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Planting Under Black Walnut Trees

Category Planting
Black walnut trees can prove to be a difficult landscaping challenge for gardeners because they are alleopathic. This can have a beneficial or harmful chemical effects on other plants. This guide is about planting under black walnut trees.
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By 1 found this helpful
April 17, 2008

Highly prized for its strength, resistance to decay, handsome wood grain, and product versatility (it's made into everything from furniture to airplane propellers) to woodworkers, black walnut trees are considered black gold. To us gardeners, however, black walnut trees can prove to be a difficult landscaping challenge.
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Understanding Alleopathy

Black walnut trees (Juglans nigra) are alleopathic, a term used to describe plant species that have beneficial or harmful chemical effects on other plants. In the case of black walnut trees, the allelopathic interaction is harmful. The toxin responsible is called juglone, and resides in large amounts in the tree's roots. Juglone is known to inhibit respiration, resulting in stunted or deformed growth, and even death, in juglone-sensitive plants. Related species such as the English walnut (Juglans regia) and butternut (Juglans cinerea) tree are also alleopathic, although to a lesser extent.

Allelopathic interactions are complex and scientists are still conducting studies to fully understand the process.

Signs & Symptoms

Plants suffering from juglone toxicity may show a variety of symptoms. The extent of damage depends on the resistance of individual plants and the amount of contact with the toxin. It's believed that susceptible plants need to come within 1/4" - 1/2" proximity to the walnut tree's roots for serious damage to occur, but even a minor amount of contact with decaying leaf litter, nut husks, or rain drops containing juglone is also thought to harm some extremely sensitive plants. Look for wilting, droopy, puckering, or twisting leaves that appear yellow or discolored. Internal stem tissues on woody plants or shrubs may also show darkening or discoloration.
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Planting Strategies

Avoid Tree Removal: Cutting down your black walnut trees is not a good solution to your landscaping woes. At least not if your tree is healthy. Juglone is not very water soluble and will stay in the soil (and any tiny feeder roots not removed) for several years. Black walnut trees provide wonderful shade, reduce CO2 while providing fresh oxygen, produce food and shelter for backyard wildlife and birds, and make an excellent windbreak due to the strength of their wood.

Create Some Distance: The roots of a large black walnut tree can extend far beyond the reaches of its canopy. There is no magic safe distance for planting, but suffice to say, the farther away the better. When in doubt, make sure you experiment with cheaper plants.

Growing In Containers: Many gardeners have found great success by growing susceptible plants above ground in attractive containers. This is your opportunity to get creative and experiment with different planters and containers.

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Clean Up Tree Debris: Leaf litter and nut husks contain only very small concentrations of juglone, but it's better to play it safe than be sorry by keeping the area around your tree raked up and free from fallen debris. If trimmed branches or bark are shredded for mulch, be sure to use them only on juglone resistant plants.

Plants To Avoid Under or Near Black Walnut Trees

Plants from the nightshade family (Solanaceae) seem especially susceptible to the effects of juglone. This includes eggplants, cabbage, green peppers, potatoes, and tomatoes.

Other susceptible plants include: Azalea, rhododendron, cotoneaster, potentilla, apple, blackberry, blueberry, cucumber, grapes (domestic), crocus, peony, black alder (Alnus); basswood; white birches; Hopa crabapple; hackberry; Amur honeysuckle; Japanese larch; lespedeza; lilac; saucer magnolia; silver maple; mountain laurel; loblolly pine; red pine; scotch pine; white pine; privet; Norway spruce; and alfalfa.

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Plants To Grow Under or Near Black Walnut Trees

There is a lot of conflicting information on which plants can be grown under black walnut trees. Plants on this list that require full sun can be grown near black walnut trees rather than beneath them. This list is meant as a general guide and it by no means complete (nor is the list above of plants to avoid). As stated earlier, allelopathic interactions are complex and your own experience may prove different depending on individual soil conditions, climate, etc. Plants on this list have either been specifically studied for their resistance to juglone or have been grown successfully under or near Black Walnut trees by fellow gardeners. As always, please feel free to comment on your own successes or failures so we can all learn!
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Vegetables

Lima beans; snap beans; beets; carrots; corn; onions; melons; parsnips; and squash.

