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Black Walnut Toxicity

Find more information and trees and plants tolerant of black walnut toxicity.

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Black Walnut Toxicity

Black walnut (Juglans nigra) is considered one of our most valuable native hardwood lumber trees and is often used in large-scale landscapes. In the smaller-scale home landscape, however, the leaves and fruits are considered by some to be a messy nuisance. Furthermore, while many plants can grow well in proximity to a black walnut, certain plant species’ growth is inhibited by this tree. The term “allelopathy” refers to the relationship between plants in which one plant produces a substance that inhibits the growth of sensitive plants nearby.

Source of Toxicity

Black walnuts produce a chemical called juglone, which occurs naturally in all parts of the tree, especially in the buds, nut hulls, and roots. The leaves and stems contain smaller quantities of juglone, which is leached into the soil after they fall. The highest concentration of juglone occurs in the soil directly under the tree’s canopy, but highly sensitive plants may exhibit toxicity symptoms beyond the canopy drip line. Because decaying roots can release juglone, toxicity may occur for several years after a tree has been removed.

Other trees closely related to black walnut, such as butternut, pecan, shagbark hickory, and English walnut also produce juglone, but at concentrations lower than black walnut. Rarely do these trees affect juglone-sensitive plants.

Symptoms

Most toxicity symptoms arise when juglone-sensitive plants are placed within the walnut’s root zone, an average of 50 to 60 feet from the trunk of a large tree. Plants sensitive to juglone show signs of wilting, yellow leaves, and stunted or slow growth. They eventually die. Many highly sensitive plants cannot tolerate even a small concentration of juglone and die within a few months. Unless one is aware of the toxicity problem, it is easy to blame these symptoms on other disease or nutritional problems. Unfortunately, there is no cure once plants are affected.

Plants Sensitive to Black Walnut Toxicity

Annuals and vegetables: asparagus, cabbage, eggplant, flowering tobacco, pepper, petunia, potato, tomato

Herbaceous perennials: autumn crocus, baptisia, columbine, lily (Asian hybrids), peony, rhubarb

Shrubs: blueberry, red chokeberry, cotoneaster, Amur honeysuckle, hydrangea, lilac, privet, potentilla, rhododendron, yew, and some viburnum shrub species

Trees: European alder, white birch, crabapple species, hackberry, larch, linden, saucer magnolia, mugo pine, red pine, white pine, Norway spruce, silver maple, and some viburnum tree species

Control

Juglone-sensitive landscape plants should be located away from black walnut trees to avoid damage. Raised beds near trees will help minimize toxicity, but care should be taken to keep leaves, twigs, branches, and nuts out of the bed. Black walnut leaves, bark, and wood chips should not be used as landscape mulch or composted on garden plants. The best alternative is to choose plants from the list below. Be aware that some plants are aggressive or invasive in certain parts of the country.

Trees Tolerant of Black Walnut Toxicity

Listed by common name (Scientific name)

Boxelder (Acer negundo)

Japanese maple (Acer palmatum and cultivars)

Red maple (Acer rubrum)

Sugar maple (Acer saccharum)

Ohio buckeye (Aesculus glabra)

Yellow buckeye (Aesculus octandra)

Serviceberry (Amelanchier spp.)

Pawpaw (Asimina triloba)

Sweet birch (Betula lenta)

Yellow birch (Betula lutea)

River birch (Betula nigra)

American hornbeam (Carpinus caroliniana)

Hickory (Carya spp.)

American chestnut (Castanea dentata)

Southern catalpa (Catalpa bignonioides)

Redbud (Cercis canadensis)

Fringe tree (Chionanthus spp.)

Flowering dogwood (Cornus florida)

Hawthorn (Crataegus spp.)

Persimmon (Diospyros virginiana)

American beech (Fagus grandifolia)

Honey-locust (Gleditsia triacanthos)

Silverbell (Halesia carolina)

Witch-hazel (Hamamelis spp.)

Sweet-gum (Liquidambar styraciflua)

Tulip tree (Liriodendron tulipifera)

Cucumbertree (Magnolia acuminata)

Tupelo (Nyssa sylvatica)

Sycamore (Platanus occidentalis)

Wild plum (Prunus americana)

Pin cherry (Prunus pensylvanica)

Black cherry (Prunus serotina)

White oak (Quercus alba)

Scarlet oak (Quercus coccinea)

Shingle oak (Quercus imbricaria)

Northern red oak (Quercus rubra)

Black oak (Quercus velutina)

Staghorn sumac (Rhus typhina)

Black locust (Robinia pseudoacacia)

Willow (Salix spp.)

Sassafras (Sassafras albidum)

American elm (Ulmus americana)

Slippery elm (Ulmus rubra)

 

Shrubs Tolerant of Black Walnut Toxicity

Listed by common name (Scientific name)

Devil’s walking stick (Aralia spinosa)

New Jersey tea (Ceanothus americanus)

Fringe tree (Chionanthus virginicus)

Pagoda dogwood (Cornus alternifolia)

Silky dogwood (Cornus amomum)

American hazelnut (Corylus americana)

February daphne (Daphne mezereum)

Forsythia (Forsythia spp.)

