Walnut Toxicity

Introduction

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.

Other Problems

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.

Plant Susceptibility

The following tables list plant species that are known to be tolerant and susceptible to juglone.

Plants Tolerant of Juglone

Common Name	Botanical Name
Actinida, bower	Actinidia arguta
Ajuga	Ajugia pyramidalis
Alumroot	Heuchera sp.
Anemone	Anemone apennia
Bean (lima, snap)	Phaseolus sp.
Beet	Beta vulgaris
Bellwort	Uvalaria sp.
Bluegrass, Kentucky	Poa pratensis
Burning Bush	Euonymus alatus
Carrot, wild	Daucus carota
Catalpa, southern	Catalpa bignonioides
Cedar, eastern Red	Juniperus virginiana
Cherry, sour	Prunus cerasus
Chickweed	Stellaria media
Clematis	Clematis sp.
Corn, sweet	Zea mays
Creeping Charlie (ground ivy)	Glechoma hederacea
Cyclamen	Cyclamen persicum
Daffodil	Narrcissus pseudonarcissus
Daisy, shasta	Leucanthemum sp.
Dandelion	Taraxacum officinale
Daylily	Hemerocallis sp.
Elm	Ulmus sp.
Euonymus, winged ‘Gaiety’	Euonymus alatus ‘Gaiety’
Fern, Christmas	Polystichum sp.
Fern, lady	Bulbinopsis bulbosa
Fescue	Festuca sp.
Forsythia	Forsythia sp.
Grape	Vitis sp.
Grass, orchard	Dactylis glomerata
Grass, redtop…	Agrostis gigantea
Hawthorn	Crataegus sp.
Hemlock	Tsuga sp.
Hemlock, poison	Conium maculatum
Hickory	Carya sp.
Honeysuckle	Lonicera sp.
Hosta	Hosta sp.
Hydrangea, wild	Hydrangea sp.
Iris	Iris x germanica
Jack-in-the-pulpit	Arissema triphyllum
Juniper	Juniperus sp.
Juniper, Arcadia	Juniperus arcadia
Lilac	Syringa sp.
Liriope	Liriope sp.
Locust, black	Robinia pseudoacacia
Maples, most types	Acer sp.
May Apple	Podophyllum emodi
Meadow Rue	Thalictrum sp.
Mock Orange	Philadelphus sp.
Narcissus	Narcissus sp.
Nightshade	Solanym aviculare
Oak	Quercus sp.
Olive, autumn	Elaeagnus umbellata
Onion	Allium cepa
Ostrich Fern	Maltcuria struthiopteris
Pachysandra	Pachysandra sp.
Parsnip	Pastinaca sativa
Pawpaw	Asimina sp.
Periwinkle (myrtle)	Vinca minor
Persimmon	Diospyros virginiana
Phlox	Phlox sp.
Poison Ivy	Rhus radicans
Primrose	Primula vulgaris
Quince	Pseudocydonia oblonga
Raspberry, black	Rubus occidentalis
Redbud	Cercis canadensis
Rose, wild	Rosa sp.
Solomon’s Seal	Polygonatum odoratum
Soybean	Glycine max.
Speedwell, creeping	Veronica filiformis
Sycamore	Platanus occidentalis
Timothy	Phleum pratense
Trillium	Trillium sp.
Violet, dogtooth (Trout lily)	Erythronium americanum
Viburnum	Viburnum sp.
Virginia Creeper	Parthenocissus quinquefolia
Walnut	Juglans sp.
Wheat	Triticum sp.
White Clover	Trifolium repens
Wood Sorel, creeping	Oxalis corniculata
Zinnia	Zinnia elegans

Plants Susceptible to Juglone

Common Name	Botanical Name
Alder, black	Alnus sp.
Alfalfa	Medicago sativa
Apple/Crabapple	Malus sp.
Asparagus	Asparagus officinalis
Azalea	Rhododendron sp.
Basswood/Linden, American	Tilia americana
Birch, white	Betula papyrifera
Blackberry	Rubus sp.
Blueberry	Vaccinium sp.
Cabbage	Brassica oleracea
Chrysanthemum	Chrysanthemum sp.
Cinquefoil	Potentilla fruticosa
Clover, crimson	Trifolium incanatum
Cotoneaster	Cotoneaster sp.
Crocus, autumn (Colchicum)	Colchicum autumnale
Eggplant	Solanum melongena
Hackberry, sugar	Celtis laevigata
Honeysuckle, amur	Lonicera maackii
Huckleberry	Gaylussacia sp.
Larch, Japanese	Larix kaempferi
Lespedeza	Lespedeza striata
Magnolia, saucer	Magnolia x soulangiana
Maple, silver	Acer saccharinum
Mountain-laurel	Kalmia latifolia
Peony	Paeonia sp.
Pepper	Capsicum annuum
Pine, Scotch	Pinus sylvestris
Pine, loblolly	Pinus taeda
Pine, eastern white	Pinus strobus
Pine, red	Pinus resinosa
Potato	Solanum tuberosum
Potentilla	Potentilla neuamanniana
Privet	Ligustrum vulgare
Rhododendron	Rhododendron
Spruce, Norway	Picea abies
Tobacco	Nicotian