Yes, it is absolutely possible to overwater plants in an urban planter box, even when it has drainage holes. While drainage holes are essential for preventing water from pooling at the bottom, they are not a foolproof guarantee against overwatering. The key factor is the frequency of watering.
Overwatering occurs when the soil remains consistently soggy, depriving plant roots of oxygen. In a planter box, this can happen if you water too often, use a soil mix that retains too much moisture, or if the planter is in a location with poor air circulation and low light, which slows evaporation.
Signs of overwatering include yellowing leaves, wilting despite wet soil, and a musty smell from the soil. The most severe consequence is root rot, a fungal condition that can kill the plant.
To avoid this, always check the soil moisture before watering. Insert your finger about an inch deep. If it feels damp, wait. Ensure you use a well-draining potting mix, not garden soil, and consider adding materials like perlite for better aeration. Finally, understand the specific water needs of your plants—succulents, herbs, and leafy greens all have different requirements. Proper watering is about timing and observation, not just relying on drainage holes alone.