Pecan Tree Toxicity – Can Juglone In Pecan Leaves Harm Plants

Pecan Tree Toxicity – Can Juglone In Pecan Leaves Harm Plants

By: Amy Grant

Plant toxicity is a serious consideration in the home garden, especially when children, pets or livestock may be in contact with potentially harmful flora. The question is, are pecan trees toxic to surrounding plants? Let’s find out.

Black Walnut and Pecan Tree Juglone

The relationship between plants wherein one produces a substance such as juglone, which affects the growth of another is called allelopathy. Black walnut trees are fairly notorious for their toxic effects to surrounding juglone sensitive vegetation. Juglone doesn’t tend to leach out of the soil and may poison nearby foliage at the circumference of twice the radius of the tree’s canopy. Some plants are more susceptible to the toxin than others and include:

  • Azalea
  • Blackberry
  • Blueberry
  • Apple
  • Mountain laurel
  • Potato
  • Red pine
  • Rhododendron

Black walnut trees have the highest concentration of juglone in their buds, nut hulls and roots but other trees related to the walnut (Juglandaceae family) produce some juglone as well. These include butternut, English walnut, shagbark, bitternut hickory and the aforementioned pecan. In these trees, and specifically with regards to juglone in pecan leaves, the toxin is generally minimal and does not affect most other plant species.

Pecan Tree Toxicity

Pecan tree juglone amounts do not usually affect animals unless ingested in large amounts. Pecan juglone can cause laminitis in horses. It is not recommended that you feed pecans to the family dog either. Pecans, as well as other nut types, can cause gastric intestinal upset or even an obstruction, which can be serious. Moldy pecans may contain tremorgenic mycotoxins which can cause seizures or neurological symptoms.

If you have had problems with plant failings near a pecan tree, it may be wise to replant with juglone tolerant species such as:

  • Arborvitae
  • Autumn olive
  • Red cedar
  • Catalpa
  • Clematis
  • Crabapple
  • Daphne
  • Elm
  • Euonymus
  • Forsythia
  • Hawthorn
  • Hemlock
  • Hickory
  • Honeysuckle
  • Juniper
  • Black locust
  • Japanese maple
  • Maple
  • Oak
  • Pachysandra
  • Pawpaw
  • Persimmon
  • Redbud
  • Rose of Sharon
  • Wild rose
  • Sycamore
  • Viburnum
  • Virginia creeper

Kentucky bluegrass is the best choice for lawns near or around the tree.

So, the answer to, “Are pecan trees toxic?” is no, not really. There is no evidence that the minimal amount of juglone affects surrounding plants. It also has no impact when composting and makes excellent mulch due to its easily crushed leaves that are slow to decompose.

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Black Walnut Trees Produce Juglone, Toxic to Many Other Plants

Black Walnuts produce a substance known as juglone which is toxic to many plants, and can cause allergic reactions in humans and horses. Juglone is excreted primarily in the roots, saturating the soil in a radius of 50-60 feet or more in a mature tree. The buds and nut hulls are also high in juglone.

The relationship between plants in which one produces a substance which affects the growth of another is known as "allelopathy", a condition Pliny noted in black walnuts around 77 A.D. (Additional common landscape trees with allelopathic properties are sugar maple, tree-of-heaven, hackberries, southern waxmyrtle, American sycamore, cottonwood, black cherry, red oak, black locust, sassafrass, and American elm.) Juglone is alleopathic to many plants, while other plants are very tolerant of it. (See lists of juglone-sensitive and juglone-tolerant plants below.)

Juglone occurs in the leaves, bark and wood of black walnuts although to a lesser degree than their roots and hulls. A Black Walnut sapling can spread juglone twice the radius of its small canopy. English Walnut and Carpathian Walnut trees are sometimes grafted onto Black Walnut rootstock (Juglans nigra L.) and Butternut rootstock (Juglans cinerea L.), making those trees produce toxins as well. Because juglone is poorly soluble in water, it tends not to leach out of the soil. Many plants such as tomato, potato, blackberry, blueberry, azalea, mountain laurel, rhododendron, red pine and apple may be injured or killed within one to two months of growth within the root zone of these trees. A few other trees closely related to black walnut produce limited quantities of juglone although toxicity is seldom observed. Those trees are butternut, English walnut, pecan, shagbark and bitternut hickory.


