Viburnums, smokebush (cotinus), spirea, butterfly bush, and roses don't do well under black walnuts. But under my huge trees I grow a wide range of hostas, lilies (tiger, Oriental, regal, (but NOT Madonna), astilbe, coral bells, daylilies, passion vine, echinaceas, spiderworts, irises (tall bearded dwarf, Siberian, wild crested, Louisiana), sedums, monarda, ferns, tall phlox, violets, gingers, lots of spring wildflowers and bulbs, hibiscus, nicotiana, dicentras, Virginia bluebells, Solomon's seal, saxifragia, hydrangeas, ajuga, goldenrod, asters, and many others. Redbuds, vitex, golden rain tree, mimosa, lilacs all do well within range of the walnut roots. Tomatoes, potatoes and other members of the nightshade family wilt and die almost immediatly when exposed to juglone, however.
Hackberry trees, also somewhat alleopathic, refuse to allow snow on the mountain or any kind of lamium within the dripline of their branches while both plants do fine under black walnuts. Go figure.
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Ilex is ok, it's the water!
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