Black walnut trees, known botanically as Juglans nigra, are among several species of trees that release a chemical compound called juglone that is deadly to many species of plants. According to Iowa State University, black walnut possesses the greatest and most powerful concentration of juglone. The compound is concentrated most heavily in the tree roots, nut casings, stems and branches. It is so powerful over some plants that after a black walnut tree and its roots have been completely removed from an area the juglone that persists in the soil will limit what species can be planted there for years into the future.
Some fruiting trees and vines are widely known to be intolerant of juglone, meaning that the fruits cannot live within the drip line zone or in some cases within up to 50 feet of black walnut trees. The most well known and understood incompatibilities between black walnut and fruit species include all apple cultivars, crab apple, blackberry, boysenberry, domestic grape and blueberry vines.
A number of vegetables are known to be susceptible to the wilting and tissue death effects of juglone in the planting soil. Most notable and well understood among these are those plants in the nightshade family which cannot tolerate juglone to any degree. Nightshade plants include some of the most common and popular vegetable garden crops such as tomatoes, potatoes, peppers and eggplant. They cannot survive or thrive when planted anywhere near the drip line of the tree or its root spread in the present or recent past due to latency of the compound in the soil.
Flowering & Landscape Plants
A range or ornamental trees and plants fall prey to the effects of juglone, making them incompatible with black walnut plantings. Though the research is not exhaustive in this area, there are many well known and accepted incompatibilities including bulbs, and woody and herbaceous plants. The most well known of these are autumn crocus, azalea, chrysanthemum, hackberry, honeysuckle, Japanese larch, lespedeza, lilac, potentilla, privet and rhododendron.
Grasses & Field Crops
Several grasses or herbs are also known to be intolerant of juglone. Crop fields planted in proximity to black walnut stands or in juglone laden soil can struggle and underperform or fail entirely. The most well documented incompatibilities among this type of plant include alfalfa, crimson clover, thyme and tobacco. As juglone does not travel widely or spread in the soil easily, plantings made with at least 50 feet in clearance from black walnut roots and the canopy drip line are thought to be safe.
A number of popular landscape trees also fall prey to juglone and should not be co-located or planted within roughly 50 feet of black walnut. Trees to avoid include black alder, basswood, linden, white birch, magnolia, silver maple, mountain laurel, pine and Norwegian spruce.