Good Morning and thank you for your question. The short answer to your question is yes, walnut leaves can be composted. However I understand your concerns and there are a few things you should be aware of.

The trees in the walnut family; black walnut, butternut, hickory and pecan all contain the toxin Juglone which is harmful to many species of plants. Juglone breaks down in the leaves and the toxic effect is degraded in two to three weeks when the leaves are exposed to air, water and bacteria. Juglone is at the highest concentration through the trees roots in the soil surrounding the tree.  Avoid gardening near the walnut trees.

For compost a good rule of thumb is thirty percent greens to seventy percent browns. I use a mulching mower on the first leaves that drop to add the nutrients in my lawn areas. I shred the remaining of multiple species of trees and use as either mulch in my beds and gardens or I turn them into leaf mold which helps break up my dense clay soil.

You can get further information from the Franklin County Extension Services, 2548 Carmack Rd, Columbus, Ohio 43210.<personal data hidden>. For some good information online please go to: https//online.osu.edu/factsheet/hyg-1189-99 where you will find an informative factsheet on  composting.

I hope I have been helpful and not too chatty. Happy gardening.

Mike Heys