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.
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