Black Walnut Trees: Roots of Evil?

George and Becky Lohmiller
black-walnut-trees-roots-evil-ascending-the-giants-wikimedia-commons
Photo by Ascending the Giants: Wikimedia Commons

The black walnut tree is one of our most valuable and beautiful native trees, but it does have a “dark side,” too. (Humor required.)

The easily worked, close-grained wood of the black walnut has long been prized by furniture- and cabinetmakers for its attractive color and exceptional durability. Its logs are in such demand for veneer that “walnut rustlers” have made off with trees in the dead of night and even used helicopters in their operations.

A Brief History of Black Walnut Trees

The early settlers discovered black walnuts growing in mixed forests from Canada to northern Florida and west to the Great Plains. They found that its rich-brown heartwood was exceptionally resistant to decay and put it to use as fence posts, poles, shingles, and sills.

When surrounded by other trees in the forest, black walnut grows straight and tall with few, if any, lower branches.

When planted in the open, the tree will branch out closer to the ground, developing a spreading shape that makes it easier to harvest its sweet, round, two- to three-inch nuts.

The settlers snacked on the nutritious nuts out of hand, added them to soups and stews, and ground them into meal for baking; the hard shells provided a perfect package for storing the nuts over winter.

The “Dark Side” of Black Walnuts

Unfortunately, the black walnut’s roots, which may extend 50 feet or more from the trunk, do exude a natural herbicide known as juglone that prevents many plants from growing within their reach. 

Tomatoes, potatoes, apples, pears, berries, and some landscape plants such as rhododendrons, azaleas, and lilacs may be killed or stunted if grown in close proximity to black walnut roots.

A Great Shade Tree

In spite of this, black walnuts make great shade trees for larger properties. They commonly grow to 50 feet or taller and about as wide, and specimens of more than 100 feet have been recorded.

Black walnut’s large, fernlike foliage provides light, airy shade for those grasses and ground covers not affected by juglone. In autumn, the leaves turn bright yellow, contrasting nicely with the tree’s rugged, dark bark.

Black walnuts require a deep, fertile soil with a near-neutral or slightly acidic pH. They are pretty much disease-free and are threatened by few pests, with the exception of perhaps an occasional helicopter.

Do you have a black walnut tree? Please share your comments, questions, and advice!

Source: 

This article was originally published in March, 2008 and has been updated.

Reader Comments

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Black walnut trees

We had a beautiful black walnut tree in our back yard, but Dad had it cut down because it was killing everything else he wanted to grow back there. But the wood did make a gorgeous cedar chest. The finished wood looked like gray to black velvet. The gray just faded into small areas of dark and back again. Oh was it gorgeous.

Black Walnut Tree

After we have cut down our black walnut trees we want to plant maple trees near their stumps.

Is this a good idea.

trees

We have Black Walnut trees. They are beautiful. We take the nuts and replant to make new.

Walnut keep on giving

As a novice woodworker, the black walnut wood is beautiful. I am always looking for walnut trees that have served their useful life and now can be put to further beautiful use. Connect with me if you need to get rid of one near Dayton OH.

Black walnut trees

I nay have a few n since i have horses am considering thier removeal. Would you be interested in roing so since you want tge wood? Clean up would have to be meticulous n done in the winter

Roots of Black Walnut Tree lifting up concrete path

Our tract home was built in 1965 on land that was a walnut orchard. We have a very old Walnut tree in our front yard. It has lifted our walkway which is less than 2 ft. from the trunk. I would like to to build a new walkway approx. 16 ft. from the trunk. There are horizontal roots in that area. Can I cut those roots to allow for a walkway since it's 16 ft. from the trunk? FYI..the tree has 3 trunks that grew from the cut off center trunk before I purchased the home.

Black Walnut Roots

Hi Jan,

Yes, it would be safe to cut some of those roots without affecting the tree. Most tree roots go deep underground, leaving it plenty of routes to get water. 

The husk seems larger than last yr. around the nut this year (

does this mean the weather this winter is going to be cooler ??

walnut weather??

There is an adage that suggests a large crop of walnuts indicates a cold and snowy winter but we do not know of a proverb about the husk. Keep watch this winter and maybe you’ll see a relationship…and you can “invent” a proverb to fit the conditions!

Be careful where you plant

A previous owner planted a black walnut tree in front of my place decades ago. It's not particularly attractive, way too big for the space it's occupying, and during the years when it produces nuts (which to my relief isn't every year) there are a couple of months where you literally can't walk past my house without tripping on the things. I try to keep them picked up because I don't want anyone hurting themselves or for a dog to bite into one (the paste that coats the nut between the outer and inner shells is highly toxic) but as soon as you've collected one bushel basket full, down comes another. Add to this the fact that these trees can last for literally hundreds of years, grow to enormous size, and cutting them back only makes them produce MORE nuts, and you've got a monster on your hands.

