Sunflower seed shells can indeed be used as mulch for outdoor plant boxes, offering several benefits for gardeners. These shells are lightweight, biodegradable, and help retain soil moisture while suppressing weeds. However, there are a few considerations to keep in mind.

First, sunflower seed shells decompose slowly, making them a long-lasting mulch option. They also add organic matter to the soil as they break down, improving soil structure over time. However, avoid using salted shells, as the salt can harm plants.

Additionally, sunflower seed shells may attract birds or pests, so monitor your garden closely. For best results, layer the shells thinly (about 1-2 inches) to allow proper airflow and prevent mold growth.

In summary, sunflower seed shells are a viable, eco-friendly mulch choice for outdoor plant boxes, but proper application and monitoring are key to success.