TREES & plants

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.  The best alternative is to choose plants from the list below.

 

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

Mulberry                               Morus spp.

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

Speckled alder                       Alnus rugosa                    

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            

Euonymus                                Euonymus spp.                 

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                

Honeysuckle                            most Lonicera spp.            .                                  

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                       Dicentra 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.