5 Simple Reasons Not To Plant A Black Walnut Tree

Eastern black walnut (juglans nigra)

You may consider planting a black walnut tree for many reasons. They are attractive, provide plentiful shade, have few pest issues, produce an edible nut, and some people are even interested in the high valued lumber that can be harvested from black walnut trees. However, there are several reasons not to plant a black walnut tree in your yard which you should also carefully consider. 

Black walnut trees produce juglone, which is present in all parts of the tree. Juglone is produced by the black walnut as a way to keep other plants from competing with it. Juglone can also eliminate other plants, including several plants you may have in your home garden or landscape.

There are several other factors which make black walnut an unappealing choice for a shade tree in your yard. In this article, we will cover all of the drawbacks of planting a black walnut tree as well as how you can remove one from your property.

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What Is A Black Walnut Tree?

Black walnut trees are in the genus Juglans along with other trees such as butternut and English walnut (which is what you buy at the grocery store to eat).

Black walnut tree in fall
Black walnut tree during Fall.

This is an image of a black walnut tree at our family property during the Fall. There’s a younger black walnut growing next to it, but the large when is a mature black walnut tree.

You can tell it’s a black walnut tree by noticing some of the remnants of the leaves in their distinct leaflet orientation, which I’ll get into a bit later!

Img 2714
Notice the thin leaflets on each side of the black walnut tree pointing outwards of each other.

In this section, we will tell you about the etymology, how to identify a black walnut tree, and uses for black walnut trees. Let’s get to it!

Etymology: The Meaning Of The Scientific Name For Black Walnut

Etymology is the study of the origin of words and how their meaning has changed over time. When it comes to the etymology of scientific names, this typically means deciphering their Latin meaning.

The scientific name of a species is made up of two parts, the genus (which is capitalized) and the specific epithet (which is lower case).

For example, all walnut trees are in the same genus Juglans, but each species has its own unique specific epithet such as nigra to distinguish it from other members of the same genus.

The meaning of the scientific name for black walnut, Juglans nigra, is black nut of Jupiter.

Juglans is a combination of the Latin word Jupiter, referring to Jupiter (god of the sky), and glans, meaning acorn. This likely refers to the fact that walnut trees can grow quite large and produce an edible nut.

How To Identify A Black Walnut Tree

Black walnut tree with road in background
Black walnut tree (Juglans nigra.)

Black walnut trees are characterized by having an overall form with a straight trunk and open canopy. It can grow as tall as 120 feet but averages 50 to 70 feet tall and 50 to 70 feet wide.

Black walnut has pinnately compound leaves which have a central petiole known as the rachus and leaflets which grow on either side of the rachus. The leaves have anywhere from 9 to 23 leaflets which are just under a half foot long and about 2 inches wide. The leaflets have a serrated toothlike margin and are hairy on the underside. 

The bark is a grayish-brown to black color and mature trees have bark with ridges which form a roughly shaped diamond pattern

Black walnut trees have both male and female flowers and are primarily wind pollinated. The male flowers are yellow-green catkins, a slim, cylindrical spike, which hang down in clusters. The female flowers are white and form spikes usually in pairs. 

The fruits of black walnut trees form from the female flowers after pollination in fall, usually in pairs.

Black walnut tree zoomed in on fruit
Fruit of black walnut tree (Juglans nigra.)

They are around 2.5 inches wide when mature and covered in a thick green husk which contains juglone, a black walnut specific chemical to protect the inner nuts from predators. Beneath the husk is a hard shell, which contains the nut, and is dark brown to black in color.

We have many black walnut trees across our property spread across naturally in wooded areas – here’s a black walnut fruit example of a fallen fruit I took during the summer.

Fruit of a black walnut tree.
Fruit of a black walnut tree.

One thing about black walnuts is that they have quite the pungent smell to them which is typically thought to be the juglone giving off its signature scent to tell pests and insects to ‘stay away‘.

