Black walnut trees can be toxic to other plants

By Marilyn McKinley

OSU Extension master gardener volunteer

Black walnut is a valuable tree and an Ohio native.

Though many plants grow well in proximity to a black walnut, the tree is known to hinder the growth of others. This is due to allelopathy, a relationship between plants in which one plant produces a substance that affects the growth of another plant. In black walnuts, that substance is juglone.

Plants adversely affected will have yellowing foliage, wilting and eventually, will die.

Juglone appears in all parts of the plant. Higher concentrations are in the buds, nut hulls and roots.

Its effect ranges from 50 to 80 feet away from the tree, with the area under the dripline being the most toxic. The toxic effects can remain in the soil long after the tree has been removed.

Other trees closely related to the black walnut also produce juglone, but not enough to cause toxicity. These trees include English walnut, pecan, shagbark hickory and butternut hickory.

Here’s a partial list of plants that are sensitive: Asparagus, cabbage, eggplant, tomato, potato, rhubarb, apple, blackberry, blueberry, pear, azalea, white birch, red chokeberry, crabapple, hackberry, hydrangea, larch, lilac, silver maple, Scotch pine, white pine, rhododendron, viburnum, yew, baptisia, some peony, columbine, chrysanthemum, lily and petunia.

Plants that appear to tolerate juglone are: lima beans, snap beans, beets, carrots, corn, melons, onions, squash, black raspberry, cherry, arborvitae, catalpa, clematis, forsythia, honeysuckle, Japanese maple, rose of Sharon, astilbe, bee balm, begonia, bluegrass, blue bells, crocus, shasta daisy, daylily, ferns, wild ginger, hollyhock, hyacinth, lamb’s ear, lungwort, morning glory, most narcissus, pansy, phlox, marigold, tulip and zinnia.

If you plan to grow plants that are sensitive, your garden should be located away from any black walnut trees. Even raised beds must be situated to minimize root penetration. Care must be taken to keep any garden free of black walnut leaves and hulls. Excellent drainage can be of great help.

For humans who are sensitive, the walnut pollen is most problematic when it appears in mid-spring.

The hull of the black walnut leaves a yellow stain on your hands, but generally is not a cause for worry.

Dogs may be affected, possibly having gastritis or a bowel obstruction from eating a walnut. There is concern about dogs eating old, moldy walnuts in the yard, so rake and dispose of them if you have pets.