Landscape Plants

Red cedar; crabapple*; elm; winged euonymus; forsythia; hawthorn; hemlock; hickories; black locust; maples (most species); oaks; autumn olive; pachysandra; paw paw; persimmon; wild rose; sycamore; most viburnums; and Virginia creeper.

Fruits

Cherries and black raspberries

Flowers and Herbaceous Plants

Astilbe; bee balm; wax begonia; bellflower; bloodroot; bluebells; Kentucky bluegrass; coral bells; cranesbill; crocus*' daffodil; Shasta daisy; daylily; Dutchman's britches; ferns; fescue; hollyhock; glory-of-the-snow; hosta; iris; Jack-in-the-pulpit; lamb's ear; liriope; lungwort; pansies; pot marigold; morning glory; narcissus; phlox; sedum; Siberian squill; snowdrop; spiderwort; spring beauty; sundrops; sweet woodruff; trillium; violets; and zinnia.

* Conflicting information available so it appears on both lists.

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Questions

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April 11, 20140 found this helpful

I want to plant ground cover in the area between the street and my sidewalk. It is under two black walnut trees. So I need something the can be stepped on and is able to handle the abuse of falling walnuts and squirrels.

By Ryan F.

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January 31, 20170 found this helpful

Ivy, virginia creeper, and vinca do very well under my black walnut.

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By 0 found this helpful
October 18, 2010

I just had a black walnut tree cut down and the stump ground down, can you tell me what tree I can plant in it's place that would flower in the spring or summer? Can I plant a crape myrtle tree?

Hardiness Zone: 7a

By Linda

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October 22, 20100 found this helpful

Peter Loewer (sp?) used to have a list of plants that will live under walnut trees on his website. You might google his name or "the wild gardener" and see if it is still there.

Good luck!

Incidentally, my extension agent just told me that walnut trees are an indicator of good soil and growing conditions. I didn't know that, but should mean that whatever you can get to grow on the site will have a good chance of doing well.

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By 0 found this helpful
August 11, 2008

I want to plant a Virginia sweetspire. The area where I'm thinking of planting it has a black walnut tree in it. Does anyone know if the black walnut is toxic to Virginia sweetspires? Thanks in advance!

Kate from Berkeley Springs, WV

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August 11, 20080 found this helpful

Hi there! Black walnuts are a real pain this way... I googled "juglone poisoning" and didn't get much. I've never really been able to get much from online articles on this. If you do find a website that seems to have definitive answers on what is tolerant and what isn't, you can click on another and find the complete opposite. Here's what I've done. Go ahead and plant it near the walnut- Wait a year (I know, this part is horrible) and see how it does by mid spring next year. What most places will tell you is that if it "fails to thrive", you should move it. The sweetspire shouldn't die. (I can't say won't die, but I'm 99% sure it won't) What will happen if it's susceptible to the juglone is that it will seem listless, and might not put out as much new growth, or it'll come back smaller than it was this year. The good news is that you can move it and it will recover nicely. This is all assuming you have another place you are willing to put it if this spot doesn't work out. And honestly, the only way I've been able to fully trust whether something can tolerate it or not is to try it. You shouldn't lose the plant, and you'll have a definite answer.

Funny story- The way I discovered this black walnut toxicity was when I planted a rhodie underneath one and it slowly died. I got onto my husband, accusing him of peeing on it (It was underneath a balcony and you know how guys are) and killing it with too much acid. He swore he didn't do it, so I was forced to look up alternative causes of death. :)

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June 20, 20140 found this helpful

Does this apply to all kinds of walnuts, Manchurian walnut, Japanese walnut, and common walnut, too?

By Eric K

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