Oregon State University provides some nice photos of the characteristics that distinguish black walnut trees from other walnut trees if you’d like some more specifics.

Uses Of Black Walnut Trees

Black walnuts are edible and have a bold flavor when compared to the English walnuts which you typically find at the grocery store. You can purchase black walnuts online if you can’t find them at your local store. We love Hammons Black Walnuts which are wildly sourced and hand harvested. 

Black walnut is used for making high quality furniture and is favored for its dark, rich brown color.

Many other woods will be covered with black walnut veneer to reduce the cost of the furniture while still giving it the beautiful finish of black walnut. It is also naturally resistant to decay and insect damage. 

The juglone present in black walnut has a yellow-brown color which is used for dying clothing and other textiles.

It has also been used as a coloring agent in foods, cosmetics, and hair dye. It will also stain your hands if you remove the protective husks from black walnut fruits!

Where Do Black Walnut Trees Grow?

Black walnut trees are native to Eastern North America and grow well in hardiness zones 4 through 9. Their natural range extends in the north from Vermont through southern Ontario and west to South Dakota and in the south from Georgia through the Florida panhandle and westward over to eastern Texas. 

The USDA Forest Service provides a county-level map of the native range of black walnut.

Black walnut trees have been introduced to more than 14 countries, most of which are in Europe. It has also been introduced to Mexico, and a few countries in South America and minimally in Australia

Outside of its native range, black walnut trees are most abundant in Europe and western North America. 

Let’s dive into the 5 reasons you should NOT plant a black walnut tree!

Juglone Will Effect Other Plants In Your Yard

Juglone is a allelopathic compound produced by all plants in the family Juglandaceae. It is considered a defense mechanism. Plants use it to eliminate plants in the nearby vicinity, reducing competition for sunlight, nutrients, and water.

Juglone does begin to break down when it contacts air, which is why we can eat its nuts and use it to make furniture without encountering the negative effects. 

Black walnuts have higher concentrations of juglone than any other tree, and it is present in all parts of the plants.

Juglone concentrations in the roots are highest throughout the growing season, and it can contaminate soil as far as 60 feet away from the adult tree!

The hulls of the fruits also have very high concentrations of juglone to help deter animals from eating the nuts so they can survive and form new trees. 

Juglone can stain clothing and your hands with a yellow-brown stain. This staining property is why juglone has been used as a dye for clothing and for coloring in food, cosmetics, and hair dye. 

Juglone can impact several plants in your garden such as peppers, tomatoes, and eggplants. It can also eliminate plants in your landscape such as rhododendrons and azaleas. Further, it can impact other trees on your property such as crabapples, silver maples, and pines. 

For an extensive list of plants not to grow under your black walnut tree, check out our list of 20 plants not to grow under a black walnut tree.

Thousand Cankers Fungus (TCD)

TCD is caused by the fungus Geosmithia morbida, which was first recognized in 2008 as the cause of damage to walnut trees which had been occurring since the 1990s.

The fungus is carried by a tiny bark beetle, the walnut twig beetle (Pityophthorus juglandis), which infects the tree when it begins feeding. 

By the time TCD was recognized as the cause of walnut tree dieback, it was already spreading throughout the western United States. It was first detected in the eastern United States, the native range of black walnut, in 2010.

The Missouri Department of Agriculture provides a map showing the current spread of TCD in the United States. 

Trees with TCD will show signs such as yellowing and wilting of foliage. Beneath the bark, thousands of cankers form and ultimately girdle the tree, preventing nutrients and water from moving throughout the tree.

Though it takes several years, TCD ultimately leads to the demise of the infected tree. 

If you live in an area where TCD is present, this is another significant reason not to plant black walnut trees. You don’t want to waste time growing a beautiful shade tree just to see it meet its demise from TCD.

Pests And Insects Love Black Walnut Trees (Even With Juglone!)

Even if your black walnut tree does succumb to TCD, there are many other insects and fungi which can cause injury to the tree and decrease its aesthetically pleasing appearance. 