Black Walnuts in Husks
Black Walnut Leaves

The presence of juglone is highly concentrated immediately under the leaf canopy of black walnuts, both from the tree roots and the accumulation of dead and dying debris. Decaying roots from a dead black walnut tree can still contain juglone for many years. The leaves containing juglone may be composted where the juglone will break down in several weeks from the presence of bacteria, air and water. If you want to test the toxicity of composted walnut leaves, plant tomato seedlings in it. Tomatoes are highly sensitive to juglone and will quickly die in its presence.

Fresh sawdust and ground black walnut mulch should not be used around plants sensitive to juglone. Black walnut pollen (usually in May) has caused allergic reactions in humans and horses and black walnut bedding in horse stalls has a similar effect. However, mulch made from walnut bark is said to be safe to use on even juglone-sensitive plants after composting 6 months to a year. Frankly, I’d rather not take the chance. And of course, breakdown is directly affected by soil type, micro-organisms and drainage. Spring is a good time to check your vegetable and flower beds for new seedlings emerging from the nut hoard squirrels may have planted the previous fall. Pulling them out now before their roots take hold is quite easy.

There are some benefits to growing black walnuts. One is the delicious nutmeat, and another is the dye you can make from the hulls. Black walnuts have been recommended for pastures on hillsides in the Ohio Valley and Appalachian mountains to hold the soil and prevent erosion. They provide shade for cattle. There is also said to be a beneficial effect on growing Kentucky bluegrass in pastures.

Medicinal Properties of Black Walnuts

Black Walnut has antifungal anti-parasitic and antiseptic properties. Rubbed on the skin, Black Walnut extract is reputed to be beneficial for eczema, herpes, psoriasis, and skin parasites. External applications have been known to kill ringworm. The Chinese are said to use Black Walnut to kill tapeworms with extremely good success.

The brown stain found in the green husk of black walnuts contains organic iodine, which has antiseptic and healing properties and also makes a good insect repellant.

There are several things a gardener may consider if you have black walnuts on your property. You should locate your garden area as far from the walnuts as possible. If that is not possible, consider raised beds but build them in such a way as to prevent black walnut roots from getting in to the planting area. Keep the area clean of all leaves and other debris from the black walnut trees.

Culinary Properties of Black Walnuts

Black walnuts have a strong, rich, smoky flavor with a hint of wine. Use them any recipe that call for nuts. Caution: the black walnut flavor may overpower everything else. Combine them 1:3 with English walnuts to tone down the flavor.

Black Walnut Chicken Quiche

1 cup finely chopped cooked chicken
1 cup grated Swiss cheese
1/4 cup chopped onion
1 tablespoon flour
2 eggs, beaten
1/2 teaspoon spicy brown mustard
1 cup milk
1 cup Black Walnuts, chopped
1 baked 9-inch pie crust

Mix chicken, cheese, onion, flour and 1/2 cup Black Walnuts. Sprinkle into crust. Mix beaten eggs, milk and mustard. Pour over chicken mixture top with remaining Black Walnuts. Bake at 325 degrees for 50 minutes. 6 servings.

Photo Credits:
Thanks to hczone6 and Equilibrium for use of their photos from Plantfiles.
Black Walnut tree, iStockPhoto.com, # 3714758, Used by Permission
Black Walnuts, ©Dominika Sebjan, iStockPhoto.com #2336710, Used by Permission


