Yes, it is absolutely possible to overwater plants in a planter box even if it has drainage holes. While drainage is crucial for plant health by allowing excess water to escape, it is not a foolproof shield against overwatering. The real issue often lies in watering frequency and soil composition.
Drainage holes prevent water from pooling at the very bottom, which helps avoid root rot caused by complete saturation. However, if you water too often, the soil remains constantly wet. This deprives roots of oxygen, creates an environment for harmful fungi, and can wash away essential nutrients. Poor-draining soil, even in a well-draining pot, will hold too much moisture. Compacted soil or soil without enough perlite or sand exacerbates the problem.
Key signs of overwatering include consistently wet soil, yellowing lower leaves, wilting despite moist soil, and a foul smell from the soil. To prevent this, always check the soil moisture before watering—insert your finger about an inch deep. Water only when the top layer feels dry. Ensure you use a high-quality, well-draining potting mix specifically designed for containers. Finally, empty the saucer under your planter box shortly after watering to prevent the pot from sitting in a puddle.
Remember, drainage holes are a critical tool, but they work in partnership with your watering habits and the right soil. Proper care involves understanding your plant's specific needs and the conditions of its environment.