DO BLACK WALNUT TREES HAVE ALLELOPATHIC EFFECTS ON OTHER PLANTS? (HOME GARDEN SERIES)

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DO BLACK WALNUT TREES HAVE
ALLELOPATHIC EFFECTS ON OTHER PLANTS?
(HOME GARDEN SERIES)
Linda Chalker-Scott, Associate Professor and Extension
Horticulturist, WSU Puyallup Research and Extension Center,
Washington State University
FS325E
Photo courtesy: D. Schreiber

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DO BLACK WALNUT TREES HAVE ALLELOPATHIC EFFECTS ON
OTHER PLANTS?
Gardeners have heard that black walnut trees
(Juglans nigra; Figure 1) contain a toxic chemical
called juglone that will kill any other plants growing
nearby. This phenomenon is called allelopathy, and,
in recent years, many other possible allelopathic
chemicals have been identified. This publication will
review the current research into black walnut
allelopathy and discuss the practical significance of
gardening in the presence of black walnut trees.
What is allelopathy?
Allelopathy literally means “death to others.” In its
simplest sense, allelopathy is the ability of one plant
species to affect the growth of another through their
chemical exudates. While early researchers narrowly
defined the relationship to include only the influence
of living plants on other living plants (Rice 1974),
more recently the definition has been expanded to
include interactions within the soil environment.
Such “soil-mediated chemical interference” (Inderjit
and Foy 2001) is arguably a more relevant definition,
as roots of adjacent plants share the same soil
environment. Competition for water and nutrients,
soil microbial activity, and other environmental
conditions should be considered separately from any
possible “chemical warfare” among plants.
Therefore, allelopathy is currently understood to
involve living or dead plant parts that release
chemicals into the soil which have an effect on other
plants—positive or negative. And no allelopathic
chemical is better known—and feared—by gardeners
than juglone.
Walnut allelopathy and juglone
Writers from ancient Rome to current day have
warned that walnut trees are detrimental to the health
of nearby plants (Willis 2000). Initially, injury was
attributed to the dense shade and extensive root
systems of walnut trees. In the last 100 years,
viewpoints have shifted to claims of chemical
Figure 1. A mature black walnut tree in an arboretum. Photo by Linda Chalker-
Scott.
poisoning of the soil, despite the lack of any direct,
supporting evidence.
In the 1920s, damage to tomatoes and other crops
near walnut trees caused people to believe that toxic
chemicals were involved. However, many farmers
had fields near walnut orchards and saw no negative
effects on their crops. Still, the public perception that
walnut trees would kill other plants persisted and
grew, especially when a researcher in Virginia
noticed his garden tomatoes were struggling (Willis
2000). Aware of the lore surrounding walnut trees,
he correlated the nearby location of such a tree with
the damage he observed. Subsequently, he and other
researchers suggested that juglone, an orange
compound isolated from leaf litter and walnut hulls,
was responsible for the damage.
In 1948, the USDA issued a press release assuring
the public that walnut trees were harmless, but these
assurances failed to convince a skeptical public. In
1951, experimental field testing began: importantly,
little to no negative effects were seen (Willis 2000).
Experimentation moved to the lab, where juglone
applied to germinating seeds and seedlings was
found to cause stunting, wilting, and necrosis. The

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mode of action of juglone remains unclear (Strugstad
and Despotovski 2013), though it is thought to
disrupt photosynthetic and respiratory pathways and
interfere with water uptake (Achatz and Rillig 2014).
Experiments but no explanations
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
2014).
• 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
2016).
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
2001).
• 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).
An ecosystem approach to studying
juglone effects
Researchers interested in juglone and more widely in
allelopathy itself have suggested several suggestions
to modify conflicting, traditional approaches.
• First, researchers must accept that lab-based
bioassays that isolate juglone from the
natural environment cannot determine
whether allelopathy occurs in nature (Inderjit
and Nilsen 2003).
• Second, juglone must “accumulate to
phytotoxic levels and reach a target plant to
be of ecological relevance” (Choesin and
Boerner 1991). This may pose an
insurmountable problem for juglone work:
Sun et al. (2013) found that while walnut
trees release large quantities of juglone into
the rhizosphere, very little reaches the bulk
soil and thereby other plant roots.
• Finally, researchers “must show that
chemicals contributed by specific plants are
primarily responsible for growth inhibition in
field situations” (Inderjit and Foy 2001). This
means demonstrating conclusively that
growth inhibition is not due to competition
for soil resources, excessive shade, or any
other environmental factor.

