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
|