Black Walnuts - allelopathic?

This came up in another forum so I am posting it here.
My yard has a massive 70 foot tall Black Walnut tree. It is a small yard so everything grows near the tree or close to it. All fruit trees get mulched with Walnut leaves. Tomato beds get Black Walnut compost.
I’ve not noticed any negative effects on any of my fruits or vegetables.

This article says essentially the same thing.

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Juglans nigra - the subject of this post and the reference, are native to the eastern U.S. It is cultivated in many places.

Juglans californica sometimes called California Black Walnut is native to southern CA and valued for its disease resistance.

A discussion of these species is given in:
Aradhya, M. K. et al. 2007. Molecular phylogeny of Juglans (Juglandaceae): a biogeographic perspective Tree Genet. Genomes 3:363-378.

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My yard had a black walnut between the sidewalk and the street. One set of roots extended to one of the front gutters. No tomato or potato could live between sidewalk and street or near the gutter. Garlic, squash and other plants were severely stunted. Even thyme eventually succumbed, though parsley, chives, oregano, mint, sorrel and other herbs survived. Live BW roots is another matter compared to dead BW material. I did have BW leaves in mulch and they did not affect mulched plants much.


Juglans Nigra – that is what I have growing in my yard.

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You’re one blessed dude if you are growing tomatoes successfully under a black walnut tree.
(But, sometimes one house in the middle of 25 others that are leveled escapes in a tornado, too.)

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Black walnut…at least in my yard have got to be one of the fastest growing trees i’ve seen. I have 3 now all planted by squirrels and they have exploded in height in just a few short years. I leave them because of the shade they provide although i did prune some of the lower large branches off one of them. They also fruit pretty fast from seed…just a few years and you can have at least a handful of nuts.

Raspberries dont’ seem to care much because they grow right under one of them.

Black Raspberries at least are supposedly Juglone insensitive: Plants tolerant of black walnut toxicity | The Morton Arboretum

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Black walnut is seriously allelopathic to many species including most fruit trees. Blue Grass is insensitive to juglone and grows symbiotically with walnut. Maples are highly sensitive. Juglone is produced heaviest by the roots. Leaves have relatively little but will affect sensitive species such as solanums (tomato, potato, eggplant, pepper).


Have you found Black Walnut to hurt your tomatoes/fruit trees? In your personal observation?

I have grown tomatoes for over a dozen years on this property. I have several mature fruit trees (15 years old or more) Cherry, Plum, Apple, Fig, Persimmon as well as Mulberry, Blueberry. And many new ones (loquat, pears, quince etc.). They all do decently well.

Oh, I also have several Japanese Maples not far from the Black Walnut. They are also doing very well.

I am attaching a few excerpts from the original article I referenced:

Inconsistent results have been the bane of juglone
allelopathy research. For every report of toxicity in a
tested species, another report will find no effect.
Researchers have pointed out a number of problems
with initial assumptions and laboratory trials, which
are summarized below:
• Juglone, a highly toxic chemical, is not found
in intact tissues of black walnut trees
(Strugstad and Despotovski 2013). Instead,
living tissues contain a nontoxic precursor
called hydrojuglone, which is transformed in
the soil to make juglone (Achatz and Rillig
• Most hydrojuglone is contained in the roots
and shells of walnuts (Figure 2). There is
little in the leaves and virtually none in the
wood (Lee and Campbell 1969).
• Many researchers use artificial experimental
methods to test for allelopathy. For instance,
soilless media and laboratory extractions of
juglone from walnut tissues do not occur in
nature (Inderjit and Foy 2001).
• Allelopathic responses are enhanced when
potting media are used instead of soil. This is
attributed to the increased permeability of the
media compared to soil (Parepa and Bossdorf
The lack of field test evidence to support laboratory
results has spurred critics to insist that experimental
testing include a functional soil system to more
closely mimic what happens in nature. They note
several conditions and activities that may account for
the lack of positive field test results:
• Juglone undergoes chemical, physical, and
biological degradation in the soil (Inderjit
• Organic matter and clay particles in soils can
bind juglone, reducing its movement within
the soil (Inderjit 2001).
Figure 2. Walnut surrounded by husk. Photo courtesy of Monika Pickles.
• Juglone does not persist in soils with high
microbial activity (Jilani et al. 2008; von
Kiparski et al. 2007).

And the conclusions by the authors:

Mulch well with arborist wood chips to retain
soil moisture and to nourish beneficial soil
life including mycorrhizae.
• Enjoy your walnut trees! Not only are they
robust landscape plants they provide food
and habitat for wildlife. In areas with
thousand-cankers disease, however, they
should not be planted.
• Use walnut wood chips for mulch if you have
them. They will not harm plants and work
just as well as those from any other woody

Have you verified you actually have a black walnut.?
Pecans, white walnuts, hickory…all have some juglone…but not nearly the same quantity/dosage.

Besides raspberries, sweet potatoes, tulips, daylilies, daffodils, oriental lilies, Lenten rose (helleborus), catmint, wormwood, golden rain tree, juniperus virginiana…………are some trees and plants I know will grow under a black walnut. The jury’s out on mountain laurel and deciduous azaleas…in other words I’m trying it and they look a little sick.

I just asked a question kinda about this…need to know if a black walnut is going to hurt my muscadines. Anybody know. Wild grape is tolerant but I can’t find info on muscadines.

K8t, I have muscadines growing in and all the way through the crown of black walnut trees with no issues. I would still avoid planting a muscadine in any area where a tree’s roots will be in competition because it will reduce production of fruit.

Ramv, do things your own way. I have no interest in disagreeing. What I do have is 50 acres of black walnut trees growing and producing walnuts. I also grow about 350 tomato plants each year. If you guess that I don’t plant tomatoes in the walnuts, you are correct.


Thanks! It’s not really all that close…40’ or more but if it was going to be an issue I’d rather take out a small young tree rather than a big one that had reached them!!

I had made this post over a year ago. You can examine the photos to see if it is really an Eastern Black Walnut.

That does not look like eastern Black Walnut. Where are you physically located? That picture looks more like Juglans Hindsii.

Edit: I did a bit of research. From what I can see, that might be a hybrid between eastern black walnut and Juglans Hindsii. They were commonly available from Luther Burbank about 100 years ago.

Does anyone know of nut-free cultivars of eastern black walnut? If they existed they would be a quick-growing shade tree. They have some problems - their compound leaves have sort of messy stems and the alleopathy. But good God, the rain of Freeman maple samaras was like snow in our neighborhood this year. So even they have their problems (e.g. poor branching structure).

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I also have a friend who has a small orchard in his spacious front yard. At the corner stands a large BW. Trees within 40 ft of it are stunted. He basically gets all his fruit from past the 45 ft line. he should trench at 30 ft.

Maple helicopters are the worst…especially if you have open gutters.

MY mil needs a new tree (west side of house). She had a maple but it had dual trunks and split right down the middle. Too bad because it provided a ton of shade. I think i’m going to plant a seedling oak in its place. Red oak. Don’t see a lot of them as yard trees around here.

The red oaks around here are all dying of oak wilt. The white oaks (rounded leaves) don’t get it.

Hmmmm… I think a park near here was hit bad with that a few years back. Pretty soon we won’t have any hardwoods left! With elms, ash all toast… I do have some burr oak seedlings…they just seem a little slower growing then the reds.

“Oak wilt is always fatal to trees in the red oak group, which includes northern red oak, northern pin oak, and black oak. Trees that were infected with the oak wilt fungus this spring will begin rapidly dropping their leaves in July and August.”

I didnt’ realize it was fatal to the tree. I’ll have to check mine over (i have many).