Plants Observed Growing Under or Near Black Walnuts*

Trees
* Japanese Maples, Acer palmatum and its cultivars
* Southern Catalpa, Catalpa bignonioides
* Eastern Redbud, Cercis canadensis
* Canadian Hemlock, Tsuga canadensis
Vines and Shrubs
* Clematis 'Red Cardinal'
* February Daphne, Daphne mezereum
* Euonymus species
* Weeping Forsythia, Forsythia suspensa
* Rose of Sharon, Hibiscus syriacus
* Tartarian Honeysuckle, Lonicera tatarica, and most other Lonicera species
* Virginia Creeper, Parthenocissus quinquefolia
* ** Pinxterbloom, Rhododendron periclymenoides
* **'Gibraltar' and 'Balzac', Rhododendron Exbury hybrids
* Multiflora Rose, Rosa multiflora
* Black Raspberry, Rubus occidentalis
* Arborvitaes, Thuja species
* ** Koreanspice Viburnum, Viburnum carlesii, and most other Viburnum species
Annuals
* Pot-marigold, Calendula officinalis 'Nonstop'
* Begonia, fibrous cultivars
* Morning Glory, Ipomoea 'Heavenly Blue'
* Pansy Viola
* Zinnia species
Vegetables
* Squashes, Melons, Beans, Carrots, Corn
Fruit Trees
* Peach, Nectarine, Cherry, Plum
* Prunus species, Pear-Pyrus species
Herbaceous Perennials
* Bugleweed, Ajuga reptans
* Hollyhock, Alcea rosea
* American Wood Anemone, Anemone quinquefolia
* Jack-in-the-Pulpit, Arisaema triphyllum
* European Wild Ginger, Asarum europaeum
* Astilbe species
* Bellflower, Campanula latifolia
* **Chrysanthemum species (some)
* Glory-of-the-Snow, Chionodoxa luciliae
* Spring Beauty, Claytonia virginica
* Crocus species
* Dutchman's Breeches, Dicentra cucullaria
* Leopard's-Bane, Doronicum species
* Crested Wood Fern, Dryopteris cristata
* Spanish Bluebell, Endymion hispanicus
* Winter Aconite, Eranthis hyemalis
* Snowdrop, Galanthus nivalis
* Sweet Woodruff, Galium odoratum
* Herb Robert, Geranium robertianum
* Cranesbill, Geranium sanguineum
* Grasses (most) Gramineae family
* Jerusalem Artichoke, Helianthus tuberosus
* Common Daylily, Hemerocallis 'Pluie de Feu'
* Coral Bells, Heuchera x brizoides
* Orange Hawkweed, Hieracium aurantiacum
* Plantain-lily, Hosta fortunei 'Glauca'
* Hosta lancifolia
* Hosta marginata
* Hosta undulata 'Variegata'
* Common Hyacinth, Hyacinthus Orientalis 'City of Haarlem'
* Virginia Waterleaf, Hydrophyllum virginianum
* Siberian Iris, Iris sibirica
* Balm, Monarda didyma
* Wild Bergamot, M. fistulosa
* Grape Hyacinth, Muscari botryoides
* Sweet Cicely, Myrrhis odorata 'Yellow Cheerfulness,' 'Geranium,' 'Tete a Tete,' 'Sundial,' and 'February Gold'
* Sundrops, Oenothera fruticosa
* Senstitive Fern, Onoclea sensibilis
* Cinnamon Fern, Osmunda cinnamomea
* Peony, **Paeonia species (some)
* Summer Phlox, Phlox paniculata
* Mayapple, Podophyllum peltatum
* Jacob's-Ladder, Polemonium reptans
* Great Solomon's-Seal, Polygonatum commutatum
* Polyanthus Primrose, Primula x polyantha
* Lungwort, Pulmonaria species
* Bloodroot, Sanguinaria canadensis
* Siberian Squill, Scilla sibirica
* Goldmoss Stonecrop, Sedum acre
* Showy Sedum, Sedum spectabile
* Lamb's-Ear, Stachys byzantina
* Spiderwort, Tradescantia virginiana
* Nodding Trillium, Trillium cernuum
* White Wake-Robin, Trillium grandiflorum
* Tulipa Darwin 'White Valcano' and 'Cum Laude,' Parrot 'Blue Parrot,' Greigii 'Toronto'
* Big Merrybells, Uvularia grandiflora
* Canada Violet, Viola canadensis
* Horned Violet, Viola cornuta
* Woolly Blue Violet, Viola sororia

*These are based upon observations and not from clinical tests.
**Cultivars of some species may do poorly.


Plants That DO NOT Grow Within 50 Feet of the Drip Line of Black Walnut


Herbaceous Perennials

* Colorado Columbine, Aquilegia caerulea
* Wild Columbine, Aquilegia canadensis
* Asparagus, Asparagus offinalis
* *Chrysanthemum Chrysanthumum species (some)
* Baptisia australis
* Hydrangea species
* Lilies, Lilium species (particularly the Asian hybrids)
* Alfalfa, Medicago sativa
* Buttercup, Narcissus 'John Evelyn,' 'Unsurpassable' 'King Alfred' and 'Ice Follies'
* Peonies, *Paeonia species (some)
* Rhubarb, Rheum rhabarbarum
Trees
* Silver Maple, Acer saccharinum
* European Alder, Alnus glutinosa
* White Birches, Betula species
* Northern Hackberry, Celtis occidentalis
* Apples and Crabapples, Malus species
* Norway Spruce, Picea abies
* Mugo Pine, Pinus mugo
* Red Pine, Pinus resinosa
* Eastern White Pine, Pinus strobus
* Basswood, Tilia heterophylla
Shrubs
* Red Chokeberry, Aronia arbutifolia
* Hydrangea species
* Mountain Laurels, Kalmia species
* Privet, Ligustrum species
* Amur Honeysuckle, Lonicera maackii
* Brush Cinquefoil, Potentilla species
* Rhododendrons and Azaleas, **Rhododendron species (most)
* Blackberry, Rubus allegheniensis
* Lilacs, Syringa species and cultivars
* Yew, Taxus species
* Blueberry, Vaccinium corymbosum
* *Viburnum plicatum tomentosum 'Mariesii'
Annuals and Vegetables Transplants
* Cabbage, Brassica oleracea capitata
* Peppers, Capsicum species (some)
* Tomatoes, Lycopersicon esculentum
* Flowering Tobacco, Nicotiana alata
* Petunia species and cultivars
* Eggplant, Solanum melongena
* Potato, Solanum tuberosum
* double-flowered cole vegetables

