Planting near black walnuts

I have a number of native black walnut trees in my yard. I'd like to put some plants below them, but I've heard they leach toxins into the ground that some plants find difficult to survive. The non-native weed grasses seem to do okay under the trees. Does anyone know if it's only the walnut shells or also the leaves that creates a problem?

Anyway, I'm wondering if there are any suggestions on natives that might survive whatever natural herbicides the trees create AND ALSO are okay with the shade created by the tree.