Black walnuts are allelopathic. They produce juglone, a substance toxic to other plants. They do this to reduce competition for water, soil nutrients and sunlight. Not all plants are susceptible to the toxins produced by walnut trees.

Several related trees such as English walnut, hickories and pecan also produce juglone, but in smaller amounts compared to black walnut. Additional common landscape trees with allelopathic properties: sugar maple, tree-of-heaven, hackberries, southern waxmyrtle, American sycamore, cottonwood, black cherry, red oak, black locust, sassafrass, and American elm.

Here are plant lists of plants that are not damaged by juglone, and will therefore grow successfully under and around walnut trees.

Perennials

Achillea (yarrow)
Ajuga reptans (bugleweed)
Alcea rosea (hollyhock)
Anemone
Arisaema (Jack-In-The-Pulpit)
Asarum europaeum (European wild ginger)
Aster
Astilbe
Campanula latifolia (bellflower)
Celastrus (bittersweet)
Chrysanthemum (hardy chrysanthemum)
Doronicum (leopards bane)
Ferns
Galium odoratum
(sweet woodruff )
Geranium (cranesbill)
Helianthus tuberosus
(Jerusalem artichoke)
Hemerocallis (daylily)
Heuchera (coral bells)
Hieracium aurantiacum (orange hawkweed)
Hosta
Iris sibirica (Siberian iris)
Leucanthemum (shasta daisy)
Liriope
Lobelia
Monarda (bee balm, bergamot)
Oenothera (primrose)
Phlox paniculata (phlox)
Podophyllum (mayapple)
Polemonium reptans (Jacob's ladder)
Polygonatum commutatum
(great Solomon's seal)
Primula (primrose)
Pulmonaria (lungwort)
Rudbeckia (black-eyed Susan)
Sanquinaria canadensis (bloodroot)
Sedum
Solidago (goldenrod)
Stachys byzantina (lamb's ear)
Tradescantia (spiderwort)
Trillium
Trollius (globe flower)
Verbascum (mullien)
Vernonia (ironweed)
Veronica (speedwell)
Viola (violet)

Annuals

Begonia
Calendula (pot marigold)
Helianthus (sunflower)
Impatiens
Ipomoea (morning glory)
Viola (pansy)

Bulbs

Crocus
Daffodil/Narcissus
Eranthis hyemalis (winter aconite)
Galanthus nivalis (snowdrop)
Hyacinthus (hyacinth)
Muscari (grape hyacinth)
Scilla
Tulipa (tulip)

Shrubs

Amelanchier (serviceberry)
Arborvitae
Berberis (barberry)
Daphne
Forsythia
Hamamelis (witchhazel)
Hibiscus syriacus
(rose of Sharon)
Hydrangea
Hypericum (St. John'swort)
Ilex (holly)
Juniperus virginiana
(juniper, eastern red cedar)
Ligustrum (privet)
Physocarpus (ninebark)
Rhododendron
Rhus (sumac)
Rosa (native species)
Rubus
(blackberry, raspberry)
Sambucus (elderberry)
Syringa (lilac)
Viburnum

 

Trees

Acer negundo (box elder)
Acer (maples)
Aesculus glabra (Ohio buckeye)
Asimina trilobata (pawpaw)
Betula nigra (river birch)
Carya (hickory, pecan)
Catalpa
Cercis canadensis (redbud)
Chionanthus (fringetree)
Cornus (dogwood)
Fagus grandiflora (American beech)
Fraxinus americana (white ash)
Gleditsia (honeylocust)
Halesia (Carolina silverbell)
Koelreuteria (golden-rain tree)
Liquidambar (sweet gum)
Malus crabapple varieties
Nyssa sylvatica (black gum)
Picea abies (Norway spruce)
Platanus (sycamore)
Prunus serotina (black cherry)
Pyrus calleryana (ornamental pear)
Quercus (oak)
Robinia (locust)
Sassafras
Tsuga canadensis (hemlock)
Tulipifera (tulip tree)
Ulmus (elm)

Vines

Clematis
Lonicera (honeysuckle)
Parthenocissus quinquefolia
(Virginia creeper)
Vitex (grape)
Wisteria