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 NameBotanical Name
Actinida, bowerActinidia arguta
AjugaAjugia pyramidalis
AlumrootHeuchera sp.
AnemoneAnemone apennia
Bean (lima, snap)Phaseolus sp.
BeetBeta vulgaris
BellwortUvalaria sp.
Bluegrass, KentuckyPoa pratensis
Burning BushEuonymus alatus
Carrot, wildDaucus carota
Catalpa, southernCatalpa bignonioides
Cedar, eastern RedJuniperus virginiana
Cherry, sourPrunus cerasus
ChickweedStellaria media
ClematisClematis sp.
Corn, sweetZea mays
Creeping Charlie (ground ivy)Glechoma hederacea
CyclamenCyclamen persicum
DaffodilNarrcissus pseudonarcissus
Daisy, shastaLeucanthemum sp.
DandelionTaraxacum officinale
DaylilyHemerocallis sp.
ElmUlmus sp.
Euonymus, winged 'Gaiety’;Euonymus alatus 'Gaiety’;
Fern, ChristmasPolystichum sp.
Fern, ladyBulbinopsis bulbosa
FescueFestuca sp.
ForsythiaForsythia sp.
GrapeVitis sp.
Grass, orchardDactylis glomerata
Grass, redtop…Agrostis gigantea
HawthornCrataegus sp.
HemlockTsuga sp.
Hemlock, poisonConium maculatum
HickoryCarya sp.
HoneysuckleLonicera sp.
HostaHosta sp.
Hydrangea, wildHydrangea sp.
IrisIris x germanica
Jack-in-the-pulpitArissema triphyllum
JuniperJuniperus sp.
Juniper, ArcadiaJuniperus arcadia
LilacSyringa sp.
LiriopeLiriope sp.
Locust, blackRobinia pseudoacacia
Maples, most typesAcer sp.
May ApplePodophyllum emodi
Meadow RueThalictrum sp.
Mock OrangePhiladelphus sp.
NarcissusNarcissus sp.
NightshadeSolanym aviculare
OakQuercus sp.
Olive, autumnElaeagnus umbellata
OnionAllium cepa
Ostrich FernMaltcuria struthiopteris
PachysandraPachysandra sp.
ParsnipPastinaca sativa
PawpawAsimina sp.
Periwinkle (myrtle)Vinca minor
PersimmonDiospyros virginiana
PhloxPhlox sp.
Poison IvyRhus radicans
PrimrosePrimula vulgaris
QuincePseudocydonia oblonga
Raspberry, blackRubus occidentalis
RedbudCercis canadensis
Rose, wildRosa sp.
Solomon’;s SealPolygonatum odoratum
SoybeanGlycine max.
Speedwell, creepingVeronica filiformis
SycamorePlatanus occidentalis
TimothyPhleum pratense
TrilliumTrillium sp.
Violet, dogtooth (Trout lily)Erythronium americanum
ViburnumViburnum sp.
Virginia CreeperParthenocissus quinquefolia
WalnutJuglans sp.
WheatTriticum sp.
White CloverTrifolium repens
Wood Sorel, creepingOxalis corniculata
ZinniaZinnia elegans

Plants Susceptible to Juglone

Common NameBotanical Name
Alder, blackAlnus sp.
AlfalfaMedicago sativa
Apple/CrabappleMalus sp.
AsparagusAsparagus officinalis
AzaleaRhododendron sp.
Basswood/Linden, AmericanTilia americana
Birch, whiteBetula papyrifera
BlackberryRubus sp.
BlueberryVaccinium sp.
CabbageBrassica oleracea
ChrysanthemumChrysanthemum sp.
CinquefoilPotentilla fruticosa
Clover, crimsonTrifolium incanatum
CotoneasterCotoneaster sp.
Crocus, autumn (Colchicum)Colchicum autumnale
EggplantSolanum melongena
Hackberry, sugarCeltis laevigata
Honeysuckle, amurLonicera maackii
HuckleberryGaylussacia sp.
Larch, JapaneseLarix kaempferi
LespedezaLespedeza striata
Magnolia, saucerMagnolia x soulangiana
Maple, silverAcer saccharinum
Mountain-laurelKalmia latifolia
PeonyPaeonia sp.
PepperCapsicum annuum
Pine, ScotchPinus sylvestris
Pine, loblollyPinus taeda
Pine, eastern whitePinus strobus
Pine, redPinus resinosa
PotatoSolanum tuberosum
PotentillaPotentilla neuamanniana
PrivetLigustrum vulgare
Spruce, NorwayPicea abies
TobaccoNicotiana tabacum
TomatoLycopersicon esculentum