Trimming Black Walnut

We are in MN. Is there a certain time of year that black walnut trees should be trimmed, whether the branch is dead or alive?
Thank you.

Pruning black walnut trees

Prune during the dormant season. Do not apply paint or other sealants to pruning cuts. Wait until walnut trees are 3 to 4 inches in diameter. Trees up to 10 inches in diameter may benefit from pruning. 

Do not remove more than 25% of the live crown in a single pruning or prune higher than 50 percent of total tree height. (The “crown” of a tree refers to the branches extending from the trunk or main stems. The “live crown” refers to the part of the crown that has live foliage.)

Prune only branches less than 2 inches in diameter, preferably less than 1 inch. Larger pruning cuts heal slowly and may lead to decay and ring shake. Prune up to 9 feet high; if possible, prune timber trees up to 17 feet in two or more pruning operations. Stem forks, crooks, or large branches may restrict pruning height.

apples trees

I have a 25 ft black walnut tree I've grown from a nut brought from Ontario to Vancouver Island.
There is an apple tree growing within 10 ft of the tree. Will te roots of the walnut kill my apple tree and will they affect the fruit in any way.

apple and black walnut

The only “apple” trees that we can find in our information/sources that tolerates the juglone (the chemical in black walnut tree buds, nut hulls, and roots) are crabapple and mayapple. We checked several, so we would say no, the apple will not survive. Our sources suggests that the highest concentration of juglone is under the canopy but toxicity symptoms can appear on plants that are, on average, 50 to 60 feel from the tree trunk.

BTW, we are in awe of the fact that you grew your 25-foot black walnut from “seed”!

Nuts on the ground, the fall harvest

I have about 8 walnut trees on my property, with one being in a clearing. I have noticed these little white worms, quickly invading the green outer shell of the nut on this one particular tree within days of being on the ground. what are these little buggers?

black walnuts

The Black Walnuts are very large this year but many the nut isn't ripe it's dried out - what would cause this ?

Black Walnuts are also anti-parasitic!

They are at least anti-parasitic, for raccoons. (although I have seen black walnut powder listed as an ingredient in various nutritional supplements to detoxify or de-worm humans, too)... Anyway, after discovering a litter of abandoned baby raccoons, I sought out my local nature center, to take them in, and help them out. I was turned away, because they didn't take such animals, but they gave me the phone number of a certified wildlife rehabber, who did. This person was overwhelmed with the critters, so she took them with the condition that I was willing for her to teach me how to do it, and officially sub contract me, and have me do it, myself. This is how I learned that black walnuts are a favorite food of raccoons. Not only is it nutritious, but the walnuts provide a natural anti-parasite property, that helps to prevent or relieve intestinal worm infections. On a side note, let it be known that raccoons do not prefer to eat garbage, and are actually very selective about what they eat, but unfortunately, they don't always have enough areas for foraging for such foods. They actually eat a diet very similar to bears... Just on a smaller scale. I live in the country, where they have plenty to eat, and they've never gotten in my garbage. But similarly, to bears, I suppose some of them get into the habit, out of convenience. Sorry for going off topic, but perhaps this will explain why wild animals are attracted to these trees, on people's yards.

Black Walnut Tree Watcher

Not someone who knows much about trees, I was surprised to find out the trees that line my country road, where I wait for my kids at the bus stop, are actually walnut trees. I actually thought they were dropping some kind of apple-like fruit. There is a group of 7 crows that live in one of them, and I have enjoyed watching them eat the walnuts (as I now know them to be), by knocking them off the branches, and then centering them in the dirt road, along the tire ruts, for cars to run over them and crack them open. Crows are smart! It makes even more sense, realizing these are not apple-like fruit, but actually nuts. The walnut trees line a pasture for cattle and corn, alternately. They are very, very tall old trees. And indeed, they do provide a lot of shade. One of them has the remnants of a fort, built into it... a fort that must have been built some time ago... now that fort is at least 15 feet from the ground. Thank you for posting this article so I could learn more about my favorite trees.

Black Walnut Tree Losing Branches

Hi! I have a very large, tall black walnut tree at the front of my house -near the drive way. It has no signs of disease or that it is dying -- but it is dropping large branches! The branches have green leaves and walnuts and seem to be falling for no reason. Some of the branches have been so large that I have considered taking it down because it is dangerous. Have you ever seen this happen before?
Thanks!

