Black walnut tree roots produce a substance called juglone (5-hydroxy-alphanapthaquinone). This substance is highly toxic to many plants. Tomatoes, apples, rhododendrons and azaleas will perish within two months of being planted within the root range of a black walnut. Most mature walnut trees have a 60- to 80-foot toxic range around the tree that can make planting difficult. Juglone also occurs in the bark, wood and leaves of the black walnut tree. Several plant species can be successfully planted under the black walnut tree but care must be taken when choosing compatible varieties.
Locate plants away from the drip line of the black walnut tree. Many plants that are sensitive to the black walnut will grow once planted away from the roots and the drip line of the tree. This means planting at a radius of 80 feet out from the tree trunk all the way around the tree.
Choose plants that can successfully be grown under the black walnut tree. There are a large range of plants, shrubs, trees and vines that are not affected by juglone. Squash, carrots, corn and melons have been observed growing under the drip line of a black walnut tree. The pansy plant, marigolds and morning glory can tolerate juglone and grow well beside the black walnut. Plum trees, cherry trees and nectarine trees can grow beside the black walnut without adverse affects.
Build raised gardens to maintain plants within an 80 foot radius of the black walnut tree that are susceptible to juglone toxicity. Lay down material that will not allow the roots to penetrate immediately such as a layer of plywood, rocks, a concrete pad or plastic sheeting. Place fresh garden soil into the raised garden bed. All debris from the black walnut tree must be immediately removed from the raised garden beds or the plants will die from juglone exposure. Remove twigs, nuts and small leaves from any raised garden beds before the juglone can leech into the soil of the raised garden bed.
Pour a concrete barrier that the tree roots cannot penetrate to plant juglone susceptible plants behind. Driveways work as a common barrier to plant juglone susceptible plants behind. The driveway lessens the roots' impact on the topsoil because the tree roots are forced to go deeper into the soil to avoid the upper concrete layers. This makes the topsoil contain less juglone; however, care still needs to be taken to choose plants that can tolerate a small amount of juglone.
Maintain good drainage around the black walnut tree to lessen the juglone accumulation in the surrounding soil. Add mulch, sawdust or bark chips to the garden soil at a ratio of 50 percent soil with 50 percent mulch to make the soil less toxic. Mulch needs to be continuously added and tilled into the existing soil each season.