The black walnut tree (Juglans nigra L.) produces a toxic substance known as juglone (5-hydroxy-alphanapthaquinone) in its roots, buds, leaves, stems and nut shells. Plants and shrubs that are susceptible to the toxicity of the juglone will perish within two months of being planted near the black walnut tree or its roots. An adult tree can produce a juglone toxic zone up to 80 feet around its trunk. A few plants can successfully grow under and near the root system of the walnut tree.
The annual zinnia grows successfully near and under the black walnut tree. The zinnia grows 1 to 3 feet tall and is available in a wide range of colors. There are over a hundred zinnia cultivars available according to online plant encyclopedia Floridata.The plants prefer full sun with well-draining soil. They will tolerate partial shade. Zinnias requiring late-spring planting after all danger of frost has passed. Sow seeds indoors for an early start. Zinnias produce colorful blossoms throughout the summer and can withstand hot weather well with ample water.
The lamb's ear (Stachys byzantina) plant produces short furry silver leaves. The plant is unaffected when planted beside the black walnut. It grows approximately 6 to 12 inches in height. Place in full sun or partial shade for best growth results. The lamb's ear produces long flower spikes with barely noticeable flowers. It is valued for its foliage and not its blossoms. The plant can easily become a noxious weed if left unattended. It spreads with ease. Butterflies and bees congregate around the plant when blooming. Plant in well-draining soil. Keep the plant moist, but not overly wet, for best results.
Astilbes (Astilbe xarendsii) thrive under the black walnut tree. There are numerous species and hybrids available. Many are dwarf in size and others are grow several feet in height. A wide range of flower plume colors are available. The plants require constantly moist soil to thrive. Mulch should be applied to help keep the soil moist in the heat of summer. Astilbes are winter hardy and require no extra protection during the cold months.
The Bloodroot (Sanguinaria canadensis) is a perennial plant that thrives under the black walnut. It is native to the United States and widely found in the woody forests of the deep south. The plant produces a very unique sap that appears to be blood-like. The sap of the plant has been widely used as a dye. The plant stands 12 inches in height and produces waxy white flowers. Flowers blossom in the spring and measure 2 inches wide. The plant prefers partial- to full-shade conditions with moderately moist soil.