Monday, June 12, 2006

The Problem with Black Walnuts

First Published in The Landscape Contractor magazine

“What You Don’t Know About Walnuts Can Hurt Your Plants”

By Becke Davis

How many times have you planted perfectly healthy trees or shrubs into an existing landscape, only to have them die for no apparent reason? When that happens, do you call in a specialist from the County Extension Service you diagnose the cause of death? Do you have an arborist check out the remains of the plant? Or do you just bite the bullet and resign yourself to the inevitable loss of a few trees and shrubs? Nine times out of ten, you’d be safe to do that, but the 10th time you could be letting yourself in for a repeat performance.

There are a number of reasons why a relatively new plant can die -- planting too deeply (although that usually causes a slower death), herbicide contamination, insufficient watering, root burn from over-fertilization, rootballs sitting in water that won’t drain, and so on. When you are running down a checklist of possible reasons for the death of your plants, take a look at neighboring trees -- not just in your client’s yard, but in the yards of near neighbors, too. If you happen to see a black walnut (Juglans nigra) or a butternut (J. cinerea) growing nearby, that could be the source of your problem.

Black walnuts are valuable timber trees and they are relatively common in the Midwest. They can be found growing in the wild as well as in residential and commercial landscapes. The tree is hardy, the leaves are attractive, the roots help control soil erosion, and people don’t seem to mind the mess of falling walnuts. Black walnuts, butternuts and other walnut species that have been grafted onto the vigorous roots of black walnuts share a common trait: they produce a toxin called “juglone”, technically called 5-hydroxy-alphanapthaquinone. Other juglone-producing plants include shagbark hickory, mockernut hickory and the pecan. Juglone is called an “allelotoxin” and the reaction of plants to its presence in the soil is called “allelopathy”.

While some plants seem to be unaffected by the presence of juglone in the soil, other plants are highly susceptible to it. Even landscapers that are aware of the toxic properties of black walnut sometimes underestimate distance necessary to create a safe barrier for susceptible plants. Juglone is found in the the leaves, stems and branches, bark, buds, wood and primarily in the roots; some experts say it also appears in the nuts and hulls, while others insist it does not. In any case, juglone is not toxic to humans so eating walnuts carries no risk of poisoning. However, humans may suffer from allergic reactions when black walnut pollen is shed in late spring.

The fact that walnuts are fairly deep-rooted may protect some plants whose roots remain close to the soil surface. However, the roots extend in a very large radius around the tree, two to three times the radius of the crown. A young tree only 10 feet tall will have roots stretching out 20 feet or more. In a full-grown tree, the root zone can extend as far as 80 feet in diameter around the tree. Plant susceptible to juglone will be endangered anywhere within that root zone.

Experts disagree as to the risk of using sawdust or woodchips from walnut leaves, husks, branches or bark in the landscape, in mulch or even in the compost heap. While some insist that it is safe to do so, others say that applications of mulch, sawdust or compost containing any form of black walnut will eventually kill the treated plants. The presence of juglone in the soil has been traced both to living black walnut roots as well as decaying roots left in the soil from trees that have died and been removed. There are even reports of juglone entering the soil through raindrops coming off the leaves of black walnuts. Other reports say that adding nitrogen to the soil within the dripline of black walnuts can reduce the toxicity of the juglone.

Unfortunately, many reports -- coming from well-respected universities and other sources -- offer contradictory
advice, indicating that more research is needed in this area. Because of these mixed reports, using compost, mulch, shredded leaves or any landscape product containing live or decaying black walnut elements should be avoided.


The following is a partial list of plants that have been observed living within the toxic range of black walnut trees, rather than being specifically tested for juglone tolerance. Experts don’t all agree on these plants, which may be an indication that plants survive under certain conditions become more susceptible under others. In some cases, particular hybrids or species were tolerant, while others were not.

