

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

Trees

European Alder

White Birches

Hackberry

Crabapples

Norway Spruce

Shrubs

Red Chokeberry

Privet (some)

Rhododendrons

Lilacs

Yew

Vegetables

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.



Annuals

Pot Marigold

Begonia, fibrous cultivars

Morning Glory

Pansy Viola

Zinnia species

Most other annuals

Vegetables

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

Contacts :

Christopher Feeley, Forestry, (515) 294-6739, cfeeley@iastate.edu

Jean McGuire, Continuing Education and Communication Services, (515) 294-7033, jmcguire@iastate.edu

Two high resolution photos suitable for publication are available for use with this story, linked below:

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71505WalnutBlack2.jpg, 280 KB