Nearly all members of the Walnut family produce a natural chemical called juglone, which is responsible for toxic reactions in other plants within the surrounding vicinity. Black walnut and butternut produce the greatest quantity of juglone, which can make it difficult to grow susceptible species nearby.
SYMPTOMS The symptoms of walnut juglone toxicity can include: stunting of growth, full or partial wilting, and/or plant death. The level of toxicity and the specific plant symptoms will depend upon the plant species of interest. Some species are especially sensitive, while others demonstrate some tolerance to juglone and are able to grow successfully in the presence of black walnut or other juglone-producing trees (i.e. butternut, pecan, hickory).
STRATEGIES TO AVOID DAMAGE If possible, planting gardens beneath walnut trees should be avoided if possible, but if proximity to a juglone-producer is unavoidable, there are some methods to reduce the damage to landscape plants. As most of the juglone is found in the roots, buds, and nuts, damage to gardens can be minimized by creating a raised flower bed and/or a physical barrier to prevent root expansion into the proximity of susceptible plants. Also, wood chips and leaf litter from walnut trees should not be used to mulch plants and leaves should be removed from the bed as they fall. Lastly, choosing more tolerant species (often those with shallow roots) and avoiding use of very susceptible species is another approach that can allow for a more successful garden in the presence of black walnut.
TOLERANT SPECIES Note: these lists are a general guide, with native species in bold font o Actinida, bower (Actinidia arguta) o Elm (Ulmus sp.) o Ajuga (Ajugia pyramidalis) o Euonymus, winged ‘Gaiety’ (Euonymus o Bean (lima, snap) (Phaseolus sp.) alatus ‘Gaiety’) o Beet (Beta vulgaris) o Fern, Christmas (Polystichum sp.) o Bellwort (Uvalaria sp.) o Fern, lady (Bulbinopsis bulbosa) o Bluegrass, Kentucky (Poa pratensis) o Fescue (Festuca sp.) o Burning Bush (Euonymus alatus) o Forsythia (Forsythia sp.) o Carrot, wild (Daucus carota) o Grape (Vitis sp.) o Catalpa, southern (Catalpa bignonioides) o Grass, orchard (Dactylis glomerata) o Cedar, eastern red (Juniperus virginiana) o Grass, redtop (Agrostis gigantean) o Cherry, sour (Prunus cerasus) o Hawthorn (Crataegus sp.) o Chickweed (Stellaria media) o Hemlock (Tsuga sp.) o Clematis (Clematis sp.) o Hickory, bitternut (Carya cordiformis.) o Coralbells (Heuchera sp.) o Hosta (Hosta sp.) o Corn, sweet (Zea mays) o Hydrangea, wild (Hydrangea sp.) o Creeping Charlie (ground ivy) (Glechoma o Iris (Iris germanica) hederacea) o Jack-in-the-pulpit (Arissema triphyllum) o Cyclamen (Cyclamen persicum) o Daffodil (Narcissus pseudonarcissus) o Juniper, common (Juniperus communus) o Daisy, shasta (Leucanthemum sp.) o Lilac (Syringa sp.) o Dandelion (Taraxacum officinale) o Liriope (Liriope sp.) o Daylily (Hemerocallis sp.) o Locust, black (Robinia pseudoacacia)
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TOLERANT SPECIES CONTINUED SUSCEPTIBLE o Maple (Acer sp.) SPECIES o May apple (Podophyllum emodi) (very sensitive) o Meadow Rue (Thalictrum sp.) o Mock Orange (Philadephus sp.) o Alder, black (Alnus sp.) o Narcissus (Narcissus sp.) o Alfalfa (Medicago sativa) o Nightshade (Solanym aviculare) o Apple/Crabapple (Malus sp.) o Oak, bur (Quercus macrocarpa) o Asparagus (Asparagus officinalis) o Olive, autumn (Elaeagnus umbellate) o Azalea (Rhododendron sp.) o Onion (Allium cepa) o Basswood/Linden, American (Tilia o Ostrich fern (Maltcuria struthiopteris) americana) o Pachysandra (Pachysandra sp.) o Birch, white (Betula papyrifera) o Parsnip (Pastinaca sativa) o Blackberry (Rubus sp.) o Pawpaw (Asimina sp.) o Blueberry (Vaccinium sp.) o Periwinkle, myrtle (Vinca minor) o Cabbage (Brassica oleracea) o Persimmon (Diospyros virginiana) o Chrysanthemum (Chrysanthemum o Phlox (Phlox sp.) sp.) o Primrose (Primula vulgaris) o Cinquefoil (Potentilla fruticosa) o Quince (Pseudocydonia oblonga) o Clover, crimson (Trifolium incanatum) o Raspberry, black (Rubus occidentalis) o Cotoneaster (Cotoneaster sp.) o Redbud (Cercis canadensis) o Crocus, autumn (Colchicum o Rose, wild (Rosa nitida) autumnale) o Solomon’s Seal (Polygonatum odoratum) o Eggplant (Solanum melongena) o Soybean (Glycine max.) o Hackberry, sugar (Celtis laevigata) o Speedwell, creeping (Veronica filiformis) o Honeysuckle, amur (Lonicera maackii) o Sycamore (Platanus occidentalis) o Huckleberry (Gaylussacia sp.) o Timothy (Phleum pratense) o Larch, Japanese (Larix kaempferi) o Trillium (Trillium sp.) o Lespedeza (Lespedeza striata) o Violet, dogtooth (Erythronium o Magnolia, saucer (Magnolia americanum) soulangiana) o Viburnum (Viburnum sp.) o Maple, silver (Acer saccharinum) o Virginia Creeper (Parthenocissus o Mountain-laurel (Kalmia latifolia) quinquefolia) o Peony (Paeonia sp.) o Wheat (Triticum sp.) o Pepper (Capsicum annuum) o White Clover (Trifolium repens) o Pine, Scotch (Pinus sylvestris) o Wood Sorel, creeping (Oxalis corniculata) o Pine, loblolly (Pinus taeda) o Zinnia (Zinnia elegans) o Pine, eastern white (Pinus strobus) o Pine, red (Pinus resinosa) o Potato (Solanum tuberosum) o Potentilla (Potentilla neumanniana) o Privet (Ligustrum vulgare) o Rhododendron (Rhododendron sp.) o Spruce, Norway (Picea abies) o Tobacco (Nicotiana tabacum) o Tomato (Lycopersicon esculentum)
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