Date: 26/04/2018
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The Black Walnut Tree - Beauty, Strength & Toxicity.

The Black Walnut (Juglans nigra) tree is a very large, flowering tree that produces a strong central leader and a broad-spreading canopy of dense, heavy branches.  Deeply furrowed nearly black-colored bark, alternate, odd-pinnately compound leaves, uniquely drooping catkins, and an unquestionably beautiful internal wood grain, Black Walnut trees are recognized by most everyone.  Reaching heights of up to 120 feet tall, Black Walnuts often grow wider than they do tall.  Lasting for upwards of 300 years, this slow-growing, long-lived tree has long been used as a pioneering species for timber, food, and dye production.  

While there are many upsides to Black Walnut, there is also a hidden downside: a downside for plants, horses, and even humans the production of Juglone (5-hydroxy 1, 4-naphthalenedione; C10H6O3).  Formed not only in Black Walnut, the entire Juglandaceae family produces this allelopathic compound.  Nut producing Walnuts, Pecans, and Hickory trees all produce Juglone.  Found in the leaves, bark, fruit, and at even higher concentrations in the roots of Black Walnut, this organic compound has an allelopathic effect on nearby plant growth.  Juglone affects trees by acting as a respiration inhibitor, ultimately depriving nearby plant life (those sensitive to Juglone) of the needed energy required to fulfill their own metabolic activity: photosynthesis, transpiration, and respiration  Unfortunate is that Juglone affects not only plant life, but can also harm horses, dogs and livestock, and even humans:       

In horses, a crippling disease known as Acute Laminitis occurs in the hooves of horses residing in stalls that used Black Walnut chips for bedding materi Acute Laminitis causes hoof inflammation in horses ultimately leading to the animal's inability to walk. 

In dogs and livestock, as the husks drop from the tree, a threat of these animals consuming the Black Walnut fruit could become lethal.  These fallen, decomposing husks contain Penitrem A, a mycotoxin created by Penicillium Mold - a colonizing fungus that affects the fruit as it decomposes.  Scary is that Penitrem A is a neurotoxin sending its toxicity to mammals through various stages: tremors, ataxia (dysfunction of nervous system leading to loss of coordination of muscle movements), and ending with little chance of recovery

In humans, we too must be mindful of Penitrem A as well.  This fungus can leak toxins into the nuts themselves leading to a product that is not fit for consumption after an extended amount of time on the groundii.  Black Walnut also seems to be a heavy pollen tree giving allergic symptoms to susceptible humans.  The Juglone compound also stains concrete, vehicles, and roofs if left untouched for long periods of time and if touched, it will stain clothing and hands. 

In our natural world, we know the higher the diversity, the greater the chance of survival.  This diversity has allowed some plant life to co-exist alongside Black Walnut while other plants die after only a short time.  Interesting is that the more natural, native plants seem to fair better beneath a Black Walnut than our cultivated varieties of vegetable crops.  Extensive lists of both tolerant and susceptible plants can be found online, but I chose to list plants that I have experience with:

Plants Tolerant to Juglone


Plants Sensitive to Juglone








Christmas Fern

Silver Maple


Many Pines

Poison Ivy




Fescue Grass


Virginia Sweetspire








Burning Bush



Japanese Larch



Black Locust

White Birch




Norway Spruce


Working alongside Black Walnuts can be challenging in our landscapes and removing the allelopathic chemical Juglone is nearly impossible as the chemical is highly insoluble and does not move through the soil fast, even with abundant watering.  Found in the leaves, bark, fruit, and roots, Juglone makes for one very defensive chemical.  However, Juglone does begin to break down once exposed to air, water, bacteria, and time, showing to be non-toxic in leaves within two to four weeks, though it takes much longer to become non-toxic in soils and in decaying roots.

Black Walnut trees are well rooted trees that help to prevent soil erosion and provide vast amounts of dense shade.  But with these extensive root systems, the presence of Juglone in soils surrounding a tree can remain for month.  Other reports put the presence of Juglone remaining in soils for years depending on soil type and drainage.  In order to work effectively alongside Juglone, you are better suited to change the planting material rather than trying to change the soils toxic effect on susceptible plants.  Many plants placed in Black Walnut soils last one to two months before necrosis.  Raised beds make a good option to allow for vegetable gardening under a Black Walnut, so long as leaf litter and fruit remains cleared from these beds.    

Black Walnuts have a long, rich history with most anyone that has ever lived east of the Mississippi River.  In our natural and landscaped world, each unique species holds certain traits that are beneficial to that organism.  It seems that the Black Walnut is a survivor.  Planting directly beneath the drip zone of a Walnut can be more trouble than it is worth due to the presence of Juglone, but certainly with a little forethought and understanding of the allelopathic effects on specific plant material, the ability for Black Walnuts to co-exist in our yards can remain possible well into the future.

If you have any questions about how to choose the best tree for your property in Nashville or Middle Tennessee. Or  if any other tree questions or need a Tree Service, Call Us 615.832.2410 or Contact Us Here


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Community Tree Preservation
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Nashville TN 37211-4924
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