Black walnut (Juglans nigra) and, to a lesser extent, other walnuts (Juglans spp.), as well as their close relatives, the hickories (Carya spp.), release a product called juglone that is allelopathic, that is to say toxic to most other plants. Thus, the walnut suffers little from competition, as few other plants can grow well at its base (here’s a list of plants that are resistant to juglone). Juglone is present in all parts of the walnut tree except the nut itself: leaves, branches, bark, wood, and especially its roots and nut husks.
The question is therefore: should the home gardener put “toxic” black walnut leaves in their compost bin? Ohio State University looked into the matter and the reply is yes: juglone decomposes within weeks in contact with air, water and bacteria. If the leaves are finely shredded, decomposition is even quicker. And once decomposed, walnut leaves apparently give a compost of excellent quality.
Walnut sawdust and wood chips as well as walnut husks, however, are slower to decompose: it is best to compost them for 6 months before using them in the garden to be sure the juglone has thoroughly decomposed.