Black walnut trees produce a toxic substance called juglone, says the University of Wisconsin Extension. Juglone occurs in every part of the walnut tree, but occurs in greater concentrations in the buds, nut hulls and the roots of the plant. Planting around walnut trees can be tricky, since some plants such as tomato, potato, eggplant, pepper and ornamental plants such as lilac, peony, rhododendron and azalea are highly sensitive to juglone. Certain considerations when planting around walnut trees will reduce the chance of poisoning your garden.
Plan your garden as far away from the walnut tree as possible to prevent hulls, leaves and shoots from falling into the garden, and to prevent roots from the walnut tree from entering the garden soil.
Choose plants that are tolerant of juglone and will not die if they come in contact with buds or nut hulls. Onions, beets, squash, melon, carrot, beans, corn and parsnips are all tolerant vegetables, according to Penn State University. Contact your local extension office for a list of local shrubs or ornamental plants suitable for the area that are juglone-resistant.
Plant your garden in a raised planter bed to reduce the chance of tree roots entering the garden. Dig a long trench that is no more than 4 feet wide and till the subsoil. Place timbers around the trench and fill it with the removed soil and a layer of topsoil. Bury the timbers slightly for extra protection against tree roots entering the garden.