The Juglans Nigra, more commonly known as the Black Walnut tree, is a fairly valuable hardwood tree. For that reason, it is a popular choice in some larger landscape design plans. For smaller projects, (e.g., your backyard), however, the leaves and fruits of the black walnut tree can seem to be more trouble than than the tree itself is worth. Beyond the black mess that the fruit can scatter on the ground, there is actually a pretty long lost of plants that just don’t grow well near Black Walnut trees. This is because of a condition known as “alleopathy.” In short, alleopathy occurs in plant environments when one plant in the environment produces a chemical that undermines the growth of certain other plants in the area.
What Causes Black Walnut Toxicity?
In a word, “juglone.” This is the chemical that Black Walnut trees produce all throughout the tree, particularly in the roots, buds, and nut hulls. You’ll also find juglone in the leaves and stems, but in smaller amounts. The juglone from the leaves and stems works its way into the soil when the leaves fall. For this reason, juglone occurs in the greatest concentration in the soil below the tree’s canopy. That said, some plants that are particularly sensitive to juglone can still present symptoms even if they are not under the canopy. Simply removing a Black Walnut tree isn’t always an immediate fix, either. After the tree is gone, the decaying roots can still emit juglone. This can present problems for juglone-sensitive plants years after the tree itself is gone.
The Black Walnut tree is not the only tree that produces juglone. You’ll also see it in English Walnut, Shagbark Hickory, Pecan, and Butternut. That’s because these trees are related to the Black Walnut. That said, they don’t produce nearly the same amount of juglone. In fact, the amounts are so little that they rarely, if ever, lead to problems in sensitive plants.
Symptoms of Black Walnut Toxicity
If you suspect Black Walnut toxicity, you’ll first want to examine the proximity of effected plants to the Black Walnut tree. The root zone is going to be the real hot spot. If you have a large Black Walnut tree, the root zone could extend out as far as 60 feet. If your plants are in that zone, symptoms will include, slow growth, yellow leaves, wilting, and finally death. Some plants are so sensitive that even the smallest amount of juglone can be very toxic, leading to death in a matter of just months. These symptoms can parallel that of basic nutrient deficiency, which is why you want to check proximity to your Black Walnut tree(s) first. That said, you can’t treat effected plants with nutrients. In fact, there is no known cure for effected plants.
So Which Plants are Sensitive to Black Walnut Toxicity?
The list of plants that are sensitive is pretty extensive and contains many common plants. We’ve broken it down into categories for you.
Trees: Crabapple Species, European Alder, Hackberry, Larch, Linden, Mugo Pine, Norway Spruce, Red Pine, Saucer Magnolia, Silver Maple, White Birch, White Pine, and some Viburnum tree species.
Shrubs: Amur Honeysuckle, Blueberry, Cotoneaster, Hydrangea, Lilac, Potentilla, Privet, Red Chokeberry, Rhododendron, Yew, and some Viburnum shrub species.
Herbaceous perennials: Autumn Crocus, Baptisia, Columbine, Lily (Asian Hybrids), Peony, and Rhubarb.
Annuals and Vegetables: Asparagus, Cabbage, Eggplant, Flowering Tobacco, Pepper, Petunia, Potato, and Tomato.
How to Control Black Walnut Toxicity
First, and it should be pretty obvious by now, place juglone-sensitive plants at a safe distance from your Black Walnut tree(s). If you are in a fairly small space and can’t get too far from the Black Walnut tree(s), try planting in raised boxes. This will help to keep the soil free from juglone emitted by tree roots. That said, you’ll have to take special care to keep those beds free from Black Walnut twigs, leave, branches, and nuts, as they can bleed juglone into the soil in the beds. Further, avoid using anything Black Walnut-related (e.g., wood chips, bark, leaves, etc.) as mulch or compost. Finally, your best bet is to work with the juglone-tolerant plants listed below.
These Plants are Tolerant to Black Walnut Toxicity
Common Name / Scientific Name
American Beech / Fagus Grandifolia
American Chestnut / Castanea Dentata
American Elm / Ulmus Americana
American Hornbeam / Carpinus Caroliniana
Black Cherry / Prunus Serotina
Black Locust / Robinia Pseudoacacia
Black Oak / Quercus Velutina
Box Elder / Acer Negundo
Cucumber Tree / Magnolia Acuminata
Flowering Dogwood / Cornus Florida
Fringe Tree / Chionanthus Spp.
Hawthorn / Crataegus Spp.
Hickory / Carya Spp.
Honey-Locust / Gleditsia Triacanthos
Japanese Maple / Acer Palmatum & Cvs.
Mulberry / Morus Spp.
Northern Red Oak / Quercus Rubra
Ohio Buckeye / Aesculus Glabra
Pawpaw / Asimina Triloba
Persimmon / Diospyros Virginiana
Pin Cherry / Prunus Pensylvanica
Red Maple / Acer Rubrum
Redbud / Cercis Canadensis
River Birch / Betula Nigra
Sassafras / Sassafras Albidum
Scarlet Oak / Quercus Coccinea
Serviceberry / Amelanchier Spp.
Shingle Oak / Quercus Imbricaria
Silverbell / Halesia Carolina
Slippery Elm / Ulmus Rubra
Southern Catalpa / Catalpa Bignonioides
Staghorn Sumac / Rhus Typhina
Sugar Maple / Acer Saccharum
Sweet Birch / Betula Lenta
Sweet-Gum / Liquidambar Styraciflua
Sycamore / Platanus Occidentalis
Tulip-Tree / Liriodendron Tulipifera
Tupelo / Nyssa Sylvatica
White Oak / Quercus Alba
Wild Plum / Prunus Americana
Willow / Salix Spp.