Black walnut dropping branches

We consulted a cooperative extension for your question: The weight alone of walnuts typically does not cause limbs to crack and fall. There may be an underlying pest or disease problem. How old is the tree? Do you see any yellowing or dying leaves? There is a disease of walnut trees called Thousand Cankers Disease caused by the Walnut Twig Beetle recently spotted in Ohio. Following is a link to an OSU announcement about the Walnut Twig Beetle:
https://ag.purdue.edu/aganswers/Pages/archive.aspx?story=229#.U8RE_bFAQrw

Stacy, you can consult a cooperative extension service in your area. Click on your state for details: https://www.almanac.com/content/cooperative-extension-services
It may be that you need to consult an arborist.

Good luck!

 

Early frost

I have a walnut tree that was starting to leaf out this spring, when we received bitter cold weather and heavy wet snow for 3 days. The leaves turned black and crisp. Now it's the first is September and it has no leaves at all. I've been keeping suckers trimmed along base. I fertilized the tree with deep root waterer this summer. It still is green when I knock it. Am I wasting my time? Or may it come back next spring? It was beautiful last year. Also I have bulbs, bleeding hearts, cosmos, California poppies growing underneath. I miss my tree. Thank you

Fail to leaf after spring frost

This one is beyond our ken, E, but we found some information that may shed some light on your tree’s failure to leaf. (Sit down; there’s a lot to read.)

This page, from the Oregon State cooperative extension service addresses frost situations and more: http://horticulture.oregonstate.edu/system/files/u1473/em8907.pdf

Purdue says plainly: “Black walnut is very susceptible to light freezes…” See here: https://www.extension.purdue.edu/extmedia/FNR/FNR-119.html

You can, and should, consult your local cooperative extension service for local advice. See here and choose your state: https://www.almanac.com/content/cooperative-extension-services

Walnuts planted in 1972

Planted 100 seedlings in 1972, then another 3,500 in 1987, of which over 1,000 (drought year in 1988) are still alive and now growing well. This article says the seeds are sweet and two to three inches. Black walnuts have a strong, bordering on bitter, flavor, and in my experience average about an inch in diameter. Some say that they are a slow growing tree. I've got some second-generation volunteers now approaching 50 feet in height, had to remove one second generation tree last year that measured over 20 diameter at the base, about 40 feet through the crown. The hard shell may well protect the nuts for storage, but they must be protected from mice that will chew right through the shell, even chewing through the walls of a plastic container to get to them. Nuts planted in the fall will often sprout the following year, as the shell opens along the seam. Squirrels will "discover" the new seedlings and pull them to get at the nut, although they are also responsible for burrying more nuts than I've ever planted.

Black Walnut tree leaves

Composting: Can you compost the leaves? There was a study that OSU did. That they were safe to compost. They lose the juglone over time of composting.
The one thing is can you put that compost around tomato plants?

black walnut compost

As you mention, juglone is the substance in black walnut trees that can cause severe damage and even kill solanaceous crops (tomatoes, potatoes, peppers and eggplant). However, not all plants are susceptive to the chemical. Edibles that are tolerant of juglone include corn, beans, onions, beets and carrots.

Other trees that carry juglone include butternut, hickories, and pecan; black walnut has the highest concentration of it.

Walnut leaves can be composted because the juglone toxin breaks down when exposed to air, water and bacteria. The toxic effect can be degraded in two to four weeks. In the soil, breakdown may take up to two months after the living walnut tree has been removed. Mulch or woodchips from black walnut are not recommended for plants sensitive to juglone. However, composting the woodchips for a minimum of six months allows the chemical to break down to a safe level even for plants sensitive to juglone.

Black walnut's outer shell

This article, the comments and your well-stated answers were a joy to read. My question is why does the hard green outer shell have such a pungent citrus smell?

Something to Ponder....

I have a larger mature black walnut tree in my garden. I have had wonderful luck with Hostas and most spring bulbs. I have tulips, daffodils, iris and hyacinths. There are several guides for tolerant plants.

Then I tried putting in a KOI POND.....Walnut trees kill fish. Does anyone know if they are safe for frogs????

Black Walnut Tree Toxicity

Hi Michelle,

We’re sorry to hear about your koi pond! It’s hard to say for sure if the black walnut tree toxins will affect frogs: It is poisonous to dogs and horses, but not to cats. The area of affected soil is usually 50 to 60 feet surround the tree, depending on its age. 

increase in the amount of spring unisex flowering??

this year we have an extreme amount of the green flowering type of powder like strings falling from the trees. Does the increase of this make an increase in the the likely amount of nuts they will produce?

hydranga

Need to know if they will grow by Black walnut trees !!???

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