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.


PLANTS THAT ARE BELIEVED TO BE TOLERANT OF JUGLONE

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.

PLANTS KNOWN TO BE SUSCEPTIBLE TO JUGLONE

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.)


SOURCES:

“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, http://dev.cce.cornell.edu/yates/MG4.25.01.htm

“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