Arborvitae (Thuja spp.)
Astilbe (Astilbe spp.)
Bee Balm (Monarda didyma)
Begonia (Begonia spp.)
Black Locust (Robinia spp.)
Bleeding Heart (Dicentra spp.)
Bugleweed (Ajuga spp.)
Catalpa (Catalpa spp.)
Clematis (Clematis spp.)
Crocus (Crocus spp.)
Daphne (Daphne spp.)
Daylily (Hemerocallis spp.)
Elm (Ulmus spp.)
Euonymus (Euonymus spp.)
Forsythia (Forsythia spp.)
Hawthorn (Crataegus spp.)
Hemlock (Tsuga spp.)
Hickory (Carya spp.)
Honeysuckle (Lonicera spp. - most)
Japanese Maple (Acer japonica)
Juniper (Juniperus spp.)
Lamb’s Ears (Stachys spp.)
Maple (Acer spp. - most species)
Morning glory (Ipomoea spp.)
Oak (Quercus spp.)
Pachysandra (Pachysandra spp.)
Pansy (Viola spp.)
PawPaw (Asiminia spp.)
Redbud (Cercis spp.)
Siberian Iris (Iris siberica)
Sycamore (Platanus spp.)
Viburnum (Viburnum spp. - most)
Virginia Creeper (Parthenocissus quinquefolia)

Some reports list hostas, impatiens and ferns as being tolerant of juglone but the research on these is not complete. Crabapples and apples (Malus spp.) appear to have mixed reactions to juglone, since some experts list this species as tolerant, and others list it as extremely susceptible.
There are several reports of apple trees dying from the effects of juglone, though, so plant Malus specimens near walnuts at your own risk.


Basswood (Tilia heteropylla)
Blackberry (Rubus spp.)
Blueberry (Vaccinium corymbosum)
Chrysanthemum (Dendranthema spp. - some)
Cinquefoil (Potentilla spp.)
Daffodils (Narcissus hybrids and cultivars such as ‘King Alfred’ and ‘Ice Follies’)
Doublefile Viburnum (Viburnum plicatum var. tomentosum ‘Mariesii’)
Eastern White Pine (Pinus strobus)
European Alder (Alnus glutinosa)
Flowering Tobacco (Nicotiana spp.)
Hydrangea (Hydrangea spp.)
Lilac (Syringa spp.)
Lilies (Lilium spp. - especially Asian hybrids)
Mugo Pine (Pinus mugo)
Northern Hackberry (Celtis occidentalis)
Norway Spruce (Picea abies)
Ornamental Cabbage (Brassica oleracea)
Peony (Paeonia spp. - some)
Petunia species and cultivars (Petunia spp.)
Red Chokeberry (Aronia arbutifolia)
Red Pine (Pinus resinosa)
Rhododendrons and Azaleas (Rhododendron spp.)
Silver Maple (Acer saccharinum)
Tulip hybrids and cultivars (Tulipa spp. - some such as ‘Merry Widow’ and ‘West Point’)
Wild Columbine (Aquilegia canadensis)
White Birch (Betula spp.)
Yew (Taxus spp.)


“Black Walnut Toxicity to Plants, Humans and Horses,” by Richard C. Funt and Jane Martin, Ohio State University Extension Service Fact Sheet HYG-1148-93

“Walnut and Its Toxicity Explored,” by Tom Rood, Cornell University Cooperative Extension - Yates County, April 25, 2001,

“Walnut Trees and Your Garden: Effects of Juglone,” by Don Janssen, Extension Educator, University of Nebraska Cooperative Extension Service, Lincoln, Lancaster County, NE

“Black Walnut Allelotoxin Query: Plant & Pest Diagnostic Service,” PP&L Diagnostic Lab, Purdue University, West Lafayette, IN


Don said...

Excellent article, thank you for your research. I came upon your blog researching black walnuts because a client of mine has a large on in his yard and it's killing EVERYTHING I plant. I knew of the problems with BW but was wholly unprepared for the scope and quickness of the devastation to shrubs and perennials planted within 80 feet.

