Juglone Toxicity

All parts of a black walnut tree exude juglone, but the roots, buds and nut hulls emit the most. The highest levels of juglone concentrations occur in the soil directly beneath the canopy, which can spread out up to 100 feet with Juglans nigra and up to 60 feet with the Northern California black walnut variety. Although juglone doesn't affect humans at all, sensitive plants growing beneath the canopy frequently experience wilting, yellowing leaves, drooping foliage, stunted plant growth and even death.

Shrubs

When planting shrubs beneath your tree's canopy, choose juglone-resistant plants that can also handle low-light conditions. Many sumac and currant species tolerate both the chemical and partial shade. Look for smooth sumac (Rhus glabra), a flowering shrub winter hardy in USDA zones 3 through 9, or clove currant (Ribes odoratum), a North American native that prefers USDA zones 4 through 8. Clove currant shrubs feature fragrant, golden-yellow blooms in the spring and edible fruit in the summer. The blue-green, round leaves turn drab yellow tones in the fall. The smooth sumac offers more attractive fall foliage, with the fern-like leaves turning vibrant orange to red shades in the fall.

Vines

Planting juglone-resistant vines around your black walnut tree is a great way to fill in the empty spaces. Shade-loving choices include the Virginia creeper (Parthenocissus quinquefolia), a vine winter hardy in USDA zones 3 to 9, and sweet autumn clematis (Clematis terniflora), which prefers USDA zones 5 through 9. Although vigorous climbers, both species can also sprawl along the ground to form a dense groundcover. Virginia creeper foliage emerges in the spring with a slight purple tint, matures to a drab green in the summer and then turns attractive purple to crimson shades in the fall. Inconspicuous flower clusters bloom in the spring, giving way to dark berries. Sweet autumn clematis features aromatic, white flowers that cover the foliage from late summer through October.

Ferns

Ferns can add both color and texture to the space around your black walnut tree, and various shade-loving varieties also tolerate juglone. The lady fern (Athyrium filix-femina) and the male fern (Dryopteris filix-mas) both grow well in USDA zones 4 through 8, thriving in partially shady to fully shady sites. Male ferns are large, vigorous plants that bear arching, medium-green fronds that reach about 3 feet in length. The lady fern's fronds reaches about the same size, but looks more delicate with its lighter green, finely divided blades.