Black walnuts have a long history of cultivation on American farms and, to a lesser extent, in gardens all over the East and Midwest. They are beautiful trees, with upright trunks and wide canopies. They bear delicious (though hard to crack) nuts, and they are very valuable as timber trees — a single straight trunk can be worth thousands of dollars.
Unfortunately, black walnuts are not completely garden-friendly. The extensive surface roots compete with anything planted over them, the thick leaf cover produces dense shade, and all parts of the tree exude a chemical called juglone, which inhibits the growth of a great many plants.
Vegetables in the solanaceous group — tomatoes, peppers, eggplants, and potatoes — seem particularly sensitive to joglone, as are many perennials, including columbines, peonies, and chrysanthemums. Hydrangeas appear to hate juglone. So do rhododendrons and azaleas, lilacs, and lilies. Ditto pines and birches, apples, and blueberries.
Because juglone-laden feeder roots extend far beyond the tree’s canopy, and juglone-containing leaves and nut hulls also tend to get spread around, none of the plants listed above is likely to do well unless it’s at least 50 feet away from the drip line of a black walnut and 100 or more feet away from the trunk.
So what’s left? Quite a lot, really. Kentucky bluegrass and black raspberries actually seem to thrive when planted near (but not under) black walnuts. If the soil drainage is good and other growing conditions are right, gardeners have also had success with cucurbits (squash, melon, and cucumber), as well as beans, beets, and carrots.
The list of possible flowers is longer, starting with spring-flowering bulbs (except crocus) and woodland wildflowers such as Jack-in-the-pulpit, sweet woodruff, and cranesbill. Many hostas appear to be up to the challenge, and asters, day lilies, coralbells, and Siberian iris are also likely bets. Euonymous usually does o.k., and so do Japanese maples, viburnums, redbuds, and hemlocks.
One warning: As the recurrence of words like “seems” and “possible” suggests, these pros and cons are based on observations, not controlled experiments. And to complicate matters further, the amount of juglone in the soil can vary considerably, depending on such factors as soil type, drainage patterns, soil microorganisms, and the age of the tree.
So, if there’s something you’re dying to try under a black walnut, try it. But watch carefully. Affected plants will quickly show signs of stress and should be moved before they start dying, too.