Absolutely, it is not only possible but very common to overwater plants in a planter box that lacks drainage holes. The absence of an escape route for excess water means it pools at the bottom, creating a saturated environment that suffocates the roots. This condition, known as root rot, prevents roots from absorbing oxygen and nutrients, ultimately killing the plant.
Key signs your plant is overwatered include consistently wet soil, yellowing and dropping leaves, a musty smell from the soil, and soft, mushy stems or roots. To rescue an overwatered plant, immediately stop watering. Tip the box to carefully drain any standing water. Improve air circulation around the soil and, if severe, consider gently removing the plant to check the roots. Trim any black or mushy roots and repot into fresh, dry soil, ideally in a container with drainage.
To prevent this, be extra cautious with watering. Always check the soil moisture by feeling an inch or two below the surface—water only when it's dry. Using a well-draining soil mix and adding a layer of pebbles at the bottom (though not a complete fix) can create a small reservoir. The best long-term solution is to drill drainage holes if possible or use the no-drainage pot as a decorative cache pot, placing a functional nursery pot with holes inside it.