If you have one or more black walnut trees in your yard, you may have noticed that some plants have difficulty surviving near the trees.
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The roots of black walnut trees produce a toxic substance called juglone which adversely affects plants that are sensitive to it. Butternut trees are relatives of black walnut trees, and they too produce juglone. Many plants are highly sensitive to juglone, but there are some that will tolerate it.
Plants that cannot tolerate juglone will show symptoms such as yellowing, wilting foliage. Juglone acts as a respiration inhibitor, sapping a plant's energy and leaving it unable to breathe. Plants that cannot tolerate juglone will eventually give up and die. Plants that are extremely sensitive to juglone may fail in as little as two weeks.
Although juglone is produced in the trees, roots, all parts of a black walnut tree contain the toxin, with the strongest concentration in the buds and nut hulls. Black walnut trees have a habit of dropping leaves, nuts and twigs from late summer through autumn and this debris adds to the juglone levels in the soil beneath the trees. Rain dripping from the leaves also adds juglone to the soil, making the entire drip zone beneath the tree a hazardous environment for juglone-sensitive plants.
Cutting down the offending tree won't solve the problem, as the roots will continue to release juglone into the soil, making the area toxic for several years after the tree is gone. Debris from black walnut trees should not be added to compost, nor should the wood or bark be used for mulch.
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English walnut trees produce a small amount of juglone, but usually not so much that it is toxic to surrounding plants. Carpathian walnut trees, however, are sometimes grafted onto the rootstock of either black walnut or butternut trees. If a Carpathian walnut tree is grafted onto a juglone-producing tree, the Carpathian walnut will also produce toxic juglone.
The good news is that not all plants are sensitive to juglone. Many trees, shrubs, flowers and vegetables will grow near a black walnut tree, although even some of the juglone-resistant plants will still struggle if they are directly beneath the tree.
Plants which are extremely sensitive to juglone won't thrive within fifty feet of the drip line of black walnut trees. These include hydrangeas, silver maple, white birches, apple trees, Norway spruce, Mugo pine, mountain laurels, most azaleas, lilacs, blueberries and blackberries, cabbage and broccoli, peppers, tomatoes, potatoes and rhubarb.
These are some of the plants that will tolerate juglone to some extent: Eastern redbud, hickory and oak trees, red cedar, Southern catalpa, peach, cherry, plum and pear trees, Rose of Sharon, Thuja arborvitae, Euonymous, Virginia Creeper and most viburnums.
Juglone-tolerant vegetable plants include squash, melons, beets, corn, carrots, onions and parsnips. Some of the juglone-olerant flowering plants include calendula, morning glory, zinnias, hollyhocks, iris, ferns, most daffodils and narcissus, astilbe, crocus, snowdrops, Jack-in-the-pulpit, bloodroot, spiderwort, coral bells, monarda, and some hostas.
If you like to garden, black walnut trees would not be a good choice as landscape trees. But if the trees already exist on your property, you can still garden if you take a bit of extra care when choosing and placing plants for your landscape.