Absolutely, it is entirely possible—and quite common—to overwater plants in an urban planter box, even when it has drainage holes. While drainage is a critical first line of defense, it's not a foolproof guarantee against overwatering. The real issue lies beneath the surface. Excess water can linger in the lower soil layers, creating a perpetually soggy environment that roots cannot escape. This lack of oxygen leads to root rot, a condition where roots suffocate and decay, ultimately killing the plant.
Several factors in urban settings compound this risk. Planter boxes, especially deeper ones, can create a "perched water table" at the bottom, trapping moisture. Using a potting mix that retains too much water or lacks sufficient perlite or coarse sand exacerbates the problem. Furthermore, the urban microclimate—such as a shaded balcony or a sheltered corner—can slow evaporation dramatically, meaning the soil stays wet for much longer than expected.
The key to success is to water based on the plant's needs and the soil's condition, not a fixed schedule. Before watering, check the soil moisture by inserting your finger about two inches deep. If it feels damp, wait. Ensure your planter box has ample drainage holes and is filled with a well-draining, high-quality potting mix. Elevating the box slightly with pot feet can also improve airflow and drainage. By understanding that drainage is just one part of the equation, you can avoid the common pitfall of overwatering and keep your urban garden thriving.