Black walnut (Juglans nigra) is considered one of our most valuable native hardwood lumber trees and is often used in large scale landscapes. However, in the smaller-scale home landscape, 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, there are certain plant species whose 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, stunted or slow growth, and eventually death. 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.
PLANTS TOLERANT OF BLACK WALNUT TOXICITY
TREES
Common name Scientific name
Box elder Acer negundo
Japanese maple Acer palmatum & cvs.
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
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. Johnwort 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 & cvs.
Southern arrowwood Viburnum dentatum
Black-haw Viburnum prunifolium
Yucca Yucca spp
Prickly-ash Zanthoxylum americanum
EVERGREENS
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
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
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-pyeweed 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.