20 Plants Not To Grow Under A Black Walnut Tree
Black walnut trees are an excellent choice for a shade tree and are found in many landscapes and parks throughout the United States. While these trees have some positive attributes like shade, fruit for wildlife, and the possibility of wood harvesting profits, black walnuts come with a curse: juglone.
Juglone is a natural toxin produced by black walnuts to eliminate competition from other plants. You shouldn’t plant flowers and vegetables under a black walnut tree, such as asparagus, rhubarb, peppers, tomatoes, eggplant, potatoes, hydrangea, peonies, lilacs, blueberries, azalea, and true lilies.
Whether you’re thinking about planting a black walnut or already have one of these monstrosities growing, we’ll cover the plants you can and can’t grow beneath them. Let’s get to it!
Why Does Juglone Affect Plants Under A Black Walnut Tree?
As mentioned, juglone is a naturally produced toxin that black walnuts (along with butternut, pecan, and hickory) produce. Almost every part of a black walnut tree will produce juglone. The roots, twigs, bark, and fruits all produce this brutal chemical.
But why does it cause some plants to wilt while others do just fine?
The production of juglone to stunt or eliminate competition is known as allelopathy. Even a black walnut that is cut down could still be contaminating the soil a decade later with the decomposition of its roots.
A study done in 1998 looked at potential ways to minimize the destructiveness of juglone by using root barriers. What they found was that the concentration of juglone after using root barriers was significantly lower, basically just a trace.
However, the concentration of juglone increased significantly within the barrier due to a higher concentration of roots.
Root barriers like the Bamboo Shield Root Barrier are excellent tools if your black walnut is not too big and hasn’t quite established itself in your yard. Simply dig and place the barrier around the tree roots. This encourages them to grow down instead of out.
Let’s move on and talk about all the plants you shouldn’t grow beneath a black walnut tree.
Plants You Shouldn’t Grow Beneath A Black Walnut Tree
Whether you are looking to spruce up the landscape beneath a black walnut, or you’re planning to grow a garden nearby, you’ll want to know what you can and can’t plant beneath a black walnut.
Juglone acts by blocking the exchange of carbon dioxide and oxygen. Plants that are affected by juglone often show telltale signs such as wilting and yellowing leaves according to Iowa State University. Unfortunately, this process cannot be reversed and the plant will eventually die off.
To prevent your plants from croaking, avoid these 20 plants when choosing something to grow beneath a black walnut:
Oven-roasted asparagus with spicy cayenne pepper or mouth-watering parmesan. Mmm, delicious!
Asparagus are perennials, meaning they will come back up year after year. They’re pretty hardy and are harvested early in the spring.
Signs of juglone affecting asparagus can be hard to spot as the plant doesn’t exactly have leaves. However, look for stunted growth or yellowing of the stalk.
Rhubarbs are put in lots of tasty treats like pies and jams. They add color and zing to your garden, providing both aesthetic and nutritional value.
Rhubarbs have large leaves that are not used in harvesting. Signs of juglone will include yellowing, wilting, or twisted leaves.
Cabbages are like little dinosaur eggs sitting in a nest of leaves, just waiting to be picked and made into sauerkraut or sauteed in a vegetable medley.
Similar to rhubarb, cabbage leaves will show yellowing and begin to wilt if exposed to juglone. The growth of the cabbage may also be stunted.
Not the peppers! It’s true, juglone can detrimentally affect peppers in your garden. Unfortunately, this applies to pretty much every type of pepper – green, red, chili, cayenne.
Most peppers grow on stalks that contain leaves so look out for the usual suspects: stunted growth, yellowing or twisting leaves, and wilted-looking plants.
Yep, essentially all of our favorite garden vegetables are susceptible to juglone. Boo. Garden vegetables known as solanaceous crops are particularly affected by juglone.
Members of this family include peppers, eggplant, potato, and, yes, tomatoes. While deer, raccoons, and coyotes will munch on tomatoes, black walnut will do just as much damage.
If you’ve never seen an eggplant plant, you might be surprised by its appearance. They look like regular old houseplants in a pot with ridged leaves and multiple shoots coming out. The only difference is the gigantic berry growing off the shoots.
