[ihsw2]

English walnut differs from black walnut in that the juglones -- the protective gas emitted by walnut trees -- are emitted less. Furthermore, it is easy to confuse water stress with juglone gas stress, so sometimes these symptoms can be remedied with more water.

Given what the parent commenter has said about having to experiment with different plants, it stands to reason that they have a healthy black walnut tree in their garden rather than a regia.