Witch-Hazel / Hamamelis Spp.
Yellow Birch / Betula Lutea
Yellow Buckeye / Aesculus Octandra
Common Name / Scientific Name
American Bladdernut / Staphylea Trifolia
American Hazelnut / Corylus Americana
Beauty Bush / Kolkwitzia Amabilis
Black Raspberry / Rubus Occidentalis
Black-Haw / Viburnum Prunifolium
Currant / Ribes Spp.
Devil’s Walking Stick / Aralia Spinosa
Elderberry / Sambucus Spp.
Euonymus / Euonymus Spp.
Exbury Rhododendron / Rhododendron Hybrids
February Daphne / Daphne Mezereum
Forsythia / Forsythia Spp.
Fragrant Sumac / Rhus Aromatica
Fringe Tree / Chionanthus Virginicus
Honeysuckle / Most Lonicera Spp.
Juniper / Juniperus Spp.
Korean Spice Viburnum / Viburnum Carlesii & Cvs.
Maple-Leaved Viburnum / Viburnum Acerifolium
Mock-Orange / Philadelphus Spp.
New Jersey Tea / Ceanothus Americanus
Ninebark / Physocarpus Opulifolius
Pagoda Dogwood / Cornus Alternifolia
Prickly-Ash / Zanthoxylum Americanum
Purple-Flowering Raspberry / Rubus Odoratus
Rose-Of-Sharon / Hibiscus Syriacus
Shining Sumac / Rhus Copallina
Shrubby St. Johnwort / Hypericum Prolificum
Silky Dogwood / Cornus Amomum
Smooth Sumac / Rhus Glabra
Southern Arrowwood / Viburnum Dentatum
Speckled Alder / Alnus Rugosa
Spicebush / Lindera Benzoin
Wild Hydrangea / Hydrangea Arborescens
Wild Rose / Rosa Spp.
YuccaYucca SppEvergreensCommon NameScientific Name
Chinese Juniper / Juniperus Chinensis
Common Juniper / Juniperus Communis
Eastern Hemlock / Tsuga Canadensis
Eastern Red Cedar / Juniperus Virginiana
Common Name / Scientific Name
Bittersweet / Celastrus Spp.
Clematis / Clematis Ssp.
Dutchman’S Pipe / Aristolochia Durior
Greenbriar / Smilax Spp.
Honeysuckle Vine / Lonicera Spp.
Virginia Creeper / Parthenocissus Spp.
Wild Grape / Vitis Spp.
Wisteria / Wisteria Spp.
Herbaceous Perennials, Spring Wildflowers and Bulbs
Common Name / Scientific Name
Aster / Aster Spp.
Astilbe / Astilbe Spp.
Bee Balm / Monarda Spp.
Bellflower / Campanula Spp.
Bellwort / Uvularia Spp.
Black-Eyed Susan / Rudbeckia Spp.
Bleeding Heart / Dicentra Spctabilis
Bloodroot / Sanguinaria Canadensis
Bugleweed / Ajuga Spp.
Bush Clover / Lespedeza Spp.
Buttercup / Ranunculus Spp.
Christmas Fern / Polystichum Spp.
Chrysanthemum / Chrysanthemum Spp.
Cinnamon Fern / Osmunda Cinnamomea
Coral Bell / Heuchera Spp.
Cranesbill / Geranium Spp.
Daffodil / Selected Narcissus Spp.
Daylily / Hemerocallis Spp.
Dog’s Tooth Violet / Erythronium Spp.
Epimedium / Epimedium Spp.
Evening Primrose / Oenothera Spp.
False Dragonhead / Physostegia Spp.
Fragile Fern / Cystopteris Fragilis
Gentian / Gentiana Spp.
Goldenrod / Solidago Spp.
Grape Hyacinth / Muscari Spp.
Hellebore / Helleborus Spp.
Hollyhock / Alcea Rosea
Hosta / Hosta Spp.
Jack-In-The-Pulpit / Arisaema Triphllyum
Jacob’s Ladder / Polemonium Reptans
Joe-Pyeweed / Eupatorium Spp.
Lady Fern / Athyrium Spp.
Lamb’s Ears / Stachys Byzantina
Leopard’s Bane / Doronicum Spp.
Lilyturf / Liriope Spp.
Lobelia / Lobelia Spp.
Lungwort / Pulmonaria Spp.
Mayapple / Podophyllum Peltatum
Meadow Rue / Thalictrum Spp.
Peppermint / Mentha Piperita
Perennial Sunflower / Helianthus Spp.
Primrose / Primula Spp.
Purple Coneflower / Echinacea Purpurea
Rattlesnake Fern / Botrychium Spp.
Sensitive Fern / Onoclea Sensibilis
Siberian Iris / Iris Sibirica
Siberian Squill / Scilla Sibirica
Snowdrop / Galanthus Nivalis
Solomon’s Seal / Polygonatum Spp.
Speedwell / Veronica Spp.
Spiderwort / Tradescantia Virginiana
Spring Beauty / Claytonia Spp.
Stonecrop / Sedum Spp.
Summer Phlox / Phlox Paniculata
Sweet Woodruff / Galium Odoratum
Toothwort / Dentaria Spp.
Trillium / Trillium Spp.
Tulip / Selected Tulipa Spp.
Violet / Viola Spp.
Virginia Waterleaf / Hydrophyllum Virginianum
Wild Ginger / Asarum Spp.
Windflower / Anemone Spp.
Winter Aconite / Eranthis Hyemalis
Wood Fern / Dryopteris Spp.
Yarrow / Achillea Spp.