The Ferment Farm

Why Not Use Walnut Leaves in Mulching?

Question: Walnut leaves in mulching?

Daniel, I love your website on mulching, but have a question. You say: “use the least amount of walnut leaves. I use them sparingly and well mixed.”

Are they harmful? Unfortunately, we have only 3 large walnut trees on our yard (one has fantastic nuts! the other two so-so) and I was hoping to use them.

Thanks. Robert

Hi Robert!

Thank you so much for your feedback!

Walnut leaves are not that good. Sorry. I think they make the soil slightly toxic from too much juglone unless allowed to decompose completely for about 2 years first. Also little walnut trees popup (which are still easier than weeding). I end up with bags of walnut leaves anyways and still use them, just spread/ mixed with others.

Thanks for the question…after a bit of research I can now see why. The info below should be helpful. Remember you can get as many other types of leaves in the fall when others bag theirs for pickup. Just make a big pile of your walnut leaves somewhere to compost for a while, then use them down the road.

Happy Gardening!

Daniel

Research & Excerpts from Other Websites:

Garden-Services.com: WALNUT—MULCH MATERIAL

1. http://www.garden-services.com/mulch.htm

Walnut leaves, hulls, sawdust or wood chips shouldn’t cause a toxicity problem if the material is allowed to compost actively for several months before using. Turn leaves, hulls, chips and sawdust frequently and keep moist during the compost period. An inactive pile should not be used for approximately one year.

Fresh leaves, hulls, sawdust or wood chips may release juglone and injure desirable plants.

Walnut hulls can be added to an active compost pile. Composting heat and bacterial action will destroy the plant-toxic juglone compound. Hulls should be composted several months before using.

The smaller the hull pieces, the quicker they will compost. Break up pieces as small as possible before adding to the compost pile.

Virginia Cooperative Extension Research

2. http://www.ext.vt.edu/pubs/envirohort/426-326/426-326.html

Note: Leaves of the black walnut tree (Juglans nigra) are an exception due to the presence of juglone, a chemical that inhibits growth of many plants. While walnut roots and hulls cause most of the problems, the leaves also contain smaller quantities. Avoid using leaves collected from under black walnut trees as garden mulch. However, if leaves are obtained from a municipal collection source, the quantity of black walnut leaves likely will be diluted sufficiently that no injury should be observed. Several other nut trees also produce small quantities of juglone, and problems with sensitive plants are seldom seen even when growing under those tree canopies.

But What About Black Walnut Leaves?

3. But What About Black Walnut Leaves?! http://www.whyy.org/91FM/ybyg/fallleaves.html

First off, yes—black walnut trees do contain a natural substance called juglone that inhibits the growth of many plants (or just plain kills them). It’s contained in every part of the tree—bark, wood, leaves—but is strongest in the roots. In fact, those roots are SO ‘full of it’ that Dr. Paul Roth, Professor Emeritus, Department of Forestry, Southern Illinois University, warns that even if the trees are cut down, the roots will continue the walnut’s “alleopathic” effect on other plants for several years. Plants noted for dying quickly within this range include such favorites as tomatoes, peppers, eggplant, potatoes and blueberries; petunias, azalea, viburnum, hydrangea and rhododendron.

From Gardening Forum

4. http://www.gardening-forum.net/general/Walnut_Leaves_491295.html

Busy shredding a huge mountain of leaves to use as mulch and mostly in a huge compost pile. There are a few English Walnut leaves. Now, somewhere I either read or heard that Black Walnut leaves were toxic to garden plants and shouldn’t be used for either mulch or in compost. Is that true? Or is the English Walnut a baddie too?

Ohio State University

5. http://ohioline.osu.edu/hyg-fact/1000/1148.html

The roots of Black Walnut (Juglans nigra L.) and Butternut (Juglans cinerea L.) produce a substance known as juglone (5-hydroxy-alphanapthaquinone). Persian (English or Carpathian) walnut trees are sometimes grafted onto black walnut rootstocks. 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. The toxic zone from a mature tree occurs on average in a 50 to 60 foot radius from the trunk, but can be up to 80 feet. The area affected extends outward each year as a tree enlarges. Young trees two to eight feet high can have a root diameter twice the height of the top of the tree, with susceptible plants dead within the root zone and dying at the gins. The juglone toxin occurs in the leaves, bark and wood of walnut, but these contain lower concentrations than in the roots. Juglone is poorly soluble in water and does not move very far in the soil. [from Ohio State University Extension Fact Sheet HYG-1148-93 by Richard C. Funt and Jane tin]

The Ohio State University Extension and the American Horticultural Society have reported that R. nudiflorum, Pinxterbloom Azalea, and Exbury Azaleas Gibraltar and Balzac will grow near Black Walnut and Butternut trees.

Black Walnut Toxicity to Plants, Humans and Horses

6. https://washtenawcd.org/sites/default/files/documents/black_walnut_toxicity_to_plants_1.pdf

HYG-1148-93
Richard C. Funt
Jane Martin

Not all plants are sensitive to juglone. Many trees, vines, shrubs, groundcovers, annuals and perennials will grow in close proximity to a walnut tree. Certain cultivars of “resistant” species are reported to do poorly. Black walnut has been recommended for pastures on hillsides in the Ohio Valley and Appalachian mountain regions. Trees hold the soil, prevent erosion and provide shade for cattle. The beneficial effect of black walnut on pastures in encouraging the growth of Kentucky bluegrass (Poa pratensis L.) and other grasses appears to be valid as long as there is sufficient sunlight and water.

Gardeners should carefully consider the planting site for black walnut, butternut, or persian walnut seedlings grafted to black walnut rootstock, if other garden or landscape plants are to be grown within the root zone of mature trees. Persian walnut seedlings or trees grafted onto Persian walnut rootstocks do not appear to have a toxic effect on other plants.

Horses may be affected by black walnut chips or sawdust when they are used for bedding material. Close association with walnut trees while pollen is being shed (typically in May) also produce allergic symptoms in both horses and humans. The juglone toxin occurs in the leaves, bark and wood of walnut, but these contain lower concentrations than in the roots. Juglone is poorly soluble in water and does not move very far in the soil.

Walnut leaves can be composted because the toxin breaks down when exposed to air, water and bacteria. The toxic effect can be degraded in two to four weeks. In soil, breakdown may take up to two months. Black walnut leaves may be composted separately, and the finished compost tested for toxicity by planting tomato seedlings in it. Sawdust mulch, fresh sawdust or chips from street tree prunings from black walnut are not suggested for plants sensitive to juglone, such as blueberry or other plants that are sensitive to juglone. However, composting of bark for a minimum of six months provides a safe mulch even for plants sensitive to juglone.

Plants Observed Growing Under or Near Black Walnut Trees

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
Narcissus: ‘Yellow Cheerfulness,’ ‘Geranium,’ ‘Tete a Tete,’ ‘Sundial,’ and ‘February Gold’
Sundrops, Oenothera fruticosa
Sensitive 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 Drip Line of Black Walnut