Using walnut shells as mulch for outdoor plant boxes is an interesting idea, but it comes with both advantages and potential drawbacks. Walnut shells are a natural, organic material that can help retain soil moisture, suppress weeds, and add a unique aesthetic to your garden. However, there are important considerations before using them.

Benefits of Walnut Shell Mulch:

1. Weed Suppression: The dense texture of crushed walnut shells can block sunlight, reducing weed growth.

2. Moisture Retention: Like other mulches, walnut shells help the soil retain water, reducing the need for frequent watering.

3. Aesthetic Appeal: Their rustic look adds a decorative touch to plant boxes.

Potential Drawbacks:

1. Juglone Toxicity: Walnut shells contain juglone, a natural compound that can inhibit the growth of certain plants (e.g., tomatoes, peppers). Research plant compatibility before use.

2. Slow Decomposition: Walnut shells break down slowly, which means they won’t enrich the soil quickly like compost or wood chips.

3. Sharp Edges: Crushed shells may have sharp edges that could deter pets or barefoot gardeners.

Best Practices:

- Use aged or composted walnut shells to reduce juglone levels.

- Avoid using them near juglone-sensitive plants.

- Combine with other mulches for balanced soil health.

In conclusion, walnut shells can be a sustainable mulch option for outdoor plant boxes, but their suitability depends on your garden’s specific needs. Test a small area first to observe plant reactions.