You may find your plantings withering and dying beneath the canopy of your black walnut tree. Not just any plant can survive the toxicity of the black walnut. For some reason, Kentucky bluegrass and black raspberries thrive in the atmosphere created by this tree, while others may not fare so well and many will be killed by it.
The roots of the black walnut tree produce juglone, which creates toxic conditions in the soil that can be deadly to some species of plants. Some plants that are sensitive to juglone yellow, wither and die if they are even in the proximity of the tree's root system. If you have a black walnut tree in your landscaping, it would be best to learn what can and cannot be planted under it to save yourself time and money.
Trees and Shrubs
Unfortunately, there is no guarantee that they will do well, but several trees and shrubs have grown under or near a black walnut tree. The trees and shrubs found surviving near a black walnut tree include the Japanese maple, redbud, rose of Sharon, aborvitae, clematis, daphne, euonymous, forsythia, hawthorn, hemlock, red and sugar maples, scarlet and red oaks, sycamore, most viburnums and the Virginia creeper.
Annuals and Perennials
Several annuals and perennials have been found living in the proximity of the black walnut tree. Hopefully, you will have good luck in planting them near your tree. They include the pot marigold, begonia, morning glory, pansy, zinnia, balm, bellflower, wild bergamot, bloodroot, some bluebells, bugleweed, coral bells, cranesbill, crocus, daylily, Dutchman's breeches, some ferns, wild ginger, glory-of-the-snow, most grasses, grape-hyacinth, orange hawkweed, herb Robert, many hostas, hollyhock, Jack-in-the-pulpit, Jacob's ladder, Jerusalem artichoke, lamb's-ear, leopard's-bane, lungwort, mayapple, merrybells, phlox, polyanthus primrose, snowdrop, Solomon's-seal, spring beauty, spiderwort, Siberian squill, stonecrop, sundrop, sweet woodruff, sweet Cicely, trillium, many violets, Virginia waterleaf and winter aconite. Vegetables that have survived beneath the black walnut are beans, carrots, corn, squash and melons.
To determine where not to put plants that may be sensitive to the toxicity of your black walnut tree, assume a 60 to 80 foot radius from the trunk of your mature tree. Young trees up to 8 feet tall have a root radius of twice the height of the tree. The toxic radius grows as the tree grows. Even some plants that are reportedly resistant to the effects of juglone cannot be planted too close to your black walnut tree.
If you remove your black walnut tree, the remaining roots will keep the ground toxic for some time afterward. Similarly, if you use mulch made from the bark of a black walnut tree, allow it to compost for 6 months before using it on plants sensitive to juglone.
Several plants are known to be sensitive to the toxicity of your black walnut tree and should not be planted under or near it. These plants include columbine, asparagus, lilies, alfalfa, rhubarb, silver maples, white birches, apples, crabapples, Norway spruces, several pines, lilacs, yews, blueberries, blackberries, honeysuckle, rhododendrons and azaleas. Vegetables that will suffer under a black walnut tree include cabbage, peppers, tomatoes, eggplant and potatoes.