Both Powdery mildew (Phyllactinia guttata) and Anthracnose (Gnomonia leptostyla) are fungi which impact black walnut causing leaves to become discolored and fall from the tree early.

Anthracnose can also damage the stems and the fruit of black walnut, decreasing the quality and edibility of the nuts. 

Crown gall (Agrobacterium tumefaciens) impacts the roots and root crown of black walnut causing light colored bulges. Damage can lead to stunted growth and rarely the loss of the tree. 

There are also several species of eriophyid mites that cause galls to form on black walnut trees when they feed on the tree. Galls are abnormal growths which can come in many shapes and sizes depending on which mite species creates them. Galls can cause discoloration and distortion of the stems and leaves, and some also cause lower fruit yield.

The University of Missouri discovered three new mite-induced galls on trees in Missouri as recently as 2015. So there could be even more mite-induced galls on black walnut trees which remain to be discovered in the future!

Black Walnut Trees Are Very Messy

Fallen black walnuts on ground

Sure, all deciduous trees require cleanup when they lose their leaves in the fall. However, there are a few factors that make the cleanup from black walnut trees more difficult than your typical shade tree.

Cleaning Up The Leaves

First of all, their leaves can quickly multiply in your yard as they begin to break down. Since black walnut trees have pinnately compound leaves, each leaf actually has up to 23 leaflets. This gives black walnut leaves the potential for a single leaf to turn into 23 leaves when the leaflets start detaching from the rachus. 

You’ll also have to think about where to put all of those leaves.

Since the black walnut leaves do contain the chemical juglone, you have to compost them for long enough to let the juglone decompose so your compost doesn’t eliminate the plants you use it on.

Keep this in mind if you plan to add the leaves to your usual compost pile.

Due to the juglone in the black walnut leaves, you’ll also want to keep them from blowing all over your property or the juglone could blow around, causing problems for plants in areas you thought were far enough away from the tree to prevent juglone effects.

Cleaning Up The Fruit

The fruit from black walnut will also create a mess for you to clean up in fall. Black walnuts are slightly smaller than a baseball and have a soft outer shell called the hull which encases the hard shell which protects the nut in the center. 

As the fruit decays, the hulls will separate from the shells, multiplying the mess the fruit will make after they fall. If squirrels get involved and extract the nuts from the hard outer shell, your mess multiplies even further as each fruit is broken down into multiple pieces for you to clean up. 

A healthy mature black walnut tree can produce up to several hundred pounds of black walnuts per year. That is a big mess for you to clean up, especially if you have a healthy, well-producing tree!

Not to mention when the fruit falls, it could easily act like hail towards any structures on your property. Yikes!

Just like the leaves, fruits of black walnut also contain juglone, with the hulls having the highest concentration of any part of the tree.

You’ll want to carefully consider where you dispose of the fruit you clean up to prevent spreading juglone to other parts of your property.

Mature Black Walnut Trees Are Quite Large

The black walnut is a fast-growing tree. Once established, black walnut trees can gain 12 to 24 inches in height each year. On average, black walnut trees can grow to be 50 to 70 feet tall and 50 to 70 feet wide. Wild black walnut trees can grow over 100ft with optimal growing conditions!

The size of a mature black walnut tree can cause issues in your lawn, even though most grasses are resistant to juglone, by shading out sunlight and competing for water and nutrients. 

A mature black walnut tree reaching a height of 70 feet and width of 70 feet will ultimately create an area of at least 4900 square feet where plants susceptible to juglone effects won’t be able to grow. That’s a pretty large area to create in your yard where you won’t be able to plant species which are sensitive to juglone!

How To Get Rid Of Black Walnut Trees

So, if you have a black walnut tree – here are a few ways to get rid of it in your yard.

If you’re not sold on chopping down that black walnut tree yet, take a look at our piece on the reasons to cut down your walnut tree which specifically reviews black walnut trees!

