Yes, it is absolutely possible to over-water plants in urban planter boxes, even when they have drainage holes. While drainage holes are essential for preventing water from pooling at the bottom and causing fatal root rot, they are not a foolproof guarantee against over-watering.
The key factor is frequency. Consistently adding more water before the soil has a chance to dry out creates a perpetually soggy environment. Drainage holes will remove excess water from the bottom, but the soil mass itself can remain waterlogged. This suffocates the roots by depriving them of oxygen, leading to root rot—a condition where roots turn brown, mushy, and unable to absorb nutrients or water. Ironically, the symptoms of over-watering (wilting, yellowing leaves) often mimic those of under-watering.
Several urban factors exacerbate this risk. Planter boxes in sheltered balconies or corners may not receive enough wind or sun to aid evaporation. Using dense, moisture-retentive potting mixes or pots that are too large for the plant can also hold water for too long. The best practice is to always check the soil moisture before watering. Insert your finger about an inch or two into the soil. If it feels damp, wait. Water only when the top layer feels dry. Choosing a well-draining potting mix and ensuring your planter box has unobstructed drainage holes are your first lines of defense, but mindful watering is the true secret to thriving urban plants.