The area affected extends outward each year as a tree enlarges. Young trees two to eight feet high can have a root diameter twice the height of the top of the tree, with susceptible plants dead within the root zone and dying at the margins. Not all plants are sensitive to juglone. Many trees, vines, shrubs, groundcovers, annuals and perennials will grow in close proximity to a walnut tree. Black walnut has been recommended for pastures on hillsides in the Ohio Valley and Appalachian mountain regions.
Trees hold the soil, prevent erosion and provide shade for cattle.
Gardeners should carefully consider the planting site for black walnut, butternut, or persian walnut seedlings grafted to black walnut rootstock, if other garden or landscape plants are to be grown within the root zone of mature trees. Persian walnut seedlings or trees grafted onto Persian walnut rootstocks do not appear to have a toxic effect on other plants. Horses may be affected by black walnut chips or sawdust when they are used for bedding material. The juglone toxin occurs in the leaves, bark and wood of walnut, but these contain lower concentrations than in the roots. Juglone is poorly soluble in water and does not move very far in the soil.