Black walnut trees, along with many trees in that family including hickory and pecan, produce a chemical toxin called juglone. Juglone will kill most plants placed near a black walnut tree or even where a black walnut tree once stood. The toxin is carried primarily in the roots, buds and nut hulls but can also persist in the soil many years after a walnut tree has died or been removed. Few plants are immune to the effects of juglone and so planting under or near the trees requires thoughtful preparation and careful plant selection.
Select one or more of the following plant species known to be tolerant of juglone to use in planting near or under your black walnut tree: Arborvitae, catalpa, clematis, crabapple, daphne, euonymous, forsythias, hawthorn, hemlock, hickory, honeysuckle, junipers, black locust, Japanese maple, pachysandra, pawpaw, persimmon, redbud, rose of Sharon, wild rose, viburnum or Virginia creeper.
Place large tree specimens outside of the black walnut tree canopy so that there is not crowding of the canopies over time. Proper spacing will also allow for sunlight and fresh air flow to reach the trees planted around the black walnut.
Underplant small shrubs and creeping or vining plants under the canopy of the tree starting roughly two to three feet out from the trunk.and extending to past the drip line of the tree or as far as desired to create a blanket effect around the tree. Space the plants according to their mature size to ensure there is no overcrowding and the soil will not need to be disturbed frequently by removal of overgrown plants.