[Paul R.]

Oh, yes. Walnut trees and related species release juglone. I had wondered why my hickory leaf / nut (husk) mulch seemed to do in our tomatoes, until I found this out. Now I just use it (the mulch) for a worm-bed.

[shibumi]

"Oh, yes. Walnut trees and related species release juglone."

That's why these guys are always hanging out by them.