Black Walnut Toxicity: Juglone and What to Plant Nearby

Black walnuts (Juglans nigra) produce juglone, a natural compound toxic to many plants—a phenomenon known as allelopathy, noted by Pliny in 77 AD. Juglone concentrates in roots (extending 50–60+ feet from mature trees), buds, and nut hulls. Leaves, bark, and wood contain lesser amounts. English and Carpathian walnuts grafted onto black walnut or butternut rootstock also produce toxins. Juglone is poorly water-soluble, persisting in soil for years even after tree removal.

Juglone-Sensitive Plants (Injured or Killed)

Highly susceptible: tomato, potato, blackberry, blueberry, azalea, mountain laurel, rhododendron, red pine, apple. Many others—see full lists below.

Juglone-Tolerant Plants (Observed Growing Near Black Walnuts)

Trees: Japanese maples, southern catalpa, eastern redbud, Canadian hemlock.
Vines & Shrubs: Clematis 'Red Cardinal', February daphne, euonymus, weeping forsythia, rose of Sharon, tartarian honeysuckle, Virginia creeper, pinxterbloom azalea, 'Gibraltar'/'Balzac' Exbury azaleas, multiflora rose, black raspberry, arborvitae, Koreanspice viburnum.
Annuals: Pot marigold 'Nonstop', fibrous begonias, morning glory 'Heavenly Blue', pansy, zinnia.
Vegetables: Squashes, melons, beans, carrots, corn.
Fruit Trees: Peach, nectarine, cherry, plum, pear.
Herbaceous Perennials: Bugleweed, hollyhock, wood anemone, jack-in-the-pulpit, wild ginger, astilbe, bellflower, some chrysanthemums, glory-of-the-snow, spring beauty, crocus, Dutchman's breeches, leopard's-bane, crested wood fern, Spanish bluebell, winter aconite, snowdrop, sweet woodruff, herb Robert, cranesbill, grasses, Jerusalem artichoke, daylily 'Pluie de Feu', coral bells, orange hawkweed, hostas, hyacinth 'City of Haarlem', Virginia waterleaf, Siberian iris, bee balm, wild bergamot, grape hyacinth, sweet cicely, sundrops, sensitive fern, cinnamon fern, some peonies, summer phlox, mayapple, Jacob's ladder, Solomon's seal, polyanthus primrose, lungwort, bloodroot, Siberian squill, goldmoss stonecrop, showy sedum, lamb's ear, spiderwort, nodding trillium, white wake-robin, Darwin tulips, merrybells, Canada violet, horned violet, woolly blue violet.

Plants That Fail Within 50 Feet

Perennials: Columbines, asparagus, some chrysanthemums, baptisia, hydrangea, lilies (especially Asian hybrids), alfalfa, daffodils, some peonies, rhubarb.
Trees: Silver maple, European alder, white birches, hackberry, apples/crabapples, Norway spruce, mugo pine, red pine, eastern white pine, basswood.
Shrubs: Red chokeberry, hydrangea, mountain laurel, privet, Amur honeysuckle, cinquefoil, most rhododendrons/azaleas, blackberry, lilacs, yew, blueberry, 'Mariesii' viburnum.
Annuals/Vegetables: Cabbage, peppers, tomatoes, flowering tobacco, petunia, eggplant, potato, double-flowered cole crops.

Lists based on observation, not clinical trials. Cultivar variation exists.

Managing Juglone

• Compost walnut leaves—juglone breaks down in weeks. Test with tomato seedlings.
• Avoid fresh sawdust/mulch from black walnut around sensitive plants. Bark mulch composted 6–12 months may be safe.
• Keep garden beds clear of walnut debris. Pull volunteer seedlings early.
• Use raised beds with root barriers if planting near walnuts.

Benefits & Uses

• Delicious nutmeat; hulls make dye and contain organic iodine (antiseptic, insect repellent).
• Recommended for erosion control on hillsides; shade for livestock; may benefit Kentucky bluegrass.
• Medicinal: antifungal, antiparasitic, antiseptic. Used for eczema, herpes, psoriasis, skin parasites, ringworm, tapeworms (Chinese medicine).
• Culinary: Strong, smoky flavor. Blend 1:3 with English walnuts to mellow. Recipe: Black Walnut Chicken Quiche included.

Sources: Ohio State University juglone-tolerant/sensitive lists.
https://web1.msue.msu.edu/msue/iac/greentip/blackwal.htm
https://www.ext.vt.edu/pubs/nursery/430-021/430-021.html

Photo credits: hczone6, Equilibrium (PlantFiles); iStockPhoto #3714758, #2336710.