Is it ok to leaves black walnut leaves on my lawn if I mulch them with my mower? I do remove them from perennial beds.
Thank-you for contacting Toronto Master Gardeners with your question.
You seem to be aware that Black walnut (Juglans nigra) produces a toxin, called juglone, which leaches into the soil from the tree roots and also from fallen leaves, branches and decaying fruits. It can take a year for the toxin to dissipate if the leaves are composted, so leaving them on the grass over the winter and into the spring, even if they are shredded, could be a risk. It would be better to put them into your compost where they can decompose for at least a year. This is the optimum situation in case of susceptible plants in the area, but having said that, most grasses are actually tolerant of juglone. Depending on the layout of your yard and garden, you may not have a problem.
You may wish to consult this Ontario government-produced article on Walnut toxicity.