Clinic Tools Plants Tolerant of Black Walnut Toxicity

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1 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

2 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 Box Elder Japanese Maple Red Maple Sugar Maple Ohio Buckeye Sweet Buckeye Serviceberry Pawpaw Sweet Birch Yellow Birch River Birch American Hornbeam Hickory Chestnut Catalpa Acer negundo Acer palmatum & cvs. Acer rubrum Acer saccharum Aesculus glabra Aesculus octandra Amelanchier spp. Asimina triloba Betula lenta Betula lutea Betula nigra Carpinus caroliniana Carya spp. Castanea dentata Catalpa bignonioides

3 Redbud Fringetree Flowering Dogwood Hawthorn Persimmon Beech Honeylocust Carolina Silverbell Witchhazel Sweetgum Tuliptree Cucumber Tree Mulberry Blackgum Sycamore American Plum Pin Cherry Wild Black Cherry White Oak Scarlet Oak Shingle Oak Red Oak Black Oak Staghorn Sumac Black Locust Willow Sassafras American Elm Slippery Elm Cercis canadensis Chionanthus spp. Cornus florida Crataegus spp. Diospyros virginiana Fagus grandifolia Gleditsia triacanthos Halesia carolina Hamamelis spp. Liquidambar styraciflua Liriodendron tulipifera Magnolia acuminata Morus spp. Nyssa sylvatica Platanus occidentalis Prunus americana Prunus pensylvanica Prunus serotina Quercus alba Quercus coccinea Quercus imbricaria Quercus rubra Quercus velutina Rhus typhina Robinia pseudoacacia Salix spp. Sassafras albidum Ulmus americana Ulmus rubra

4 SHRUBS Speckled Alder Alnus rugosa Hercules-club 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 Smooth Hydrangea Hydrangea arborescens Shrubby St. Johnwort Hypericum prolificum Juniper Juniperus spp. Beautybush Kolkwitzia amabilis Spicebush Lindera benzoin Honeysuckle most Lonicera spp.. Mockorange 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 Raspberry Rubus odoratus

5 Elderberry Bladdernut Mapleleaf Viburnum Koreanspice Viburnum Arrowwood Viburnum Blackhaw Viburnum Adam s Needle Prickly Ash Sambucus spp. Staphylea trifolia Viburnum acerfolium Viburnum carlesii & cvs. Viburnum dentatum Viburnum prunifolium Yucca spp Zanthoxylum americanum EVERGREENS Chinese Juniper Common Juniper Red Cedar Shingle Oak Arborvitae Eastern Hemlock Juniperus chinensis Juniperus communis Juniperus virginiana Quercus imbricaria Thuja spp. Tsuga canadensis VINES Dutchman s Pipe Bittersweet Clematis Honeysuckle Vine Virginia Creeper Greenbriar Wild Grape Wisteria Aristolochia durior Celastrus spp. Clematis ssp. Lonicera spp. Parthenocissus spp. Smilax spp. Vitis spp. Wisteria spp.

6 HERBACEOUS PERENNIALS, SPRING WILDFLOWERS AND BULBS Yarrow Bugleweed Hollyhock Windflower Jack-in-the-Pulpit Wild Ginger Aster Astilbe Lady Fern Rattlesnake Fern Bellflower Chrysanthemum Spring Beauty Fragile Fern Toothwort Bleeding Heart Leopard s Bane Wood Fern Purple Coneflower Epimedium Winter Aconite Dog's Tooth Violet Joe-pyeweed Snowdrops Sweet Woodruff Gentian Achillea spp. Ajuga spp. Alcea rosea Anemone spp. Arisaema triphllyum Asarum spp. Aster spp. Astilbe spp. Athyrium spp. Botrychium spp. Campanula spp. Chrysanthemum spp. Claytonia spp. Cystopteris fragilis Dentaria spp. Dicentra spctabilis Doronicum spp. Dryopteris spp. Echinacea purpurea Epimedium spp. Eranthis hyemalis Erythronium spp. Eupatorium spp. Galanthus nivalis Galium odoratum Gentiana spp.

7 Cranesbill Perennial Sunflower Hellebores Daylily Coral Bell Hosta Virginia Waterleaf Siberian Iris Bush Clover Lilyturf Lobelia Peppermint Bee Balm Grape Hyacinth Daffodil Evening Primrose Sensitive Fern Cinnamon Fern Summer Phlox False Dragonhead Mayapple Jacob s Ladder Solomon s Seal Christmas Fern Primrose Lungwort Buttercup Black-eyed Susan Bloodroot Siberian squill Geranium spp. Helianthus spp. Helleborus spp. Hemerocallis spp. Heuchera spp. Hosta spp. Hydrophyllum virginianum Iris sibirica Lespedeza spp. Liriope spp. Lobelia spp. Mentha piperita Monarda spp. Muscari spp. selected Narcissus spp. Oenothera spp. Onoclea sensibilis Osmunda cinnamomea Phlox paniculata Physostegia spp. Podophyllum peltatum Polemonium reptans Polygonatum spp. Polystichum spp. Primula spp. Pulmonaria spp. Ranunculus spp. Rudbeckia spp. Sanguinaria canadensis Scilla sibirica

8 Stonecrop Goldenrod Lamb s Ears Meadowrue Spiderwort Trillium Tulip Bellwort Speedwell Violet Sedum spp. Solidago spp. Stachys byzantina Thalictrum spp. Tradescantia virginiana Trillium spp. selected Tulipa spp. Uvularia spp. Veronica spp. Viola spp.