Nut shells can be used in the garden as a natural mulch to retain soil moisture, suppress weeds, and insulate plants, or as a soil amendment to improve soil structure and drainage. Crushed peanut shells act as a natural pest deterrent, while some nut shells, like pistachio shells, can improve aeration at the bottom of pots. Walnut shells should be used with caution, as they contain juglone, a substance toxic to some plants.