Black Walnuts (Juglans nigra) contain the toxic chemical juglone, that is exuded from all parts of the plant. Black walnut trees load their roots, buds, and nut hulls with the juglone toxin. Leaves and stems have smaller amounts of juglone. The toxin seeps into the soil and susceptible companion plants will turn yellow, wilt, and sometimes die. The soil under black walnut trees is very rich in juglone due to leaf decay, nuts and shells and other organic matter of the black walnut. Collecting and removing this organic litter can significantly reduce content of this toxin. Don't forget, that even if the tree is removed, the decaying root system will keep the soil toxic for many years. Other trees of the same family like butternut, pecan, shagbark hickory, and English walnut also produce juglone, but at concentrations lower than the black walnut. Rarely do these trees affect juglone-sensitive plants.
Fortunately there are plenty of native (and some non-native) trees, shrubs and plants, that will tolerate black walnut's toxicity. So we can still enjoy the beautiful canopy of this magnificent tree and all layers of vegetation underneath including flowering perennials. Plants offered in this category are juglone tolerant.
PLANTS SENSITIVE TO BLACK WALNUT TOXICITY - don't plant these close to black walnuts!
Annuals and vegetables: asparagus, cabbage, eggplant, flowering tobacco, pepper, petunia, potato, tomato
Herbaceous perennials: autumn crocus, baptisia, columbine, lily (Asian hybrids), peony, rhubarb
Shrubs: blueberry, red chokeberry, cotoneaster, hydrangea, lilac, potentilla, rhododendron, yew, and some viburnum shrub species
Trees: European alder, white birch, crabapple species, hackberry, larch, linden, saucer magnolia, mugo pine, red pine, white pine, Norway spruce, silver maple, and some viburnum tree species
Source : Morton Arboretum