Did I say berry? Yep, eggplants are a fruit in the berry family! The more you know…
Eggplants require abundant light, so planting them anywhere near a black walnut is not only a bad idea due to juglone, but the black walnut will also cast it in shade for about 50 feet.
Taters are also a part of the solanaceae family. They are considered a root vegetable, a starch, and a favorite at mealtimes. Any mealtime!
This versatile vegetable is sensitive to juglone and will not due well planted beneath black walnut trees. Just like eggplants, potatoes need full sun and will not do well in the shade of a large tree either.
The final sensitive vegetable on our list is peas. These little pods full of goodness are grown in cool weather, unlike most vegetables. They are planted in March and need a period of coolness to mature.
Peas require full to partial sun and thrive in slightly acidic soil. Unfortunately, these are the growing requirements of black walnut as well. Planting the two in the same vicinity will result in wilting, yellowing, and the withering of your pea plants.
This striking flower is considered an herb and has drooping flowers of red and yellow according to Michigan State University. It is less sensitive to juglone than the garden plants listed above, but it will be affected if planted beneath black walnut.
Columbine requires full to partial shade, so if they are planted far enough away (and we mean far! Over 50 feet is preferred) from black walnut, they should be able to survive even if some shade is thrown on them.
The end of Summer and the cooling down of Fall can be a real downer for those that live in the 4-season states. We all know what it means: Winter is coming.
But during Fall we get a special treat in the form of mums. These flowering plants are BURSTING with color that gives us a little hope before winter crushes our souls.
Not all chrysanthemums are vulnerable to juglone. In general, the more herbaceous mums can withstand juglone, but the perennials and bulbs cannot according to Kansas State University.
Another beautiful flowering plant that’s bursting with color is the hydrangea. These Summer & Fall bloomers come in blue, white, pink, green, red, and purple.
Hydrangeas are especially attractive to butterflies, which can make them a perfect addition to your landscape. Just be sure to plant them far away from black walnut trees!
Peonies are sun-loving plants that bloom in the spring and summer, announcing that winter is finally over and we can put away our sweatshirts and snow boots.
These plants come in pretty pink, purple, yellow, white, and red flowers. They are sensitive to Juglone and should not be planted beneath black walnut trees.
Lilacs come in a variety of colors, but the flowers most often seen are purple. This flowering plant has more uses than just to look pretty in the spring and summer.
Lilac is used for astringents and can be applied to cuts and rashes to help heal and soothe the wounds. Black walnut trees are not a good partner for lilacs, who are sensitive to juglone.
Yet another flowering plant that is sensitive to juglone is Mountain Laurel. They only bloom for a short time, but when they do, you’ll be sure to know.
Mountain laurels can range from white to pink and purple. The leaves are usually canoe-shaped and the flowers are ringed in purple or pink dots.
Unlike most of the plants on our list, mountain laurel is tolerant of shade. If planted far enough away from black walnut, they can survive in the shadow of trees.
Unlike strawberries and raspberries, blueberries are true berries. They grow on cute little flowering plants in small bundles similar to grapes.
Blueberry plants require full sun, so it’s not recommended to plant next to any tree, especially a juglone-producing black walnut tree!
Viburnum encompasses a large group of flowering plants. Some are deciduous, losing their leaves each fall, while others are evergreen.
No matter what species you choose, viburnum in general is sensitive to juglone and will wither away beneath a black walnut tree.
Believe it or not, azaleas and rhododendrons are two different plants. However, they both belong to the rhododendron genus in scientific terms.
Azaleas are beautiful flowering plants and unfortunately, they are sensitive to juglone and will struggle beneath a black walnut.
Moving on to the herb family…wait, lilies are herbs? While you don’t want to pick these flowers and eat them, they are part of the herbaceous genus, which is where most herbs are placed!
The word lily is thrown around a lot in the gardening world, but only a few are true lilies. Asiatic lilies, trumpet lilies, and Canada lilies are all true lilies. Peace lilies and daylilies are just fakers.
True lilies are sensitive to juglone and should not be planted near black walnut or any other tree that produces juglone.
This herbaceous flowering plant is wrapped in Greek mythology according to the University of Missouri. The lore says that Narcissus was an exceptionally handsome young man. When he saw his reflection in a river, he could not look away until he eventually withered away.
It’s said that a narcissus flower grew where he perished. Hence the word narcissism is associated with the narcissus flower.
