Subject: Walnut leaves in mulching? Back to Fall Prep.
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.
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.
Exert from other websites:
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.
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.
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.
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?
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
Richard C. Funt
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 margins.
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 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
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'
Squashes, Melons, Beans, Carrots, Corn
Peach, Nectarine, Cherry, Plum
Prunus species Pear-Pyrus species
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 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 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
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.
The authors express their appreciation to Drs. M. Scott Biggs, Department of Horticulture and Crop Science, and Harry Hoitink, Department of Plant Pathology, for their review and additional comments.