Extension Educator, Horticulture
Yeh, give me some space! Plants have their own way of saying "back off'. Some plants can actually release chemicals that interfere with the growth of other plants near them; kind of their own herbicide. The fancy term is allelopathy and refers to the beneficial or harmful effects of one plant on another plant by the release of chemicals from plant parts such as roots, stems, leaves or fruit. Ever wonder why a particular plant just never grows well. It could that its neighbor is quietly poisoning it.
As we know more about plant growth we are learning how neighboring plants can effect plant growth. According to University of Florida Extension a chemical called ailanthone, isolated from Tree of Heaven, reportedly possesses herbicidal activity similar to glyphosate and paraquat. Glyphostae is the stuff found in Round Up™. A once common garden practice was to follow crops with rye or wheat. We now know both of these chemically suppress weeds when used as cover crops or when crop residues are used as mulch. Broccoli residue interferes with growth of related crops such as cabbage or cauliflower that follow.
Black walnut is the poster child for not being a good neighbor. Black walnut trees naturally contain a chemical called juglone which can inhibit the growth of some plants. The largest amount of juglone is found in the walnut's buds, nut hulls and roots. However, even walnut leaves and stems contain a small concentration of juglone.
Symptoms of black walnut toxicity include leaf yellowing, wilting, stunted growth and eventual death. However, plants can exhibit these same symptoms for other reasons.
Living roots of walnuts release small quantities of juglone. Therefore, plants that are extremely sensitive will not grow well anywhere within the area of tree root growth. On a large tree the roots could extend well beyond the tree's canopy.
The largest quantities of juglone are usually right under the tree's canopy. Some sensitive plants may live near a black walnut, but will not live directly under it.
The amount of juglone present also depends on soil type, drainage and soil microbes. Some species may not survive well because of the competition for light and water under a tree canopy. Even after a walnut tree has been removed, the decomposing leaves and roots will still release some juglone for years.
Gardens should be located away from black walnuts if possible or consider using plants observed as tolerant to juglone. Raised beds may be an option if sensitive plants are desired. Care should be taken to keep beds free of walnut leaves and hulls. Leaves, bark or wood chips should not be used as mulch around sensitive plants. Materials should be composted and aged for at least 6 months before use.
Constructing physical root barriers may also be an option. Toxicity also seems to be minimized by excellent soil drainage.
The following lists of plants sensitive and tolerant to juglone should be used as a guide and not regarded as definitive. Few plants have been researched for tolerance or sensitivity to juglone. Lists are based on observations detailed in published sources.
Plants observed to be sensitive to juglone: vegetables - asparagus, cabbage, eggplant, pepper, potato, tomato; fruit - apple, blackberry, blueberry; landscape plants - azalea, rhododendron, hydrangea, lilac, saucer magnolia, white pine, potentilla, peony, privet and yew.
Plants observed to be tolerant of juglone: vegetables - lima beans, snap beans, beets, corn, onions, parsnips, squash; fruit - cherry, black raspberry; landscape plants - red cedar, crabapple, burning bush, forsythia, hawthorn, pachysandra, redbud, most viburnums and winter creeper.
Perennial plants include astilbe, barrenwort, bergenia, bleeding heart, coralbells, daffodils, daylily, ferns, iris, phlox, shasta daisy and blue squill. Spring wildflowers include bloodroot, rue anemone, European and wild ginger, hepatica, Jack-in-the-pulpit, lungwort, Solomon's seal, trillium and violets.