At the bottom of an urban planter box, start with a drainage and separation layer rather than filling it straight with soil. First, check that the box has drainage holes and that it sits slightly above the ground, balcony, or roof surface so excess water can escape. For a deep planter, add a 2–5 cm layer of washed gravel, expanded clay pebbles, or crushed brick. For a shallow or weight-limited urban planter, use a thin drainage mat instead of thick gravel so the roots still have enough room and the box stays lighter.
Cover the drainage material with a permeable geotextile or filter fabric. This lets water pass through while preventing soil from washing into the drainage layer and clogging it. If you are making a self-watering or wicking planter, use a water reservoir and wicking fabric rather than a conventional gravel layer. For wooden planter boxes, line the sides with a water-resistant liner, but do not block the drainage holes.
In urban settings, keep the bottom layer as lightweight as practical, especially on balconies and roofs. Avoid filling the bottom with ordinary garden soil, sand, or a sealed plastic sheet, because these can cause waterlogging, add unnecessary weight, or stop drainage. Once the drainage layer and filter fabric are in place, add a good-quality lightweight potting mix and then plant. This simple bottom setup helps your urban planter drain well, retain healthy soil, and support stronger plant growth.