An urban planter box needs bottom drainage first: several unobstructed drainage holes, a slightly raised base, and a free-draining potting mix that lets excess water move out quickly. For a long box, aim for holes about 1/2 inch across every 6–12 inches along the bottom, plus one near each end. If the box has no holes, drill them—unless it is a sealed decorative cachepot, in which case use it only as a sleeve around a nursery pot that already drains. Cover holes with mesh or a broken shard so soil does not wash out, but never seal them.
Second, give the box a drainage layer only as part of a system. A 1–2 inch layer of coarse gravel, lightweight expanded clay, or a commercial drainage mat can help water move toward the holes, especially in deep boxes, but it cannot replace holes. Put a geotextile or landscape fabric between the drainage layer and the soil to stop fines from clogging it. In shallow balcony boxes, a drainage mat or slatted base often works better than heavy gravel. Raise the box on feet or spacers so runoff can escape and air can reach the bottom.
Use a quality container mix, not garden soil. It should be loose, porous, and moisture-retentive without staying soggy; look for a mix containing bark, peat or coir, perlite, and maybe sand or grit. A little compost is fine, but heavy clay soil in a planter box is a recipe for waterlogging. Match the mix to the plants: thirsty vegetables may want more compost, while lavender, rosemary, and succulents need extra grit and sharper drainage.
Watering and runoff management matter as much as the holes. Water until it just runs from the bottom, then stop; empty any saucer or tray within 30 minutes so the box is not sitting in water. Check that the holes are not blocked by roots, soil, or a saucer pressed against the base. In rainy climates, make sure the box has an overflow path and position it so water does not pool under it. A slight slope toward the drainage holes helps, but do not rely on slope alone.
Self-watering or reservoir planter boxes can work well in cities, but they still need an overflow outlet. The reservoir should be below the soil, with a fill tube and overflow hole, so excess water can escape instead of drowning roots. If you use a wicking system, use a free-draining mix and keep the reservoir level controlled. For most urban planter boxes, simple bottom drainage plus a raised base is the most reliable.
Finally, remember that good drainage is not the same as drying out. Mulch the surface to slow evaporation, group pots to shade each other, and check moisture before watering. If water pours straight through and the mix dries too fast, the mix may be too coarse or the box too small; if water sits on top, the mix is too dense or the holes are blocked. The goal is a planter box that accepts water easily, holds enough for the plants, and lets the excess leave quickly.