Answer: Black walnut trees, Juglans nigra, are considered allelopathic due to a substance called juglone put out by primarily by their roots. Some particularly sensitive plants such as tomatoes are also affected by the black walnut leaves, stems, and nuts. When properly composted, however, the juglone seems to become harmless. Since home composting is a somewhat variable process, I would suggest allowing it to sit for at least a year prior to applying it to the garden, especially if you have added a large quantity of the shells and/or hulls.
If the shells are from an "English" walnut (the type we normally buy at the grocery store for cooking and fresh eating), there is no danger in using the shells in your compost.
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