Mechanical Control

Mechanical control means removing the plants using a mechanical method such as mowing them down with a lawn mower or cutting them down with a chainsaw. 

If you’re dealing with newly emerging black walnut trees, either saplings from seeds or root sprouts from an adult tree that’s been removed, you can mow these down if there is a large number of them. If there are only a few, it is better to pull saplings up by the root to keep any root sprouts from occurring.

Follow up mowing with a chemical application, described below, to make sure the roots don’t remain living in the soil which could cause more root sprouting.

If you’re dealing with a mature black walnut tree, it is best to consult a professional about removal. Removing any full grown tree should always be handled by a professional.

We recommend you use a certified arborist to ensure the job gets done right!

The best time to cut down your walnut tree is when it is dormant during the winter and early spring. A freshly cut tree can attract bark beetles to the area, which could end up feeding on nearby trees that you want to keep. A tree without leaves can also make an arborist’s job of removing it easier.

Of course, if there’s issues your black walnut tree is causing then it should be taken down ASAP.

Chemical Control

Chemical control is what it sounds like: controlling an unwanted pest with the use of a pesticide. In this case, black walnut trees are the pest and you’ll use an herbicide to remove them.

Once the mature tree is removed, it is a good idea to immediately apply an herbicide such as glyphosate to the tree stump to help prevent root suckers and stump sprouts from forming if any growable black walnut remains. We like this Concentrated Weed & Grass Killer which has 41% glyphosate. 

Anytime you use herbicides, make sure to follow the directions carefully to prevent harm to yourself and the environment.

Make sure to keep an eye on the area where the black walnut tree was growing and take care of any saplings or root suckers as soon as you see them.

Saplings can be easily removed by hand, but make sure you wear protective gardening gloves. Root suckers can be treated with an herbicide like glyphosate. 

Remember, don’t immediately start planting juglone sensitive plants after removing your black walnut tree. It can take up to 5 years for the roots to decay and the juglone to disappear completely from the soil!

Your arborist will know best!

That’s All We’ve Got!

Black walnut trees can be beautiful, native shade trees in the landscape. They have many uses from being edible, making high end wood furniture, and as dye for textiles. 

However, there are some drawbacks to planting black walnut trees on your property. 

The juglone they produce can negatively impact other plants including certain vegetables, ornamental plants, and other trees on your property.

There are also several insects and fungi which can impact the health of black walnut, some of which can lead to its ultimate demise. Black walnut trees also make quite a mess in your yard in the fall and will grow quite large.

We recommend you consult a professional if you already have a black walnut tree you want to remove from your property. Make sure you follow up with the proper pesticide to keep root suckers from growing back. 

If your heart is still set on having a walnut tree in your yard, depending on where you live, you might consider an alternative such as English walnut which has some similar characteristics as black walnut but doesn’t produce as much juglone. 

If you’re interested in finding out if an English walnut tree might be an alternative for you, check out the 5 Best Places To Plant A Walnut Tree (And How To Do It) which provides details on where you can plant an English walnut tree!

Reference

Hejl, A.A., Einhellig, F.A. and Rasmussen, J.A., 1993. Effects of juglone on growth, photosynthesis, and respiration. Journal of Chemical Ecology, 19(3), pp.559-568.

Michler, C.H., Woeste, K.E. and Pijut, P.M., 2007. Black walnut. In Forest trees (pp. 189-198). Springer, Berlin, Heidelberg.

Rietveld, W.J., 1983. Allelopathic effects of juglone on germination and growth of several herbaceous and woody species. Journal of Chemical Ecology, 9(2), pp.295-308.

Tisserat, N., Cranshaw, W., Putnam, M.L., Pscheidt, J., Leslie, C.A., Murray, M., Hoffman, J., Barkley, Y., Alexander, K. and Seybold, S.J., 2011. Thousand cankers disease is widespread in black walnut in the western United States. Plant Health Progress, 12(1), p.35.

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