Allelopathic Toxins and the
Black Walnut (Juglans nigra)
By: Kathleen A. Travis
Ecology
Fall 2000
History
• Pliny the Elder (A.D. 37) a Roman
naturalist, noted the harmful effects of
Black Walnut trees on the plants around
them.
• Native Americans would often use
juices from the hull of the Walnut
to poison and catch fish in
small streams and ponds.
Location
• Black Walnut trees are found in the
Midwest and East in the United States.
• Some varieties of the Black Walnut are also
found in Europe.
Plant Allelopathy
• Plant allelopathy: the secretion or
admission of a toxin or chemical from one
plant in order to inhibit the growth of
another plant
• Allelopathic chemicals themselves are a
part of a diverse group of substances such
as alkaloids, terpenes, and organic
cyanides which can be naturally
synthesize and released by plants.
Mechanisms of Release
• Exuded through glands on roots or in
leaves
• Released by the decay of fallen leaves
and dead roots
• Leached from the leaves when rain or
fog passes over the leaf and drips off
• Volatilized and then attached to soil
particles
Jugalone Toxin
• Found in the roots, leaves, and in the
husks surrounding the seeds of Black
Walnut trees
• Will not kill everything, but it
does affect a variety of plants
commonly used for
landscaping and various
garden vegetables
Symptoms of Walnut Toxicity
• Symptoms begin with the wilting of the
terminal shoots followed by an
irreversible wilting of the rest of the
plant. (callus build in xylem)
• If the plant has only a mild reaction to
the toxin, poor flower development and
stunted growth are noted.
Experimental Objective
• The objective of this experiment was to
test the tolerance levels of some
grasses used for landscaping purposes
in this region to the jugalone toxins. A
few other garden (tomatoes and
potatoes) and floral (hostas and day
lilies) plants were also tested for
susceptibility.
Procedure
• Grasses chosen: Alliance Rye, Annual
Rye, Kentucky Bluegrass, Wolfpack Tall
Fescue, and Creeping Red Fescue.
• The grass seeds were planted and allowed
to grow in Bellarmine's Green House for
two weeks. Each variety of grass was
planted in a separate pot, and four pots of
each type of grass were grown.
Walnut Extract
•
•
•
•
Solution #1, one walnut : one liter H2O
Solution #2, three walnuts : one liter H2O
Solution #3, five walnuts: one liter H2O
Husk cut up before soaking to increase the
amount of surface area
• Soaked in a refrigerated room for two days
with stirring every 12 hours
• Walnuts removed and the solution
was refrigerated until its use (3 days)
• Solution #1, solution #2, and solution #3
were each added to one of the pots
containing each variety of grass
• One tomato and one potato were treated
with solution #2, then solution #1
• One day lily and one hosta were both
treated with solution #3.
Problem!
• No response form the grass!
• So...one whole walnut was added to each
pot.
• These Walnuts were allowed to naturally
degrade over a period of two weeks.
• A clear response could be seen after two
weeks.
Results
Plant
Final Height of Controls Solution #1 Solution #2
Alliance Rye
7-8 inches
No reaction No reaction
Annual Rye
7-8 inches
No reaction No reaction
Kentucky Bluegrass
4-5 inches
No reaction No reaction
Wolfpack Tall Fescue
10-12 inches
No reaction No reaction
Creeping Red Fescue
5-6 inches
No reaction No reaction
Tomato
Some yellowing dead
Potato
Some yellowing dead
Hosta
Day Lily
-
Solution #3
Whole Walnut
No reaction
Inhibited Growth (3-5in)
No reaction
Inhibited Growth (3-5in
No reaction Inhibited Growth (3in) and half dead
No reaction Most grass dead remaining strands 5in
No reaction
dead
dead
dead
No reaction
No reaction
No reaction
No reaction
Plants Susceptible and
Resistant to the Jugalone Toxin
Susceptible to Jugalone Toxin
Apple
Tomatoes
Azaleas
Wild Hydrangea
Blackberries
Blueberries
Cabbage
Cotoneaster
Evergreens
Hydrangea
Laurels
Mountain laurel
Norway spruce trees
Peppers
Potatoes
Rhododendron
Silver maple
Resistant to Jugalone Toxin
Astibe
Hollyhocks
Beans
Hostas
Black raspberry
Jacob's ladder
Bugleweed
Japanese maple
Canadian hemlock Lungwort
Carrots
Melons
Christmas fern
Multiflora rose
Clematis
Pansies
Common Hyacinth Pot-marigold
Crocuses
Snowdrops
Daffodills
Squash
Darwin tulips
Summer phlox
Dayliles
Weeping forsythia
Epimediums
Grape hyacinths
Additional Information
• Some of the test subjects were also
allelopathic plants
• Many turf grasses are allelopathic
including Perennial Rye, Kentucky
Bluegrass, and Creeping Red Fescue.
• Other allelopathic plants include
Chrysanthemums, varieties of apples,
Annual Sunflowers (Helianthus annus),
quack grass, crabgrass, nut sedge,
Canada thistle, and lamb's-quarters.
Conclusions
• This experiment has examined one aspect
of the jugalone toxin in Black Walnuts, its
toxicity. We have been able to deduce
which grasses of the test subjects are
most conducive to life under a Black
Walnut tree and those that should be
avoided. Perhaps in time, the inhibited
grasses would be able to develop
resistance to the toxin if allowed to grow in
its presence for a long duration.
One Final Note
• In recent years more work has been done
in the field of plant alleopathy. This work
has lead to the discovery of medicinal
uses for the jugalone toxin. Jugalone is
considered an antiseptic, a germicide, a
parasitic, and a laxative. Perhaps more
disease fighting uses can be found for
allelopathic chemicals, as they are
examined further.
Works Cited
• Abraham, D. "The Green Thumb."
Consumer's Research Magazine. April 1992: 37-38.
• "The Curse of the Black Walnut." Country Journal.
December 1997: 80.
• Fuchs, Lucy. "Natural Connections." American
Horticulturist. February 1995: 14-16.
• Meyer, P.W. "Underground Battles." Horticulture.
October 1990: 11-13. 5.
• Orr, Tamra B. "Black Walnut." Better Nutrition. March
1999: 44-45.
The End