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Juglone: which plants a black walnut poisons, how far its reach goes, and what to plant beneath it
Why a black walnut kills tomatoes and azaleas, how far and how long its juglone reaches, what to do with leaves and hulls, and which natives thrive under it.
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Where a walnut keeps its jugloneHow far juglone reaches, and for how longWhat juglone does to a tomatoLeaves, hulls and wood chipsWhat a walnut gives backHow far to trust the tolerant listsNative trees and shrubs that take jugloneWhat to plant beneath a walnutCommon questionsSourcesJuglone is a compound that black walnut makes in all its parts, and it stunts or kills sensitive plants such as tomatoes, potatoes, blueberries, azaleas and rhododendrons. Planted among a walnut’s roots, they can be dead within one to two months. The roots of a mature tree carry juglone 50 to 80 feet from the trunk, well past the drip line. Roots left in the ground go on releasing it for several years after the tree is cut. Most of the trees that share the walnut’s woods grow beside it unharmed, and so do many native shrubs, the plants to set beneath one.
Where a walnut keeps its juglone
The tree holds the most juglone in its buds, nut hulls and roots, and less in its leaves and stems. The living tree makes it as hydrojuglone, a colorless, harmless form. Air or soil turns it into juglone, and so does a sensitive plant that takes it up. There it slows respiration and starves the plant’s cells of energy. Juglone is one of the chemical defenses of trees.
Black walnut makes far more than its relatives. Pecan, English walnut and the hickories make juglone too, but so little that they rarely harm sensitive plants. Butternut makes less than black walnut, though tomatoes still belong outside its drip line.
How far juglone reaches, and for how long
Live roots release juglone, and so do dead ones as they rot. Fallen leaves and hulls leach it into the soil, and rain carries it off the leaves onto the plants below. It moves little through soil, so a sensitive root meets it where it grows within a quarter to half an inch of a walnut root.
Juglone therefore goes as far as the roots, and they outrun the crown. Around a large tree the root zone averages 50 to 60 feet from the trunk and can reach 80. A young walnut 2 to 8 feet tall can spread its roots as far out on each side as it is tall. The zone widens as the tree grows, so a plant that thrived beside a young walnut can die years later. The growth estimator shows how fast a walnut gains height.
The soil under the crown holds the most. Juglone builds up in wet, airless soil, and well-drained soil rich in organic matter feeds the microbes that break it down.
Cutting the tree does not end it. A small walnut cut to the ground usually sprouts from the stump, and its living roots keep releasing juglone. The roots of a dead tree release it for several years as they rot, and digging them all out is next to impossible.
What juglone does to a tomato
Tomatoes are especially susceptible. The sensitive lists name them among many others:
- Vegetables: tomato, potato, pepper, eggplant, asparagus, cabbage and rhubarb.
- Fruits and shrubs: apple, blackberry, blueberry, azalea, rhododendron, mountain laurel and lilac.
- Trees: crabapple, white birch, basswood and several pines and spruces.
A poisoned plant yellows and wilts, in part or all over, and its leaves may brown and twist. The water-conducting tissue in its stem is often discolored. The wilt can begin within days of planting in the root zone, and water does not cure it:
“Wilting due to lack of water occurs slowly and can be reversed with watering. Juglone-induced wilting often occurs rapidly, even when ample soil moisture is present.” (Penn State Extension, Landscaping and Gardening Around Walnuts and Other Juglone Producing Plants)
Extra water can ease the first wilting, but later the leaves brown and the plant dies. Verticillium wilt and herbicide injury look much the same, so first ask whether a walnut, yours or a neighbor’s, stands within 80 feet. Nothing cures a poisoned plant, so move it out of the root zone.
Leaves, hulls and wood chips
Leaves and hulls carry juglone into any bed they land in, so rake them off beds of sensitive plants. Pull the walnut seedlings that come up there while they are small.
