I've got both bearded and PCI's in the root zone of a black walnut to no ill affects. They are supposed to have allelopathic properties that inhibit the growth of plants, but it hasn't been true for me. I've got Canna, Euonymus, PCI's, Cornus, Phormium, Carex, Heuchera, Bamboo, Podocarpus, Pittosporum, Ilex, Spirea, Lobelia... all doing fine.
