By Jennifer Copley (Last Updated 8 March 2011)
Black walnut leaves, bark, and hulls contain a substance called juglone that is toxic to some plants. However, this toxin can be broken down through exposure to bacteria, water, and air. For black walnut leaves, at least 2-4 weeks of composting is required to get rid of the toxin (or a couple of months sitting in regular soil). Sawdust or chips from black walnut tree prunings and black walnut hulls should be composted for at least 6 months (and preferably longer) to ensure safety, so you may wishto set up a separate system for walnut-related materials. If you’re not sure that the toxin has been eliminated, plant tomato seedlings in the compost to see if they’ll grow.