Yes, it is absolutely possible to overwater plants in an urban planter box, even one with drainage holes. While drainage holes are essential for preventing waterlogging by allowing excess water to escape, they are not a foolproof guarantee against overwatering. The key factor is the frequency and volume of watering, not just the container's design.
Overwatering occurs when the soil is kept consistently too wet, depriving plant roots of oxygen. This creates an environment where harmful fungi thrive, leading to root rot—a condition often fatal to plants. Drainage holes remove excess water *after* you water, but if you water too often, the soil never has a chance to dry out adequately between sessions. The roots then sit in perpetually damp, oxygen-poor soil.
Signs of overwatering in your planter box include yellowing leaves, wilting despite wet soil, soft and mushy stems, and a musty odor from the soil. To avoid this, always check the soil moisture before watering. Insert your finger about an inch deep. If it feels moist, delay watering. Ensure your planter has sufficient, unobstructed drainage holes and use a well-draining potting mix, not garden soil. Choosing planters with adequate size and material that breathes, like terracotta, can also help regulate moisture. Remember, in urban gardening, the mantra "less is more" often applies to watering—it's easier to add water than to remove it.