Only use them if they are UNSALTED, or the salt will kill off the natural bacterias and microorganisms in the compost. Pistachio shells take ages to break down.
|This shell was in my planter soil. Still hasn't broken down.
Benefit: they help retain water and make the soil lighter! I'm so impressed. Yay!
Around Christmas time, I crack walnut, almond, hazelnut, chestnut, Brazilian-nut shells. Instead of tossing them out with uncompostable kitchen waste, I'll be composting them too.