I was thinking that the lack of undergrowth below the trees must be artificial, but then on the Wikipedia page about walnuts I saw:
"[The trees] secrete chemicals into the soil to prevent competing vegetation from growing."
"[The trees] secrete chemicals into the soil to prevent competing vegetation from growing."
Very mediocre results with my tomatoes and cucumbers this year, but it was very, very dry.
Oh, and zucchini. But you can almost plant zucchini on asphalt and it would find a way to produce bushels of the stuff...
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.
Presumably the same oils that deter insects from feeding on the leaves / needles tend to leach out into the soil over time. And all things being equal, the leaves are most likely to fall near the trunk, making it an excellent delivery mechanism, too.