The black walnut tree roots and leaves secrete a substance called juglone (5-hydroxy-alphanapthaquinone). This substance enters the soil and works as a respiratory inhibitor to most plants, trees and shrubs that grow within a 50 to 80 foot radius out from the black walnuts trunk. Plants that are planted within this area perish within one to two months. Not all plants suffer a toxic reaction to juglone. A few plants, trees and shrubs can be planted with black walnut trees. Many humans and horses can also suffer from an allergic reaction to juglone, which can cause respiratory symptoms, particularly in May when the tree releases pollen.
Hollyhocks are a herbaceous biennials that can thrive beside black walnut trees. The plant can often grow eight feet tall with wonderful pink, red, white and yellow blossoms. Hollyhocks are a favorite for hummingbirds and butterflies.
Spanish bluebells are tiny bulbs that are easy to naturalize under walnut trees. They bloom most commonly in blue but can also be purchased in white and pink.The blossoms bloom in spears that are lightly fragrant. Spanish bluebells thrive in the shade and blossom in the spring.
Astilbes produce unique plume flowers that look like large feathers. They bloom in red, pink, salmon and white. They are a perennial that is quit hardy and immune to black walnuts. The plant also thrives in the shade.
European Wild Ginger
European wild ginger is a wonderfully hardy, creeping ground cover that is an evergreen. It thrives in shady conditions. It grows from rhizomes and spreads quite fast.
The canterbury bellflower produces lovely bell shaped flowers on spirals. The blossoms can be pink, white, purple, blue, lavender and rose. The plant can easily grow four feet in height. Canterbury bells are hardy and will flourish beside black walnuts. They do like full sun so try to plant them in a sunny location.
Summer phlox is a lovely perennial that can grow three feet in height. The plant sports lavender, pink or white globes of flowers. Butterflies enjoy summer phlox. The plant can easily spread two feet.
Lambs ear is a wonderful grown cover that flourishes near black walnuts. The lambs ear produces soft, fuzzy foliage that is almost silver in appearance.
The bloodroot plant has been used in herbal medicine for centuries. The plant is a low growing green foliage plant with a white flower. It is quite hardy.
Sundrops produce bright yellow blossoms. It is a hardy little perennial plant that grows six inches in height.
The Siberian Iris is a deep dark purple flowering plant that is quite hardy. It grows from rhizomes that naturalize easily. The plant stands 36 inches in height.