Your concern about water runoff from urban planter boxes is very common and important. Many planter boxes are sold separately from their trays or saucers. You often need to purchase a matching drip tray or a slightly larger saucer to catch excess water. This setup is crucial for preventing messy runoff onto balconies, patios, or indoor surfaces.
The primary purpose of a tray is to protect your surfaces. However, it's vital to empty the tray periodically. Letting plants sit in standing water can lead to root rot. For better water management, consider planter boxes with built-in drainage systems or self-watering reservoirs. These designs often include an overflow outlet to prevent over-saturation.
To minimize runoff, use a well-draining potting mix and water your plants slowly and deeply, allowing the soil to absorb moisture. Adding a layer of mulch on top of the soil can also help reduce evaporation and slow down water flow. By choosing the right planter setup and watering wisely, you can maintain a healthy urban garden without the worry of water runoff.