Beneath Black Walnut
Will butterfly bush, lilac, dogwood, or other flowering trees and shrubs grow under a walnut tree. The tree branches are within 12 feet of the proposed planting bed.
As you know, black walnut roots, leaves and bark contain juglone. This substance is toxic to many plants. There is limited research on plants that tolerate the juglone substance. The best approach is to keep the garden at least 50 preferably 80 feet away from the tree. The following list of flowering trees and shrubs have been observed growing under black walnut trees. Redbud (select the Columbus of Minnesota strain for increased hardiness), Koreanspice viburnum, other species of viburnum, weeping forsythia and Rose of Sharon. Some varieties of the following perennials will also survive under a black walnut. Bugleweed, astilbe, Dutchman's breeches, cranesbill geranium, hosta, daylilies, Siberian iris, sundrops (Oenethera), lungwort, trillium, lamb's ear and tulips. For a more complete listing see the Ohio State University factsheet Black Walnut Toxicity to Plants, Humans and Horses.