Apart from the mythology lesson, you’ll want to know that narcissus is sensitive to juglone, though it is less sensitive than the vegetables we mentioned earlier.
Probably one of the only ‘herbs’ in this article that you’re not surprised about it being an herb. Alfalfa does not do well beneath black walnut trees as it is sensitive to the chemical juglone.
Alfalfa is often planted for livestock feed, but humans consume it as well! It is used as a garnish atop dishes and to aid in the treatment of high cholesterol.
What Plants Can Live Under A Black Walnut Tree?
We’ve covered a wide range of vegetables, fruits, herbs, and flowers that you definitely cannot plant beneath a black walnut.
So what exactly can you plant beneath these toxicant-producing trees?
In terms of vegetables, you’re safe to plant the following garden favorites beneath black walnut, according to the University of Wisconsin-Madison:
Okay, so that’s for vegetables. But what about flowers?
If you’re looking for some beautiful flowers that will brighten up the space beneath your black walnut, try some of these:
- Black-eyed Susan
Because black walnut trees are so large and provide tons of shade, if you want to plant grass beneath them you’ll need some shade-tolerant grass such as Scotts Turf Builder Grass Seed Dense Shade Mix.
This seed comes in a 7lb bag covering up to 1,750 square feet. However, be aware that this product is only good for northern regions. There are similar products for transition zones and southern regions.
Ways To Minimize Juglone Contact With Plants Under A Black Walnut Tree
If your heart is set on planting a garden but you have a big bad black walnut tree already grown in your yard, you still have some options!
These ideas are especially good for those who have smaller yards and can’t plant a garden 50-80 feet from a black walnut tree.
Use Raised Beds
You always have the option to use raised beds to plant a garden instead of doing it on ground level. Line the raised soil with something like AHG Garden Weeds Landscape Fabric.
The fabric will prevent your garden plant roots from shooting too far down or away and coming in contact with the roots of the black walnut.
The important thing to take away from this is that the soil you use for your garden should NOT be coming from the soil near your black walnut. Your best bet is to either buy new soil or obtain it from somewhere else. The soil near black walnuts will be contaminated with juglone.
Clean Up Black Walnut Tree Debris
Remember how we mentioned earlier that every part of the black walnut tree will produce juglone? Well, the longer the fallen fruit, twigs, and leaves are left on the ground, the more concentrated the amount of juglone in the soil.
Be sure to clean up any fallen fruit (and save some for yourself!), twigs, leaves, or other tree debris before it begins to decompose. This will help decrease the concentration of juglone in the soil.
Black walnuts overall are a bit more messy than regular walnut trees. If you”re interested in the difference between walnut and black walnut trees, have a look at our article: 8 Differences Between Black Walnut Trees and Walnut Trees.
That’s All For Now!
Black walnut trees provide plenty of benefits to the average yard. It provides shade, can act as a windbreak, and the fruit is delicious and edible.
The downside to black walnuts is that not all plants and vegetables are happy to be planted beneath one.
To recap, the vegetables and plants you should not plant beneath a black walnut tree include the following:
- Mountain Laurel
- True lilies
Avoid planting these flowers and vegetables and be sure to choose flowers and vegetables that are shade tolerant, as the black walnut can have a footprint of 50 feet or more!
Hierro, J. L., & Callaway, R. M. (2021, November). The Ecological Importance of Allelopathy. Annual Review of Ecology, Evolution, and Systematics, 52, 25-45. https://www.annualreviews.org/doi/abs/10.1146/annurev-ecolsys-051120-030619
Jose, S. (2002). Black walnut allelopathy: current state of the science. Chemical Ecology of Plants: Allelopathy in Aquatic and Terrestrial Ecosystems, 149-172. https://link.springer.com/chapter/10.1007/978-3-0348-8109-8_10
Jose, S., & Gillespie, A. R. (1998). Allelopathy in black walnut (Juglans nigra L.) alley cropping. I. Spatio-temporal variation in soil juglone in a black walnut-corn (Zea mays L.) alley cropping system in the midwestern USA. Plant and Soil, 203, 191-197. https://link.springer.com/article/10.1023/A:1004301309997
Scott, R., & Sullivan, W. C. (2007). A review of suitable companion crops for black walnut. Agroforestry Systems, 71, 185-193.