*Cultivars of some species may survive but will do poorly.

Thanks to Ohio State University for their lists of juglone-tolerant and juglone-sensitive plants.


Produced by trees including black walnut (Juglans nigra, USDA zones 5 to 9), pecan, black cherry (Prunus serotina, USDA zones 3 to 9) and a few others, juglone affects intolerant plants that grow within the root zone of the tree by preventing cellular respiration. Juglone is contained in most parts of the pecan tree plant, spreading into the soil through the roots, fallen leaves and nuts, as well as when rainwater passes over the plant.

Cleaning up fallen plant tissue such as broken twigs, branches, fallen leaves and nuts is one of the only ways to reduce the effects of the toxin in the soil. Rake the debris up and dispose of it.


Ministry of Agriculture, Food and Rural Affairs

Table of Contents

Introduction

Most members of the Walnut family (Juglandaceae) produce a chemical called "juglone" (5 hydroxy-alphanapthoquinone) which occurs naturally in all parts of these plants. Black walnut, pecan, hickory and others members of the family including Carya, Engelhardtia, Juglans, Platycarya and Pterocarya can produce juglone.

Black walnut and butternut produce the largest quantity of juglone and can cause toxic reactions with a number of other plant species that grow in their vicinity. Other juglone-producing species including English walnut, pecan, shellbark/shagbark/bitternut hickory, produce such small quantities of juglone that toxic reactions in other plants are rarely observed. Specific named or numbered cultivars of English walnuts and Japanese 'heartnut' walnuts that are used in commercial orchards or in landscapes are often grafted onto rootstock of native black walnut.

While many plants are tolerant to juglone and grow well in close proximity to walnut trees, there are certain susceptible plant species whose growth can be affected by walnut trees. Through observation and experience, many plant species have been classified as either 'susceptible' or 'tolerant' to walnut family members. 'Allelopathy' is a term used to describe natural interactions between plants where one plant produces a substance that affects the growth of another plant.

Experimentally, juglone has been shown to be a respiration inhibitor, which deprives sensitive plants of needed energy to enable metabolic activity. Affected plants cannot exchange carbon dioxide and oxygen properly. In affected tomatoes, xylem vessels become plugged by callus tissue, blocking upward movement of water in the plant.

Symptoms of walnut toxicity range from stunting of growth, to partial or total wilting, to death of the affected plant. The toxic reaction often occurs quickly where sensitive plants can go from healthy to dead within one or two days. Many alarmed gardeners often believe the cause of wilting is due to fungal or bacterial disease. Once wilting begins, the effect cannot be reversed. The severity of the toxic symptoms can vary depending on the plant species that is in contact with the juglone.

Walnut roots can be identified as having fairly thick bark with inner wood that quickly turns dark yellow when the bark is removed. There is also a distinctive, pungent walnut odor from the cut root. Juglone from decomposing black walnut roots can persist in the soil for more than a year after walnut trees have been removed. Walnut roots may extend 50 to 80 feet away from the outer canopy of mature walnut trees. Young walnut trees do not appear to cause toxic reactions with sensitive plants until the trees are seven to eight years old.

Raked up leaves, twigs and husks from walnut trees should be composted for one year to ensure all juglone has broken down prior to spreading into gardens or used as mulch around sensitive plants.

Gardens should be located away from black walnut and butternut trees to prevent damage to susceptible plants. Where close proximity is unavoidable (a neighbors yard) then raised garden beds can provide some protection from juglone toxicity. Care must be taken to minimize or prevent walnut tree roots from growing upwards into the raised beds. Underlying a garden with plastic or fabric weed barrier during construction can prevent tree roots from growing into raised beds.

