Growing Plants Under Black Walnuts

Prepared for Talking Plants by The Morton Arboretum

Plants Tolerant of 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. 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 "alleopathy" 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.

Plants Tolerant of Black Walnut Toxicity

Trees

Common name	Scientific Name	Common name	Scientific Name
Box Elder	Acer negundo	Carolina Silverbell	Halesia carolina
Japanese Maple	Acer palmatum, & cvs.	Witchhazel	Hamamelis spp.
Red Maple	Acer rubrum	Sweetgum	Liquidambar styraciflua
Sugar Maple	Acer saccharum	Tuliptree	Liriodendron tulipifera
Ohio Buckeye	Aesculus glabra	Cucumber Tree	Magnolia acuminata
Sweet Buckeye	Aesculus octandra	Mulberry	Morus spp.
Tree of Heaven	Ailanthus altissima	Blackgum	Nyssa sylvatica
Serviceberry	Amelanchier spp.	Sycamore	Platanus occidentalis
Pawpaw	Asimina triloba	American Plum	Prunus americana
Sweet Birch	Betula lenta	Pin Cherry	Prunus pensylvanica
Yellow Birch	Betula lutea	Wild Black Cherry	Prunus serotina
River Birch	Betula nigra	White Oak	Quercus alba
American Hornbeam	Carpinus caroliniana	Scarlet Oak	Quercus coccinea
Hickory	Carya spp.	Shingle Oak	Quercus imbricaria
Chestnut	Castanea dentata	Red Oak	Quercus rubra
Catalpa	Catalpa bignonioides	Black Oak	Quercus velutina
Redbud	Cercis canadensis	Staghorn Sumac	Rhus typhina
Flowering Dogwood	Cornus florida	Black Locust	Robinia pseudoacacia
Hawthorn	Crataegus spp.	Willow	Salix spp.
Persimmon	Diospyros virginiana	Sassafras	Sassafras albidum
Beech	Fagus grandifolia	American Elm	Ulmus americana
White Ash	Fraxinus americana	Slippery Elm	Ulmus rubra
Black Ash	Fraxinus nigra	Honeylocust	Gleditsia triacanthos

Evergreens

Common name	Scientific Name	Common name	Scientific Name
Chinese Juniper	Juniperus chinensis	Shingle Oak	Quercus imbricaria
Common Juniper	Juniperus communis	Arborvitae	Thuja spp.
Red Cedar	Juniperus virginiana	Eastern Hemlock	Tsuga canadensis

Vines

Common name	Scientific Name	Common name	Scientific Name
Dutchman's Pipe	Aristolochia durior	Virginia Creeper	Parthenocissus spp.
Bittersweet	Celastrus spp.	Greenbriar	Smilax spp.
Clematis	Clematis ssp.	Wild Grape	Vitis spp.
Honeysuckle Vine	Lonicera spp.

Shrubs

Common name	Scientific Name	Common name	Scientific Name
Speckled Alder	Alnus rugosa	Exbury Rhododendron	Rhododendron hybrids
Hercules-club	Aralia spinosa	Fragrant Sumac	Rhus aromatica
Spicebush	Lindera benzoin	Shining Sumac	Rhus copallina
New Jersey Tea	Ceanothus americanus	Smooth Sumac	Rhus glabra
Fringe Tree	Chionanthus virginicus	Currant	Ribes spp.
Pagoda Dogwood	Cornus alternifolia	Wild Rose	Rosa spp.
Silky Dogwood	Cornus amomum	Black Raspberry	Rubus occidentalis
American Hazelnut	Corylus americana	Purple Raspberry	Rubus odoratus
February Daphne	Daphne mezereum	Elderberry	Sambucus spp.
Euonymus	Euonymus spp.	Bladdernut	Staphylea trifolia
Forsythia	Forsythia spp.	Mapleleaf Viburnum	Viburnum acerfolium
Rose of Sharon	Hibiscus syriacus	Fragrant Viburnum	Viburnum carlesii & cvs.
Shrubby St. Johnwort	Hypericum prolificum	Arrowwood	Viburnum dentatum
Beautybush	Kolkwitzia amabilis	Blackhaw	Viburnum prunifolium
Honeysuckle	most Lonicera spp.	Adam's Needle	Yucca spp.
Ninebark	Physocarpus opulifolius	Prickly Ash	Zanthoxylum americanum

Herbaceous Perennials, Spring Wildflowers and Bulbs

Common name	Scientific Name	Common name	Scientific Name
Bugleweed	Ajuga spp.	Perennial Sunflower	Helianthus spp.
Hollyhock	Alcea rosea	Daylily	Hemerocallis spp.
Windflower	Anemone spp.	Coral Bell	Heuchera spp.
Jack-in-the-Pulpit	Arisaema triphllyum	Hosta	Hosta spp.
Wild Ginger	Asarum spp.	Siberian Iris	Iris sibirica
Aster	Aster spp.	Bush Clover	Lespedeza spp.
Astilbe	Astilbe spp.	Lobelia	Lobelia spp.
Lady Fern	Athyrium spp.	Peppermint	Mentha piperita
Rattlesnake Fern	Botrychium spp.	Bee Balm	Monarda spp.
Bellflower	Campanula spp.	Grape Hyacinth	Muscari spp.
Chrysanthemum	Chrysanthemum spp.	Daffodil	selected Narcissus spp.
Spring Beauty	Claytonia spp.	Evening Primrose	Oenothera spp.
Fragile Fern	Cystopteris fragilis	Sensitive Fern	Onoclea sensibilis
Toothwort	Dentaria spp.	Cinnamon Fern	Osmunda cinnamomea
Bleeding Heart	Dicentra spctabilis	Summer Phlox	Phlox paniculata
Leopard's Bane	Doronicum spp.	False Dragonhead	Physostegia spp.
Wood Fern	Dryopteris spp.	May Apple	Podophyllum peltatum
Winter Aconite	Eranthis hyemalis	Jacob's Ladder	Polemonium reptans
Joe-pyeweed	Eupatorium spp.	Solomon's Seal	Polygonatum spp.
Sweet Woodruff	Galium odoratum	Christmas Fern	Polystichum spp.
Cranesbill	Geranium spp.	Primrose	Primula spp.
Lungwort	Pulmonaria spp.	Spiderwort	Tradescantia virginiana
Buttercup	Ranunculus spp.	Trillium	Trillium spp.
Black-eyed Susan	Rudbeckia spp.	Tulip	selected Tulipa spp.
Bloodroot	Sanguinaria canadensis	Bellwort	Uvularia spp.
Stonecrop	Sedum spp.	Speedwell	Veronica spp.
Goldenrod	Solidago spp.	Violet	Viola spp.
Lamb's Ears	Stachys byzantina