Black walnut and juglone: what extension services say about plants near walnut trees
Black walnut trees make a chemical called juglone, and gardeners are told it harms plants nearby. This page reports what Wisconsin, Purdue and Washington State extension publications say, including where they disagree.
This page is for tree owners. It reports what the named sources say and is not advice about your tree: for that, ask a qualified arborist or one of the offices under Who to call.
Checked against the sources at the bottom of this page on October 9, 2026. Rules, fees and pay change: the source has the last word.
What juglone is and where it sits in the tree
The University of Wisconsin-Madison Division of Extension says black walnut makes juglone, which inhibits many kinds of plants growing near it. It says juglone occurs in all parts of the tree and is most concentrated in the buds, nut hulls and roots, with the greatest amounts directly under the tree where roots are dense and hulls and leaves rot. It adds that butternut also makes juglone, in smaller amounts. Purdue's leaflet HO-193 names the same three parts and says juglone has been shown experimentally to act as a respiration inhibitor in sensitive plants. Purdue also lists relatives that make it in smaller amounts: English walnut, pecan, shagbark hickory and bitternut hickory.
The Washington State University publication by Linda Chalker-Scott, published March 2019, gives a more cautious account. It says living walnut tissue holds a nontoxic precursor called hydrojuglone, which turns into juglone in the soil, and that the roots and nut shells hold most of it, with little in the leaves and virtually none in the wood.
How far it is said to reach
A University of Wisconsin article by Susan Mahr, posted June 25, 2004, says juglone dissolves poorly in water and does not spread far in soil, that its effect is worst at the dripline, and that the toxic zone around a mature walnut is within 50 to 60 feet of the trunk. Purdue says sensitive plants may show symptoms anywhere roots grow, with more juglone under the canopy. It adds that a very sensitive plant may not tolerate the small amount beyond the canopy, and that the amount in soil depends on soil type, drainage and soil microorganisms. Wisconsin's extension page says problems mostly occur when roots of susceptible plants meet walnut tissue, and that well-drained soil usually has fewer problems than poorly drained soil.
Symptoms
Wisconsin's extension page lists stunting, yellowing, partial to total wilting and death. The Mahr article says symptoms vary by plant, sometimes mimic diseases or physiological disorders, and that affected plants often wilt in a relatively short time after contact with walnut roots, even in moist soil. For tomato it lists yellowing, twisting and puckering of leaves and darkened stem tissue.
Sensitive and tolerant plants as the lists give them
Wisconsin and Purdue both say the lists rest on observation, not formal testing, and Wisconsin adds that a plant can be listed as sensitive on one list and tolerant on another. The two sources show this.
| Plant | Wisconsin extension page | Purdue HO-193 |
|---|---|---|
| Crabapple | Sensitive | On both its sensitive list and its tolerant list |
| White birch | Sensitive | Sensitive |
| Pine | Sensitive | Sensitive (loblolly, mugo, red, Scotch, white) |
| Lilac | Sensitive | Sensitive |
| Most maples | Tolerant | Tolerant |
| Oak | Tolerant | Tolerant |
| Eastern redbud | Tolerant | Tolerant |
| Silver maple | Not on the page | Sensitive |
Wisconsin's page also lists tomatoes, peppers and potatoes as sensitive, and beans, beets, carrots, corn, melons and squash as tolerant.
After a walnut is removed, and walnut mulch
Wisconsin's extension page says removal is not a suggested solution because roots stay in the soil and keep presenting juglone problems for sensitive plants. Purdue says toxicity can persist for some years after removal because decaying roots release juglone. The Mahr article says juglone can persist in the wood until the roots decompose, which can take five or more years.
On walnut leaves, bark and chips as mulch, the sources differ. The Mahr article says juglone breaks down when exposed to air, water and bacteria, so leaves and husks pose no threat once completely composted. Purdue says that even after composting such material may release small amounts. Wisconsin's page advises against mulching garden areas with walnut leaves or nut husks. The Mahr article and Purdue advise against using black walnut leaves, bark or wood chips as mulch.
What the Washington State review concludes
The WSU publication says field tests beginning in 1951 showed little to no negative effect, while laboratory tests on germinating seeds and seedlings found stunting and wilting. It says results are inconsistent and that lab methods such as soilless media do not occur in nature. It states that the sensitive and tolerant lists trace back to two extension publications that are observational, which correlate walnut presence with damage but do not confirm cause. It also says competition for water and nutrients, shade and other conditions should be considered separately from chemical effects.
Who to call
- Local cooperative extension office: plant questions in a given state; Wisconsin's horticulture site is one of the publishers above
- ISA Find an Arborist: the ISA directory of arborists
- Hiring an arborist: what the trade bodies say to look for
Sources
- University of Wisconsin-Madison Division of Extension, black walnut toxicity to plants, read October 9, 2026
- University of Wisconsin Master Gardener program, landscaping in spite of black walnuts, posted June 25, 2004, read October 9, 2026
- Purdue University Cooperative Extension Service, HO-193 black walnut toxicity, revised February 1994, read October 9, 2026
- Washington State University Extension, do black walnut trees have allelopathic effects on other plants, March 2019, read October 9, 2026