Excellent soil drainage will also help reduce toxicity problems, even among sensitive plant species. In well-drained soil, toxic reactions only occur when direct contact is made between walnut roots and roots of sensitive species. In poorly drained soil direct contact between roots is not necessary to cause toxic reactions since juglone moves through the soil water. It has been suggested that plants having shallow root systems are more tolerant of juglone than deep-rooted species. Tolerance to juglone by shallow-rooted species may also be attributed to better drainage of soil water in upper soil levels.

Other Problems

Horses and ponies can contract acute laminitis, an inflammation of the foot, where black walnut wood chips or sawdust is used for stall bedding. Acute laminitis and high respiratory rates in horses and ponies has also been reported where stables and paddocks are located too close to walnut trees. Pollen shedding from walnut trees can cause allergic reactions in people and horses.

Husks of fallen walnuts can become toxic to livestock, and lethal to dogs if ingested due to a mycotoxin called 'Penitrim A', which is produced by Penicillium mould. Therefore, walnut nuts showing symptoms of decomposition, such as a brown or black rotten appearance in the husks, may leak toxin into the kernels and are not fit for human consumption.

Plant Susceptibility

The following tables list plant species that are known to be tolerant and susceptible to juglone.


Black Walnut: The Killer Tree


Black Walnut trees produce juglone in its fruit, leaves and branches that can be excreted from the root system into the soil.

By Chris Feeley
Extension Forester
Iowa State University

As a forester, I very often am asked “Will black walnuts have harmful effects on nearby plants?” Like a true professional, I always give the best answer. Maybe.

In the 1880s, scientists identified a compound called juglone that is produced by black walnut trees. After conducting a few tests, the scientists demonstrated that injury and sometimes death resulted when the chemical juglone came in contact with a susceptible plant. The symptoms that they noted were yellowing leaves, wilting and eventual death of certain plants.

We now know that juglone is produced in the fruit, leaves and branches, and can be excreted from the root system into the soil. The actual concentration in each tree part varies with the season. In spring, juglone is concentrated in the actively growing leaves. The amount of juglone in the roots remains relatively high throughout the summer, and the concentration of juglone in the hulls of the fruit increases as the crop matures.

All species of the walnut family (Juglandaceae) produce juglone. This would include many native trees such as black walnut, butternut, the hickories and pecan. However, black walnuts have the highest concentration of juglone.

In most cases, the damage caused by black walnuts to other plants is a combination of the presence of juglone in the soil, and the competition for light, water and nutrients.

However, juglone can cause severe damage and even kill solanaceous crops (tomatoes, potatoes, peppers and eggplant). Fortunately, not all plants are susceptive to the chemical. Most trees, vines, shrubs, annuals, perennials, corn, beans, onions, beets and carrots are tolerant of juglone.

Gardeners who have large walnut trees near their vegetable gardens should consider an alternate site. The greatest concentration of juglone in the soil exists within the dripline of the trees. The dripline is the area between the trunk of the tree and the end of the branches. The toxic zone from a mature tree occurs on average in a 50-foot radius from the trunk. Avoid planting your garden in these areas to protect your garden from damage.

Walnut leaves can be composted because the juglone toxin breaks down when exposed to air, water and bacteria. The toxic effect can be degraded in two to four weeks. In the soil, breakdown may take up to two months after the living walnut tree has been removed. Mulch or woodchips from black walnut are not recommended for plants sensitive to juglone. However, composting the woodchips for a minimum of six months allows the chemical to break down to a safe level even for plants sensitive to juglone.

Plants Sensitive to Juglone

Herbaceous Perennials

  • Columbine
  • Asparagus
  • Chrysanthumum species (some)
  • Hydrangea species
  • Lilies (particularly the Asian hybrids)
  • Alfalfa
  • Narcissus
  • Peonies (some)
  • Rhubarb

  • European Alder
  • White Birches
  • Hackberry
  • Crabapples
  • Norway Spruce

  • Red Chokeberry
  • Privet (some)
  • Rhododendrons
  • Lilacs
  • Yew

  • Cabbage
  • Peppers
  • Tomatoes
  • Eggplant
  • Potato

Plants Tolerant of Juglone

Trees, Shrubs and Vines

Most trees, shrubs and vines can be grown near black walnut trees with little to no effect on the plant health.

