Answer: All parts of a walnut tree (bark, leaves, fruit, and roots) contain a growth inhibiting compound called juglone. While this naturally occuring chemical will degrade over time, it's probably not a good idea to compost the husks while they're still fresh. Why not put them in a separate pile and allow rainwater to leach the chemical out before you add the husks to your compost pile? Or, you can bury the husks and let them decompose under ground, away from your garden, where the juglone will not be bothersome to your plants.
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