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Evaluating evidence for juglone
allelopathy
Despite the lack of confirming evidence, websites
(Leuty 2010), Extension publications, and research
articles (Achatz and Rillig 2014; Strugstad and
Despotovski 2013) continue to claim black walnut
has allelopathic effects on garden and landscape
plants. As evidence, all of these publications cite one
or two Extension publications (Crist and Sherf 1973;
Funt and Martin 1993) that contain lists of
“sensitive” and “tolerant” garden and landscape
plants (Figure 3). These two publications are not
experimental but simply observational—meaning
that they correlate the presence of walnut trees with
damage to other species but do not confirm a
causative relationship. Furthermore, there is
substantial visual evidence in gardens and
landscapes that other plants successfully coexist with
established black walnut trees (Figure 4).
It’s important to note that you’re not likely to find
either of these two Extension publications online or
in print. As the Extension Communication Manager
for Ohio State University explains “…fact sheets that
are more than five to seven years old are not
available on Ohioline until the content of each is
reviewed by the author(s) and validated or
revised…The fact sheet you are looking for was
published in 1993 [Funt and Martin], so it is now 25
years out-of-date.
Figure 4. Turfgrass, ferns, and herbaceous and woody perennials all flourish
within the rootzone of this established black walnut tree (upper right corner).
Photo courtesy of Phil Reilly.
I don’t have a copy of the fact sheet, b/c it was
removed from circulation” (Cheryl Buck, email
communication).
Even more concerning is that the Crist and Sherf
(1973) publication does not actually exist. One of the
librarians at Cornell University reports, “Here’s our
final conclusion about the Cornell Horticulture
Extension Bulletin Walnut Wilt by Crist and Sherf.
We think that the first time it was cited, it was cited
incorrectly…It seems likely that all the other authors
who have cited it since probably never saw the
bulletin and just copied the citation. We’ve checked
Table 2: Influence of juglone on some plants (Funt & Martin 1993; Leuty 2010)
Negatively affected
Unaffected or hardly affected
Asparagus officinalis (asparagus)
Betula papyrifera (white birch trees)
Brassica oleracea (cabbage)
Lycopersicon esculentum (tomato)
Magnolia x soulangiana (saucer magnolia)
Medicago sativa (alfalfa)
Solanum melongena (eggplant)
Solanum tuberosum (potato)
Pinus strobus (white pine)
Vaccinium (blueberries)
Tilia americana (linden trees)
Allium cepa (onion)
Beta vulgaris (beets)
Pastinaca sativa (parsnip)
Phaseolus zinnia (lima and snap beans)
Prunus spp. (cherries, nectarine, peach, and plum)
Rubus occidentalis (black raspberry)
Most squashes
Zea mays (sweet corn)
Most of the hardy, fall-planted bulbs, including alliums,
crocus, daffodils, hyacinth, tulips, and a series of
ornamental plants
Figure 3. Adapted from Strugstad and Despotovski, 2013.

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the indexes of all the likely bulletins and looked at
departmental annual reports (which list research
published by members of the department) and we
can’t find any trace of an article/bulletin called
Walnut Wilt” (Betsy Elswit, email communication).
Thus, the entire body of primary evidence for black
walnut allelopathy in the landscape is attributed to
two dated Extension publications, one that has been
withdrawn from circulation and one that doesn’t
exist. These are not reliable sources of information
and should not be cited as evidence for juglone
toxicity, especially in peer-reviewed journal articles.
Action items for gardeners
• Provide adequate irrigation for landscape
plants during drier, warmer weather. All
plant roots compete for water, nutrients, and
oxygen. The denser your plantings, the more
intense competition will be.
• Plant sun-loving plants in full sun—not under
tree canopies. Shade tolerant species are the
best choice under walnut trees (Figure 5).
• 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
species.
Figure 5. An attractive selection of shade-tolerant perennials under a mature
walnut tree. Photo courtesy of Emma Murphy.
Literature cited
Achatz, M., and M.C. Rillig 2014. Arbuscular
mycorrhizal fungal hyphae enhance transport of the
allelochemical juglone in the field. Soil Biology and
Biochemistry 78:76–82.
Buck, C. 2018. Email communication. Ohio State
University.
Choesin, D.N., and R.E.J. Boerner. 1991.
Allyisothiocyante release and the allelopathic
potential of Brassica napus (Brassicaceae).
American Journal of Botany 78:1083–1090.
Crist, C.R., and A.F. Sherf. 1973. Walnut wilt.
Cornell University Horticulture Extension, Bulletin.
Elswit, B. 2018. Email communication. Cornell
University.
Funt, R.C., and J. Martin. 1993. Black walnut
toxicity to plants, humans and horses. Ohio State
University Extension Fact Sheet HYG 1148-93.
Inderjit, and E.T. Nilsen. 2003. Bioassays and field
studies for allelopathy in terrestrial plants: progress
and problems. Critical Reviews in Plant Sciences
22(3-4):221–238.
http://dx.doi.org/10.1080/713610857.
Inderjit, M.K. 2001. Soil: environmental effects of
allelochemical activity. Agronomy Journal 93:79–
84.

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Inderjit, M.K., and C.L. Foy. 2001. On the
significance of field studies in allelopathy. Weed
Technology 15(4):792–797.
http://www.jstor.org/stable/3988562.
Jilani, G., S. Mahmood, A.N. Chaudhry, I. Hassan,
and M. Akram. 2008. Allelochemicals: sources,
toxicity, and microbial transformation in soil – a
review. Annals of Microbiology 58(3):351–357.
Lee, K.C., and R.W. Campbell. 1969. Nature and
occurrence of juglone in Juglans nigra L. Journal of
Horticultural Sciences 4:31–35.
Leuty, T. 2010. Walnut toxicity. Ontario Ministry of
Agriculture, Food and Rural Affairs, Toronto, Ont.
http://www.omafra.gov.on.ca/english/crops/facts/inf
o_walnut_toxicity.htm.
Parepa, M., and O. Bossdorf. 2016. Testing for
allelopathy in invasive plants: it all depends on the
substrate! Biological Invasions 18:2975–2982.
Rice, E.L. 1974. Allelopathy. Academic Press, New
York.
Strugstad, M.P., and S. Despotovski. 2013. A
summary of extraction, synthesis, properties, and
potential uses of juglone: a literature review. Journal
of Ecosystem Management 13(3):1–16.
Sun, Y.Z., L.X. Yang, Z.Q. Wang, and J. Fan. 2013.
Temporal variations in soil juglone and soil
microbial community structure under Manchurian
walnut (Juglans mandshurica Maxim.) plantations.
Allelopathy Journal 31(1):169–179.
von Kiparski, G.R, L.S. Lee, and A.R. Gillespie.
2007. Occurrence and fate of the phytotoxic juglone
in alley soils under black walnut trees. Journal of
Environmental Quality 36:709–717.
Willis, R.J. 2000. Juglans spp., juglone and
allelopathy. Allelopathy Journal 7(1):1–55.
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