Extension News

Black Walnut: The Killer Tree

Black walnut

Note to media editors: Garden Column for the week of July 15, 2005

7/11/2005

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:

Black Walnut,  270 KB

Black Walnut 2 , 280 KB