By the way, I planted numerous dogwoods and ninebarks in this area because I found on them some lists of plants that work around BW, and the tree (old, 70', roots probably extend 100' or more) made short work of them. Killed coneflower, honesuckle, every grass I tried.

Thanks again, very helpful.

Treethyme said...

Thanks, Don -- I'm glad to hear this was helpful. Whereabouts are you? I mainly work with landscapers in the midwest -- primarily Illinois and Ohio, with some overlap in Wisconsin and Indiana.

Let me know if you have any other problems -- I'll see what I can do to help. I've worked in the landscape industry for about fifteen years now.

Judith said...

Do you think I could make raised beds according to the square foot garden method if they were about 60 feet from the base of a black walnut which had a 35 foot crown?

Sometimes the raised beds with 6 to 8 inch sides are built with a wooden bottom on poor soil or patio. Would a wooden or plastic with holes keep the toxic soil from osmosising into the good soil?

What do you think?

Treethyme said...

The Iowa State University Extension Service says this:

Vegetables susceptible to juglone include tomatoes, potatoes, peppers, and eggplants. Symptoms include reduced growth, wilting, and possibly death. The presence of large walnut trees near a vegetable garden subjects susceptible plants to double jeopardy. The presence of juglone in the soil, plus the competition for light, water, and nutrients creates an extremely stressful environment.

Fortunately, not all vegetables are injured by juglone. Corn, beans, onions, beets, and carrots are tolerant of juglone. If the garden plot receives sufficient sunlight, gardeners should be able to successfully grow these crops with timely applications of water and fertilizer.

Gardeners should plant shade tolerant annuals and perennials, such as impatiens, hosta, and ferns, near large walnut trees. (A complete list of plants susceptible and tolerant of juglone is unavailable as little research has been done in the area.)

Gardeners who have large walnut trees near their vegetable gardens should consider alternate sites. The greatest concentration of juglone in the soil exists within the dripline of the trees.

Vegetable gardens in this area will undoubtedly experience problems. Plants susceptible to juglone are occasionally damaged well beyond the dripline as the roots of walnuts may extend 2 to 3 times the crown radius (the distance from the trunk to the dripline).

Volunteer walnut seedlings which appear in or near the garden should be removed. Walnut leaves and other plant debris, which may accumulate in the garden, should be raked and removed. Sawdust or wood chips derived from walnuts should not be applied as a mulch around susceptible plants.

Black walnuts can create problems for home gardeners. Careful selection of juglone tolerant vegetables and shade tolerant annuals and perennials should help overcome these problems.

Does that help? If you need more information, let me know and I'll check with other sources.

Jessica said...

Do you have any information on lavender and Black walnut, I have been searching the internet all day and keep coming up empty handed. I have a row of black walnuts (huge old trees) lining my driveway and want to plant grosso and provence lavender nearby, but obviously don't want to kill the plants. Any suggestions or wisdom?

Treethyme said...

There is some discussion about lavender and black walnuts at this link. The biggest concern, as I believe someone mentions here, is that lavender is a sun-loving plant. You wouldn't want to place it in the shade of a black walnut.

As I mentioned in the article, though, juglone can affect plants in a pretty wide range. You might want to give it a try with one plant, just to check it out.

Judith Reidy said...

I am no expert, but on the advice of someone who has done square foot gardening I am going to use raised beds with bottoms. Go to and learn more. I am building boxes with bottoms and filling them with "Mel's Mix" which is 1/3 coarse vermiculite, 1/3 peat, and 1/3 of a blend of different kinds of compost. I will keep leaf and nut litter off the boxes later in the season. I am optimistic because of the success of my advisor.

Madelyn said...

Can anyone tell me whether they have had success with planting a Japanese Maple near a black walnut tree. I would like to plant a sangokaku maple.

Treethyme said...

I've done some checking for you, and it sounds as if your Japanese maple might be at risk if you plant it near a black walnut.

Here are some of the sources I found:

I also found contradictory sources that said Japanese maples (Acer palmatum) are immune to juglone. Not sure which I'd want to believe: