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Living with Black Walnut trees
The roots of Black Walnut (Juglans nigra L.) produce a substance
known as juglone (5-hydroxy-alphanapthaquinone). Persian (English or Carpathian)
walnut trees are sometimes grafted onto black walnut rootstocks. Many plants
such as tomato, potato, blackberry, blueberry, azalea, mountain laurel,
rhododendron, red pine and apple may be injured or killed within one to two
months of growth within the root zone of these trees. The toxic zone from a
mature tree occurs on average in a 50 to 60 foot radius from the trunk, but can
be up to 80 feet. The area affected extends outward each year as a tree
enlarges. Young trees two to eight feet high can have a root diameter twice the
height of the top of the tree, with susceptible plants dead within the root zone
and dying at the margins.
Not all plants are sensitive to juglone. Many trees, vines, shrubs,
groundcovers, annuals and perennials will grow in close proximity to a walnut
tree. Certain cultivars of "resistant" species are reported to do poorly. Trees
hold the soil, prevent erosion and provide shade for cattle. The beneficial
effect of black walnut on pastures in encouraging the growth of Kentucky
bluegrass (Poa pratensis L.) and other grasses appears to be valid as
long as there is sufficient sunlight and water.
Gardeners should carefully consider the planting site for black walnut,
butternut, or persian walnut seedlings grafted to black walnut rootstock, if
other garden or landscape plants are to be grown within the root zone of mature
trees. Persian walnut seedlings or trees grafted onto Persian walnut rootstocks
do not appear to have a toxic effect on other plants.
Close association with walnut trees while pollen is being shed
(typically in May) also produce allergic symptoms in both horses and humans. The
juglone toxin occurs in the leaves, bark and wood of walnut, but these contain
lower concentrations than in the roots. Juglone is poorly soluble in water and
does not move very far in the soil.
Walnut leaves can be composted because the toxin breaks down when exposed to
air, water and bacteria. The toxic effect can be degraded in two to four weeks.
In soil, breakdown may take up to two months. Black walnut leaves may be
composted separately, and the finished compost tested for toxicity by planting
tomato seedlings in it. Sawdust mulch, fresh sawdust or chips from street tree
prunings from black walnut are not suggested for plants sensitive to juglone,
such as blueberry or other plants that are sensitive to juglone. However,
composting of bark for a minimum of six months provides a safe mulch even for
plants sensitive to juglone.
Plants Observed Growing Under or Near Black Walnut*
Trees
 | Japanese Maples, Acer palmatum and its cultivars |
| Southern Catalpa, Catalpa bignonioides |
| Eastern Redbud, Cercis canadensis |
| Canadian Hemlock, Tsuga canadensis |
Vines and Shrubs
| Clematis 'Red Cardinal' |
| February Daphne, Daphne mezereum |
| Euonymus species |
| Weeping Forsythia, Forsythia suspensa |
| Rose of Sharon, Hibiscus syriacus |
| Tartarian Honeysuckle, Lonicera tatarica, and most other
Lonicera species |
| Virginia Creeper, Parthenocissus quinquefolia |
| ** Pinxterbloom, Rhododendron periclymenoides |
| **'Gibraltar' and 'Balzac', Rhododendron Exbury hybrids |
| Multiflora Rose, Rosa multiflora |
| Black Raspberry, Rubus occidentalis |
| Arborvitaes, Thuja species |
| ** Koreanspice Viburnum, Viburnum carlesii, and most other
Viburnum species |
Annuals
| Pot-marigold, Calendula officinalis 'Nonstop' |
| Begonia, fibrous cultivars |
| Morning Glory, Ipomoea 'Heavenly Blue' |
| Pansy Viola |
| Zinnia species |
Vegetables
| Squashes, Melons, Beans, Carrots, Corn |
Fruit Trees
| Peach, Nectarine, Cherry, Plum |
| Prunus species Pear-Pyrus species |
Herbaceous Perennials
| Bugleweed, Ajuga reptans |
| Hollyhock, Alcea rosea |
| American Wood Anemone, Anemone quinquefolia |
| Jack-in-the-Pulpit, Arisaema triphyllum |
| European Wild Ginger, Asarum europaeum |
| Astilbe species |
| Bellflower, Campanula latifolia |
| **Chrysanthemum species (some) |
| Glory-of-the-Snow, Chionodoxa luciliae |
| Spring Beauty, Claytonia virginica |
| Crocus species |
| Dutchman's Breeches, Dicentra cucullaria |
| Leopard's-Bane, Doronicum species |
| Crested Wood Fern, Dryopteris cristata |
| Spanish Bluebell, Endymion hispanicus |
| Winter Aconite, Eranthis hyemalis |
| Snowdrop, Galanthus nivalis |
| Sweet Woodruff, Galium odoratum |
| Herb Robert, Geranium robertianum |
| Cranesbill, Geranium sanguineum |
| Grasses (most) Gramineae family |
| Jerusalem Artichoke, Helianthus tuberosus |
| Common Daylily, Hemerocallis 'Pluie de Feu' |
| Coral Bells, Heuchera x brizoides |
| Orange Hawkweed, Hieracium aurantiacum |
| Plantain-lily, Hosta fortunei 'Glauca' |
| Hosta lancifolia |
| Hosta marginata |
| Hosta undulata 'Variegata' |
| Common Hyacinth, Hyacinthus Orientalis 'City of Haarlem' |
| Virginia Waterleaf, Hydrophyllum virginianum |
| Siberian Iris, Iris sibirica |
| Balm, Monarda didyma |
| Wild Bergamot, M. fistulosa |
| Grape Hyacinth, Muscari botryoides |
| Sweet Cicely, Myrrhis odorata 'Yellow Cheerfulness,' 'Geranium,' 'Tete
a Tete,' 'Sundial,' and 'February Gold' |
| Sundrops, Oenothera fruticosa |
| Senstitive Fern, Onoclea sensibilis |
| Cinnamon Fern, Osmunda cinnamomea |
| Peony, **Paeonia species (some) |
| Summer Phlox, Phlox paniculata |
| Mayapple, Podophyllum peltatum |
| Jacob's-Ladder, Polemonium reptans |
| Great Solomon's-Seal, Polygonatum commutatum |
| Polyanthus Primrose, Primula x polyantha |
| Lungwort, Pulmonaria species |
| Bloodroot, Sanguinaria canadensis |
| Siberian Squill, Scilla sibirica |
| Goldmoss Stonecrop, Sedum acre |
| Showy Sedum, Sedum spectabile |
| Lamb's-Ear, Stachys byzantina |
| Spiderwort, Tradescantia virginiana |
| Nodding Trillium, Trillium cernuum |
| White Wake-Robin, Trillium grandiflorum |
| Tulipa Darwin 'White Valcano' and 'Cum Laude,' Parrot 'Blue
Parrot,' Greigii 'Toronto' |
| Big Merrybells, Uvularia grandiflora |
| Canada Violet, Viola canadensis |
| Horned Violet, Viola cornuta |
| Woolly Blue Violet, Viola sororia |
*These are based upon observations and not from clinical tests.
**Cultivars of some species may do poorly.
Plants That Do Not Grow Within 50 Feet of Drip Line of Black Walnut
Herbaceous Perennials
| Colorado Columbine, Aquilegia caerulea |
| Wild Columbine, Aquilegia canadensis |
| Asparagus, Asparagus offinalis |
| *Chrysanthemum Chrysanthumum species (some) |
| Baptisia australis |
| Hydrangea species |
| Lilies, Lilium species (particularly the Asian hybrids) |
| Alfalfa, Medicago sativa |
| Buttercup, Narcissus 'John Evelyn,' 'Unsurpassable' 'King Alfred'
and 'Ice Follies' |
| Peonies, *Paeonia species (some) |
| Rhubarb, Rheum rhabarbarum |
Trees
| Silver Maple, Acer saccharinum |
| European Alder, Alnus glutinosa |
| White Birches, Betula species |
| Northern Hackberry, Celtis occidentalis |
| Apples and Crabapples, Malus species |
| Norway Spruce, Picea abies |
| Mugo Pine, Pinus mugo |
| Red Pine, Pinus resinosa |
| Eastern White Pine, Pinus strobus |
| Basswood, Tilia heterophylla |
Shrubs
| Red Chokeberry, Aronia arbutifolia |
| Hydrangea species |
| Mountain Laurels, Kalmia species |
| Privet, Ligustrum species |
| Amur Honeysuckle, Lonicera maackii |
| Brush Cinquefoil, Potentilla species |
| Rhododendrons and Azaleas, **Rhododendron species (most) |
| Blackberry, Rubus allegheniensis |
| Lilacs, Syringa species and cultivars |
| Yew, Taxus species |
| Blueberry, Vaccinium corymbosum |
| *Viburnum plicatum tomentosum 'Mariesii' |
Annuals and Vegetables Transplants
| Cabbage, Brassica oleracea capitata |
| Peppers, Capsicum species (some) |
| Tomatoes, Lycopersicon esculentum |
| Flowering Tobacco, Nicotiana alata |
| Petunia species and cultivars |
| Eggplant, Solanum melongena |
| Potato, Solanum tuberosum |
| double-flowered cole vegetables |
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