  • Pot Marigold
  • Begonia, fibrous cultivars
  • Morning Glory
  • Pansy Viola
  • Zinnia species
  • Most other annuals

  • Squashes
  • Melon
  • Beans
  • Carrots
  • Corn

Fruit Trees

  • Peach
  • Nectarine
  • Cherry
  • Plum

Herbaceous Perennials

  • Bugleweed
  • Hollyhock
  • American Wood Anemone
  • Jack-in-the-Pulpit
  • European Wild Ginger
  • Astilbe species
  • Bellflower
  • Chrysanthemum species (some)
  • Glory-of-the-Snow
  • Spring Beauty, Claytonia virginica
  • Crocus species
  • Dutchman's Breeches
  • Leopard's-Bane
  • Crested Wood Fern
  • Spanish Bluebell
  • Winter Aconite
  • Snowdrop
  • Sweet Woodruff
  • Herb Robert
  • Geranium
  • Grasses (most)
  • Jerusalem Artichoke
  • Common Daylily
  • Coral Bells
  • Orange Hawkweed
  • Hostas
  • Siberian Iris
  • Phlox
  • Sedum
  • Lamb's Ear
  • Spiderwort


Pecan Tree Juglone Info: Are Pecan Trees Toxic To Other Plants - garden

Q. We have three pecan trees, planted in 1996. They started producing in about ten years. The first year there were about three mature nuts and about a dozen immature nuts. The following year there were about a dozen mature nuts and a dozen immature nuts. Since then they have produced only immature nuts. What are they missing? Do they need some kind of soil additive?

A. There are a number of challenges to growing a good pecan crop in Indiana. There are two major groups of pecan tree cultivars. The southern types are not hardy enough for our area and require a greater number of growing days than can be relied upon in Indiana. The northern types can be successfully grown in southern Indiana but are further sub-divided into two types. You must have at least one from each type to assure adequate cross-pollination. Group I includes the cultivars Giles, Major, Yates and Peruque. Major is said to be partially self-fruitful, meaning it can produce a partial crop even in the absence of another cultivar. Group II includes Colby, Lucas and Posey.

Assuming that your three trees are northern-hardy types and that they are cross-pollination compatible, there are still more challenges to nut production, such as summer drought, insect and disease. Check out the following resources for additional pecan information.

Q. I live in the very southern part of Daviess County, Indiana, and would like to get some more information on growing nut trees, almond trees in particular.

A. I hate to be the bearer of bad news, but almonds are just not a good possibility for growing in Indiana. California is the only U.S. state active in commercial almond production. There are so-called hardy almonds, but even those are generally not viable for growing in Indiana. Their early spring blooming habits makes them quite susceptible to our normal spring frosts. These hardy almonds also have poor nut quality, compared to commercial almonds.

The good news is that there are several nut species that are well-adapted to Indiana: black walnut, filbert, and Chinese chestnut and in southern Indiana, perhaps some of the hardier selections of pecan and Carpathian walnut. University of Illinois Extension has a good overview of nut growing for home gardeners at http://www.aces.uiuc.edu/

vista/html_pubs/NUTGROW/nuthome.html. Also see the Indiana Nut Growers Association website mentioned in the previous question.

Q. I am an avid gardener but I cannot get big heads of broccoli to grow in my garden. Everything else seems to do well. The last couple of years I have added cow manure and some duck manure to my 50'x25' garden. In the fall I put leaves on it and till it in. I get big plants but broccoli heads no bigger than 2-3 inches in diameter. I would really like to get some big heads like you see in the grocery stores.


Walnut Trees and Your Garden (juglone)

Since ancient times, scholars have suspected that walnuts have harmful effects on nearby plants. In the 1880s, scientists isolated a compound called juglone from the fruit of walnuts. They demonstrated that injury and sometimes death result when this phytotoxic material interacts with susceptible plants.

In addition to the fruit, juglone has also been found in the leaves, branches, and roots. The actual concentration in each part varies with the season. In spring, juglone is concentrated in the rapidly growing leaves. The amount of juglone in the roots remains relatively high throughout the summer. The concentration of juglone in the hulls of the fruit increases as the crop matures. All species of the walnut family produce juglone. Black walnuts have the highest concentrations. Relatively small amounts are found in butternut, hickory, and pecan. Most toxicity problems are caused by the black walnut.

The sources of juglone in the soil include both living and decaying plant material. Rain droplets leach juglone from the buds, leaves, and twigs. The decomposition of plant debris by soil microorganisms also releases juglone. Living roots exude juglone into the surrounding soil.

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.

(This resource was added May 2003 and appeared in the Lincoln Journal Star Newspaper Sunday edition. For information on reproducing this article or using any photographs or graphics, read the Terms of Use statement)

University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office


Watch the video: Pecan From Seed To Tree