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Most members of the Walnut family (Juglandaceae) produce a chemical called "juglone" (5 hydroxy-alphanapthoquinone) which occurs naturally in all parts of these plants. Black walnut, pecan, hickory and others members of the family including Carya, Engelhardtia, Juglans, Platycarya and Pterocarya can produce juglone.
Black walnut and butternut produce the largest quantity of juglone and can cause toxic reactions with a number of other plant species that grow in their vicinity. Other juglone-producing species including English walnut, pecan, shellbark/shagbark/bitternut hickory, produce such small quantities of juglone that toxic reactions in other plants are rarely observed. Specific named or numbered cultivars of English walnuts and Japanese 'heartnut' walnuts that are used in commercial orchards or in landscapes are often grafted onto rootstock of native black walnut.
While many plants are tolerant to juglone and grow well in close proximity to walnut trees, there are certain susceptible plant species whose growth can be affected by walnut trees. Through observation and experience, many plant species have been classified as either 'susceptible' or 'tolerant' to walnut family members. 'Allelopathy' is a term used to describe natural interactions between plants where one plant produces a substance that affects the growth of another plant.
Experimentally, juglone has been shown to be a respiration inhibitor, which deprives sensitive plants of needed energy to enable metabolic activity. Affected plants cannot exchange carbon dioxide and oxygen properly. In affected tomatoes, xylem vessels become plugged by callus tissue, blocking upward movement of water in the plant.
Symptoms of walnut toxicity range from stunting of growth, to partial or total wilting, to death of the affected plant. The toxic reaction often occurs quickly where sensitive plants can go from healthy to dead within one or two days. Many alarmed gardeners often believe the cause of wilting is due to fungal or bacterial disease. Once wilting begins, the effect cannot be reversed. The severity of the toxic symptoms can vary depending on the plant species that is in contact with the juglone.
Walnut roots can be identified as having fairly thick bark with inner wood that quickly turns dark yellow when the bark is removed. There is also a distinctive, pungent walnut odor from the cut root. Juglone from decomposing black walnut roots can persist in the soil for more than a year after walnut trees have been removed. Walnut roots may extend 50 to 80 feet away from the outer canopy of mature walnut trees. Young walnut trees do not appear to cause toxic reactions with sensitive plants until the trees are seven to eight years old.
Raked up leaves, twigs and husks from walnut trees should be composted for one year to ensure all juglone has broken down prior to spreading into gardens or used as mulch around sensitive plants.
Gardens should be located away from black walnut and butternut trees to prevent damage to susceptible plants. Where close proximity is unavoidable (a neighbors yard) then raised garden beds can provide some protection from juglone toxicity. Care must be taken to minimize or prevent walnut tree roots from growing upwards into the raised beds. Underlying a garden with plastic or fabric weed barrier during construction can prevent tree roots from growing into raised beds.
Excellent soil drainage will also help reduce toxicity problems, even among sensitive plant species. In well-drained soil, toxic reactions only occur when direct contact is made between walnut roots and roots of sensitive species. In poorly drained soil direct contact between roots is not necessary to cause toxic reactions since juglone moves through the soil water. It has been suggested that plants having shallow root systems are more tolerant of juglone than deep-rooted species. Tolerance to juglone by shallow-rooted species may also be attributed to better drainage of soil water in upper soil levels.
Horses and ponies can contract acute laminitis, an inflammation of the foot, where black walnut wood chips or sawdust is used for stall bedding. Acute laminitis and high respiratory rates in horses and ponies has also been reported where stables and paddocks are located too close to walnut trees. Pollen shedding from walnut trees can cause allergic reactions in people and horses.
Husks of fallen walnuts can become toxic to livestock, and lethal to dogs if ingested due to a mycotoxin called 'Penitrim A', which is produced by Penicillium mould. Therefore, walnut nuts showing symptoms of decomposition, such as a brown or black rotten appearance in the husks, may leak toxin into the kernels and are not fit for human consumption.
The following tables list plant species that are known to be tolerant and susceptible to juglone.
Plants Tolerant of Juglone
Plants Susceptible to Juglone
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