Black walnut trees provide a special challenge for landscapers, because the roots of the trees secrete a substance called juglone that is toxic to fruits, vegetables, ornamental plants and many other species of plant life. Plants that are susceptible to juglone will gradually weaken and die over a space of a few months once they are exposed to the substance in the soil. Additionally, walnut trees block sunlight, which prevents sun-loving plants from thriving beneath the tree. Gardeners must research each plant thoroughly for light tolerance and juglone resistance before planting it beneath a black walnut tree.
Determine which plants are resistant to juglone toxins in the soil. Examples of plants that are resistant to juglone include Japanese maple, eastern redbud, rose of Sharon, Virginia creeper, morning glories, peach trees, hollyhocks, crocus, snowdrops, lambs ear and summer phlox.
Choose plants that thrive in partial to deep shade. Good plants for deep shade beneath a walnut tree include hostas, cinnamon fern and bleeding heart.
Dig shallowly beneath a black walnut tree with a garden fork to avoid damaging tree roots. Most tree roots are located in the top 24 inches of soil around a tree. Shift the planting hole of your plants if you encounter a large feeder root. Place the plant's root ball in the planting hole, and fill in around the roots of the plant with soil.
Water around the roots of landscaping plants frequently. Large trees such as walnut may steal most of the water from the soil. Watering around landscaping plants will prevent them from drying during drought conditions.
Sprinkle a balanced, granulated fertilizer around your landscaping plants once in spring and twice more during the growing season. This will help the plants thrive despite competition from walnut tree roots.
Mulch shallowly around the plants to prevent weeds from competing for moisture and fertilizer. Weeds such as pokeweed grow well in the shade as well as sunlight. Use only 2 inches of bark mulch over the roots of the tree to avoid smothering them.