Reference
Gardening Near a Black Walnut: The Juglone Problem
The short answer
Black walnut (and its relatives) produce juglone, a natural compound in all parts of the tree that inhibits sensitive plants, causing foliage to yellow, wilt, and eventually die. It is only poorly soluble in water, so it does not move far, but live roots release it continuously.
The highest juglone concentration is directly under the canopy, where root density is greatest and fallen leaves and hulls accumulate. Very sensitive plants can show symptoms anywhere within the tree's root spread, which can extend well beyond the branch tips. Because decaying roots keep releasing juglone, toxicity can persist for some years after a walnut is removed.
Purdue Extension lists plants observed to be sensitive (including tomato, potato, pepper, eggplant, asparagus, rhubarb, apple, blueberry, and shrubs like lilac, hydrangea, azalea, and rhododendron) and tolerant (including most grasses, beans, beet, carrot, corn, onion, squash, plus many tough perennials and shrubs). Treat these lists as guidance, not a guarantee.
What juglone does and how far it reaches
Purdue Extension describes juglone as a respiration inhibitor that deprives sensitive plants of the energy they need, producing foliar yellowing, wilting, and eventually death. The largest concentrations are in the walnut's buds, nut hulls, and roots, with smaller amounts in leaves and stems. Because juglone is only poorly water-soluble, it does not spread far through the soil on its own.
Distance matters. The heaviest exposure is under the canopy, where roots are dense and leaves and hulls pile up and decay. But small amounts released by live roots mean very sensitive plants can be affected anywhere within the tree's root zone, which extends beyond the dripline. Some plants tolerate the lower levels out near the edge yet die directly beneath the tree; highly sensitive ones may fail even past the canopy. And because dead roots keep releasing juglone, problems can continue for some years after the tree is gone.
Sensitive and tolerant plants (Purdue Extension lists)
Observed sensitive plants, per Purdue: vegetables including asparagus, cabbage, eggplant, pepper, potato, rhubarb, and tomato; fruits including apple, blackberry, blueberry, and pear; and landscape plants including azalea, white birch, red chokeberry, crabapple, hydrangea, lilac, saucer magnolia, silver maple, mountain laurel, several pines, potentilla, privet, rhododendron, Norway spruce, and yew. Among flowers, autumn crocus, blue wild indigo (Baptisia), columbine, lily, petunia, and some peonies and chrysanthemums are listed as sensitive.
Observed tolerant plants, per Purdue: vegetables including lima and snap beans, beet, carrot, corn, melon, onion, parsnip, and squash; fruits including black raspberry and cherry; and landscape plants including arborvitae, red cedar, catalpa, clematis, elm, euonymus, forsythia, hawthorn, hemlock, black locust, Japanese maple and most maples, oak, pachysandra, pawpaw, persimmon, redbud, rose of sharon, and Virginia creeper. Many perennials tolerate it too, including astilbe, bee balm, most grasses and Kentucky bluegrass, coral bells, cranesbill, daylily, ferns, hosta (many), Siberian iris, phlox, Solomon's-seal, and tulip. Purdue stresses these lists come from observation, not exhaustive testing, so use them as guidance rather than a definitive rule.
Practical steps if you must garden near a walnut
Purdue's advice is to locate gardens away from black walnut trees where possible. If proximity is unavoidable, raised beds built to keep walnut roots from growing up into them, and kept clear of walnut leaves and nut hulls, offer some protection. A physical barrier such as a rock wall or driveway between the tree and the garden can limit root extension into the bed.
Good drainage helps: Purdue notes that decreased toxicity is associated with excellent soil drainage even for sensitive species, so improving drainage with organic matter or a lighter soil tends to reduce problems. Do not use black walnut leaves, bark, or wood chips as mulch around sensitive plants, because even after composting they can release small amounts of juglone. When in doubt, plant from the tolerant list near a walnut and keep the sensitive crops, especially tomatoes, well away.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- Purdue University Cooperative Extension - Black Walnut Toxicity (HO-193) — Juglone is a respiration inhibitor causing yellowing, wilting, and death of sensitive plants; concentrations are highest under the canopy but sensitive plants can be affected within the root zone beyond the dripline, and toxicity persists years after removal. Sensitive plants include tomato, potato, pepper, eggplant, asparagus, rhubarb, apple, blueberry, pear, azalea, lilac, hydrangea, rhododendron, and white birch; tolerant plants include most grasses, beans, beet, carrot, corn, onion, squash, cherry, oak, redbud, forsythia, hosta, daylily, and ferns. Do not mulch sensitive plants with walnut leaves, bark, or wood chips.
- The Morton Arboretum - Black Walnut Toxicity — Juglone occurs in all parts of the black walnut, is most concentrated within the dripline, and its effects can reach roughly to a distance equal to the tree's height; sensitive plants should be placed outside the tree's dripline.