Rose-of-Sharon (Hibiscus syriacus)

Wild hydrangea (Hydrangea arborescens)

Shrubby St. John’s wort (Hypericum prolificum)

Juniper (Juniperus spp.)

Beauty bush (Kolkwitzia amabilis)

Spicebush (Lindera benzoin)

Mock-orange (Philadelphus spp.)

Ninebark (Physocarpus opulifolius)

Exbury rhododendron (Rhododendron hybrids)

Fragrant sumac (Rhus aromatica)

Shining sumac (Rhus copallina)

Smooth sumac (Rhus glabra)

Currant (Ribes spp.)

Wild rose (Rosa spp.)

Black raspberry (Rubus occidentalis)

Purple-flowering raspberry (Rubus odoratus)

Elderberry (Sambucus spp.)

American bladdernut (Staphylea trifolia)

Maple-leaved viburnum (Viburnum acerifolium)

Korean spice viburnum (Viburnum carlesii and cultivars)

Southern arrowwood (Viburnum dentatum)

Black-haw (Viburnum prunifolium)

Yucca (Yucca spp.)

Prickly-ash (Zanthoxylum americanum)

Evergreens Tolerant of Black Walnut Toxicity

Listed by common name (Scientific name)

Chinese juniper (Juniperus chinensis)

Common juniper (Juniperus communis)

Eastern red cedar (Juniperus virginiana)

Arborvitae (Thuja spp.)

Eastern hemlock (Tsuga canadensis)

Vines Tolerant of Black Walnut Toxicity

Listed by common name (Scientific name)

Dutchman’s pipe (Aristolochia durior)

Bittersweet (Celastrus spp.)

Clematis (Clematis ssp.)

Honeysuckle vine (Lonicera spp.)

Virginia creeper (Parthenocissus spp.)

Greenbriar (Smilax spp.)

Wild grape (Vitis spp.)

Wisteria (Wisteria spp.)

Herbaceous Perennials, Spring Wildflowers, and Bulbs Tolerant of Black Walnut Toxicity

Listed by common name (Scientific name)

Yarrow (Achillea spp.)

Bugleweed (Ajuga spp.)

Hollyhock (Alcea rosea)

Windflower (Anemone spp.)

Jack-in-the-pulpit (Arisaema triphllyum)

Wild ginger (Asarum spp.)

Aster (Aster spp.)

Astilbe (Astilbe spp.)

Lady fern (Athyrium spp.)

Rattlesnake fern (Botrychium spp.)

Bellflower (Campanula spp.)

Chrysanthemum (Chrysanthemum spp.)

Spring beauty (Claytonia spp.)

Fragile fern (Cystopteris fragilis)

Toothwort (Dentaria spp.)

Bleeding heart (Lamprocapnos spctabilis)

Leopard’s bane (Doronicum spp.)

Wood fern (Dryopteris spp.)

Purple coneflower (Echinacea purpurea)

Epimedium (Epimedium spp.)

Winter aconite (Eranthis hyemalis)

Dog’s tooth violet (Erythronium spp.)

Joe-pye weed (Eupatorium spp.)

Snowdrop (Galanthus nivalis)

Sweet woodruff (Galium odoratum)

Gentian (Gentiana spp.)

Cranesbill (Geranium spp.)

Perennial sunflower (Helianthus spp.)

Hellebore (Helleborus spp.)

Daylily (Hemerocallis spp.)

Coral bell (Heuchera spp.)

Hosta (Hosta spp.)

Virginia waterleaf (Hydrophyllum virginianum)

Siberian iris (Iris sibirica)

Bush-clover (Lespedeza spp.)

Lilyturf (Liriope spp.)

Lobelia (Lobelia spp.)

Peppermint (Mentha piperita)

Bee balm (Monarda spp.)

Grape hyacinth (Muscari spp.)

Daffodil selected (Narcissus spp.)

Evening primrose (Oenothera spp.)

Sensitive fern (Onoclea sensibilis)

Cinnamon fern (Osmunda cinnamomea)

Summer phlox (Phlox paniculata)

False dragonhead (Physostegia spp.)

Mayapple (Podophyllum peltatum)

Jacob’s ladder (Polemonium reptans)

Solomon’s seal (Polygonatum spp.)

Christmas fern (Polystichum spp.)

Primrose (Primula spp.)

Lungwort (Pulmonaria spp.)

Buttercup (Ranunculus spp.)

Black-eyed Susan (Rudbeckia spp.)

Bloodroot (Sanguinaria canadensis)

Siberian squill (Scilla sibirica)

Stonecrop (Sedum spp.)

Goldenrod (Solidago spp.)

Lamb’s ears (Stachys byzantina)

Meadow rue (Thalictrum spp.)

Spiderwort (Tradescantia virginiana)

Trillium (Trillium spp.)

Tulip selected (Tulipa spp.)

Bellwort (Uvularia spp.)

Speedwell (Veronica spp.)

Violet (Viola spp.)