Walnuts, Image Courtesy of Dan, FreeDigitalPhotos.net
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.
For answers to more frequently asked composting questions, see the Composting FAQ page. For a full list of gardening articles and links to garden photos, visit the main Gardening page.