Yes, it is absolutely possible to overwater plants in a planter box with drainage holes. While drainage holes are crucial for plant health by allowing excess water to escape, they are not a complete safeguard against overwatering. The key issue is the frequency of watering, not just the container's design.
When you water too often, the soil remains constantly saturated. This deprives plant roots of essential oxygen, creating an environment where harmful anaerobic bacteria and fungi thrive. This leads to root rot, a serious condition where the roots turn brown, mushy, and die. Even with drainage holes, the water has no time to drain fully between waterings, keeping the root zone waterlogged.
Signs of overwatering include consistently wet soil, yellowing and dropping leaves (often starting with lower leaves), wilting despite wet soil, and a foul smell from the soil. To prevent this, always check the soil moisture before watering. Insert your finger about an inch or two deep. If it feels moist, delay watering. Ensure your planter box has adequate, unblocked holes and use a well-draining potting mix. Remember, proper watering is about timing and soil condition, not just the presence of drainage.