Yes, it is absolutely possible to overwater plants in a planter box even if it has drainage holes. While holes are crucial for preventing waterlogging, they are not a foolproof guarantee against overwatering. The key factor is the speed at which water can exit the pot versus the rate at which you add water and the soil's ability to drain.
Several conditions can lead to overwatering despite drainage. First, using a dense, heavy potting mix that retains too much moisture will keep the roots soggy. Second, watering too frequently doesn't allow the soil to dry out between waterings. Third, a saucer placed underneath that isn't emptied traps water, allowing the bottom soil to reabsorb it. Finally, environmental factors like low light, cool temperatures, or high humidity slow down a plant's water use, leaving soil wet for longer.
The primary danger of overwatering is root rot. Saturated soil pushes out oxygen, causing roots to suffocate, decay, and become vulnerable to fungal diseases. Symptoms include yellowing leaves, wilting despite wet soil, soft and mushy stems, and a foul smell from the soil.
To prevent this, ensure you use a well-draining potting mix, water only when the top inch or two of soil is dry, always empty the drainage saucer, and choose a planter box size appropriate for your plant's root system. Good drainage holes are essential, but mindful watering habits are the true secret to healthy plants.