All five sources below keep fresh walnut debris off sensitive plants, and they split over compost and chips:
| Source | Leaves | Hulls | Bark and wood chips |
|---|---|---|---|
| The Morton Arboretum | Kept out of beds, and never mulched or composted for garden plants | Kept out of beds | Never mulched or composted for garden plants |
| Purdue University | Never a mulch for sensitive plants, since even compost may release juglone | Kept out of beds | Never a mulch for sensitive plants |
| University of Wisconsin-Madison | Kept out of beds | Kept out of beds | Never a mulch |
| Virginia Cooperative Extension | Raked up and composted, or used nowhere | Raked up and composted, or used nowhere | Composted, or used nowhere, like any part of the tree |
| Iowa State University | Safe after two to four weeks in an active pile | Safe after six months of composting | Safe after six months of composting |
Microbes with air and water break juglone down, and a working compost pile gives them both. Leaves go quickly, while hulls and chips are woody and take months. Two of the five still keep composted walnut off garden plants, so compost bound for tomatoes gets the full six months. Under the walnut, among plants that tolerate it, leave the leaves where they fall.
What a walnut gives back
Black walnut is one of the most coveted native hardwoods, and its chocolate-brown heartwood goes to veneer, fine furniture and gunstocks. The nuts are rich and in demand for baking and ice cream, if you beat the squirrels to them. Squirrels bury nuts and forget many, which is how most new walnuts start, and the kernels feed wildlife as hard mast.
An open-grown walnut may drop a few nuts at 4 to 6 years old, and large crops wait until 20 to 30. A second walnut nearby improves the crop, as the pollination planner shows. Blue Whale Trees sells black walnut as saplings and as nuts from heavy-bearing parents. Plant it 50 feet from the house and the vegetable garden, and the site check tests a spot at that distance.
How far to trust the tolerant lists
Every list of sensitive and tolerant plants comes from what people have seen in gardens, orchards and woods, and few plants have been tested. One arboretum says so plainly:
“Much of what is known is based on one study done many decades ago. This means that any list regarding this topic is incomplete and some of the information may be anecdotal.” (The Morton Arboretum, Black Walnut Toxicity)
So the lists part at the edges. The Morton Arboretum lists hackberry and rhododendron as sensitive, while Wisconsin lists hackberry as tolerant. Penn State lists pinxterbloom azalea, a native azalea of the rhododendron genus, as tolerant. One list even puts crabapple on both sides.
The lists agree on the walnut’s own woods. Of the eight trees the Forest Service names as black walnut’s chief associates, seven are on at least one tolerant list and on no sensitive one. Basswood, the eighth, grows with walnut in the wild, and three lists still call it sensitive.
Native trees and shrubs that take juglone
These shrubs and small trees of ours are on at least two tolerant lists, and no list names them as sensitive:
| Plant | Kind | Zones | Height, feet | Light |
|---|---|---|---|---|
| American hazelnut | Shrub | 4–9 | 8–12 | Sun to part shade |
| Black elderberry | Shrub | 3–9 | 6–12 | Sun to part shade |
| Blackhaw viburnum | Shrub | 3–9 | 12–15 | Sun to shade |
| Ninebark | Shrub | 2–8 | 5–10 | Sun to part shade |
| Spicebush | Shrub | 4–9 | 6–12 | Sun to shade |
| Witch hazel | Shrub | 3–8 | 12–20 | Sun to shade |
| American plum | Small tree | 3–8 | 15–25 | Sun to part shade |
| Downy serviceberry | Small tree | 4–9 | 15–25 | Sun to part shade |
| Eastern redbud | Small tree | 4–9 | 20–30 | Sun to part shade |
| Flowering dogwood | Small tree | 5–9 | 15–30 | Sun to shade |
| Pawpaw | Small tree | 5–9 | 15–30 | Sun to shade |
| White fringetree | Small tree | 4–9 | 12–20 | Sun to part shade |
Among larger trees, the hickories and black locust are on all five lists, and American persimmon is on three.
What to plant beneath a walnut
Plant the walnut’s own understory: the shade-tolerant shrubs and small trees from the table, in the leaves the tree drops. Keep tomatoes and the rest of the sensitive list 50 to 80 feet out, or grow them in a raised bed lined against roots and filled with new soil. A driveway or a stone wall between the garden and the tree also limits the roots.
Every plant in the Shade Understory Pack is on a tolerant list except eastern leatherwood, which no list rates either way. The companion species tool starts from the walnut and shows the natives that share its floodplains and rich slopes, with the shrubs that live in its shade. It leaves out every azalea, rhododendron, blueberry, crabapple and hackberry we grow, and basswood, which the arboretum quoted above lists as sensitive. It puts first, and marks, the plants on the arboretum’s tolerant lists, and matches the rest by habitat, sun and soil.
