The question I’ve been asked most often since I became a Master Gardener is “Why won’t anything grow near my black walnut tree?”
Black walnut trees release a chemical called juglone that can damage plants growing near them. Leaves yellow and wilt and sometimes the entire plant dies. Juglone is released from walnut trees through roots, through leaves that drop to the ground, through husks of nuts, and even from rain drops that drip through the tree. If your neighbor has a black walnut and the squirrels use your yard for a burial ground, that may explain why your lilies-of-the-valley aren’t doing so well.
Juglone is not very soluble in water so it doesn’t spread very far in soil. The edge of the tree’s crown, called the dripline, often has the most juglone. But the roots of a black walnut tree often reach twice as far as the dripline. In general, the toxic zone around a mature walnut tree is within 50-60 feet of its trunk.
The good news is that there are some plants that are more tolerant of juglone than others. These plants can be grown near black walnuts: American arborvitae, ash, barberry, American beech, black birch ‘Heritage’, river birch, boxelder, Ohio buckeye, catalpa, black cherry, crabapple, daphne, flowering dogwood, elderberry, American elm, forsythia, fringetree, hawthorn, hibiscus, hickory, honeylocust, hydrangea, lilac, many maples, ninebark, oaks, callery pear, eastern red cedar, privet, serviceberry, Norway spruce, sumac, some viburnums, aster, astilbe, bee balm, begonia, calendula, clematis, coral bells, crocus, Virginia creeper, daffodils, Shasta daisies, daylilies, ferns, hardy geraniums, hollyhocks, hosta, morning glory, phlox, rudbeckia, sedum, sunflower, tulips, violets, wild grapes and yarrow.
The following plants are very sensitive to juglone and should be grown away from black walnut trees: tomato, potato, pepper, eggplant, rhubarb, asparagus, apple, pear, strawberry, domestic grape, pine, some birches, hackberry, silver maple, saucer magnolia, linden, pine, lilac, potentilla, azalea, rhododendron, lily, columbine, chrysanthemum, forget-me-not and lily of the valley.
There are many websites with comprehensive lists of both juglone-tolerant and juglone-sensitive plants. Search under “black walnuts juglone” if you have a question about a particular plant.
Of course, the best way to prevent this problem is not to plant a black walnut tree in your yard, especially if it is a small lot where it can damage gardens and nearby landscape plants. If you already have a black walnut tree on your property, grow the more sensitive plants away from the tree. Do not use any part of the walnut tree as compost or mulch for other plants. Cutting down the tree won’t solve the problem immediately either. The juglone will persist until the roots are dead and decomposed, which can take five or more years.
Black walnuts aren’t the only trees that create juglone. Butternut, English walnut, shagbark hickory, sugar maple, tree-of-heaven, hackberry, American sycamore, cottonwood, black cherry, red oak, black locust and American elm also produce juglone, but fortunately, in relatively small amounts.
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