specifically designed drainage aggregates. This layer should be 2-3 inches deep and sits directly over the drainage holes.
Crucially, place a permeable barrier over this base layer. A simple, inexpensive landscape fabric or a piece of burlap works perfectly. This critical step prevents the soil above from washing down and clogging the drainage layer, while still allowing water to pass through freely.
Now, add your soil. Use a high-quality, well-draining potting mix formulated for containers—never dense garden soil. For enhanced aeration and moisture retention, consider amending the mix with perlite or coconut coir. Fill the box, leaving about 1-2 inches of space below the rim for watering.
The final, often overlooked layer is a thin top dressing of fine gravel, bark mulch, or moss. This helps retain soil moisture, suppresses weeds, and protects the soil surface from erosion during watering.
This method—coarse drainage base, fabric separator, quality soil, and top mulch—creates a healthy environment for roots by mimicking natural soil conditions, ensuring excess water escapes quickly while maintaining ideal moisture and aeration for your urban plants.