Common questions
What vegetables grow near a black walnut tree?
Beans, beets, carrots, corn, melons, onions, parsnips and squash are on the tolerant lists. Tomatoes, peppers, potatoes, eggplant, cabbage, asparagus and rhubarb belong outside the root zone.
Can I plant hydrangeas near a black walnut?
The native wild hydrangea is on two tolerant lists. Hydrangeas in general appear on three sensitive lists and one tolerant list, so keep other kinds outside the root zone.
Can I compost black walnut leaves and hulls?
In an active pile, leaves are safe after two to four weeks, and hulls and chips after six months or more. Some extension services keep walnut compost off garden plants altogether, so give compost for tomatoes the full six months.
Does black walnut kill grass?
No: Kentucky bluegrass and tall fescue grow well under walnuts except in drought, sometimes better than in the open lawn. The soil under a walnut tends to be less acid than the soil beyond its roots, which may explain it.
Sources
- Black Walnut Toxicity: The Morton Arboretum: juglone in every part of the tree and most in the buds, hulls and roots, less in leaves and stems, the most under the crown, a root zone averaging 50 to 60 feet, toxicity for several years after removal, relatives that rarely harm sensitive plants, the sensitive and tolerant lists and their basis in one old study, Norway spruce and hydrangea among the sensitive plants and wild hydrangea among the tolerant, and no walnut leaves, bark or chips as mulch or compost for garden plants.
- Black Walnut Toxicity: University of Wisconsin-Madison Extension, Plant Disease Diagnostics Clinic (Ann Joy, Brian Hudelson and Laura Jull, 2024): effects to 80 feet, the symptoms and the discolored water-conducting tissue, wilting within days of planting or years later as a young tree grows, confusion with Verticillium wilt and herbicide injury, no cure, root pieces releasing juglone for several years, leaves and hulls kept out of beds, seedlings pulled, bark and wood never a mulch, the sensitive list, and hackberry and vegetables on the tolerant list.
- Landscaping and Gardening Around Walnuts and Other Juglone Producing Plants: Penn State Extension (Dan Roman and Jim Sellmer, updated 2026): the harmless precursor converted by sensitive plants and by oxidation, respiration inhibited, the quotation on wilting and its later course, poor solubility, wet airless soil and microbes, persistence for years, raised beds lined against roots and filled with new topsoil, and the tolerant lists with pinxterbloom azalea and wild hydrangea.
- Growing Tomatoes in the Home Garden: Ohio State University Extension (Brad Bergefurd, HYG-1624, 2017): tomatoes and other sensitive plants stunted or killed within one to two months in the root zone, a toxic zone averaging 50 to 60 feet and up to 80, widening as the tree grows, and young trees whose roots spread across twice their height.
- Black Walnut Toxicity: Purdue University Cooperative Extension Service (Michael N. Dana and B. Rosie Lerner, HO-193): juglone as a respiration inhibitor, poor solubility, live and decaying roots, the most juglone under the crown, relatives that rarely harm plants, a rock wall or driveway as a barrier, compost that may still release juglone, lists built on observation, basswood, mountain laurel and hydrangea among the sensitive plants, and crabapple on both lists.
- The Walnut Tree: Allelopathic Effects and Tolerant Plants: Virginia Cooperative Extension (Bonnie Appleton and Roger Berrier, Publication 430-021, 2000): hydrojuglone turned to juglone by air and soil, the chocolate-brown heartwood, rain carrying juglone off the leaves, roots meeting within a quarter to half an inch, roots beyond the drip line, drainage and organic matter, debris raked up and composted, the tolerant list, and lawn grasses and higher pH under walnuts.
- Black walnut hulls and shells in the compost?: Ask Extension, answered by Aaron Steil of Iowa State University Extension and Outreach (2021): walnut leaves safe after two to four weeks of composting, hulls, shells and chips after six months, and hulls slow to break down because they are woody.
- Black Walnut: USDA Forest Service, Silvics of North America: tomatoes especially susceptible, the chief associated species, stumps of small trees that sprout, timber for veneer, furniture and gunstocks, nuts for baked goods, ice cream and wildlife, regeneration from nuts squirrels bury, bearing ages, and bluegrass under walnuts.