Planting under black walnut trees can be detrimental to certain plants. The black walnut tree (Juglans nigra) contains the phytotoxin juglone which can have a deleterious effect on plants when they are planted near the black walnut. Juglone is present in the roots, leaves, fruit and branches of Black Walnut. Other species in the walnut family also contain juglone but in lesser concentrations. They are, in decreasing order of concentration: English walnut (Juglans reaia), butternut (Juglans cineraria), shagbark hickory (Carya ovata), and pecan (Carya illinoinensis).
Symptoms of walnut toxicity range from stunting to partial or total wilting to death of the affected plant. Frequently, susceptible plants in the home garden may be growing quite well when in just a day or two, they suddenly wilt or die. In an all too frequent scenario, a homeowner plants a successful garden for several years near a young black walnut. Then one year, susceptible garden plants nearest the tree become affected. Each year more of the garden suffers until it becomes impossible to grow plants such as tomato or potato. With herbaceous plants, the wilt closely resembles a wilt caused by a bacterial or fungal pathogen. For example, symptoms on tomato include yellowing of foliage, twisting and internal discoloration. These symptoms are identical to those of Verticillium or Fusarium Wilt.
Factors Involved in the Toxic Action
According to a 1967 study, the juglone acts on susceptible plants by inhibiting respiration; the plant simply cannot breathe properly.
While juglone is present in the leaves, fruit hulls, bark and roots of walnut trees, a problem usually arises only when the roots of these species are in very close proximity or contact with the roots of susceptible plants. A 1925 report noted that a walnut tree’s root distribution could be mapped without removing any soil, merely by observing the wilting pattern in a tomato planting. More recent investigations have substantiated this phenomenon. The average limit of the toxic zone from a mature tree is about 50 to 60 feet, but plants as far away as 80 feet have also been injured.
If the soil is removed from the root zone and all the roots of the walnut are carefully screened out, susceptible plants can thrive in this soil. Thus, the toxin apparently is not secreted from the walnut roots into the surrounding soil. However if a tomato crop is planted in soil where juglone leaves, husks, bark or root pieces have been incorporated, the susceptible tomato plants will rapidly wilt as their roots contact the walnut tissues.
While some plants are susceptible to juglone, others are not and can event thrive in the area of walnuts. The factors affecting susceptibility are not clear. However, deep rooted plants are more commonly affected by juglone than shallow-rooted plants. The following lists note plants which have been shown to be susceptible and not susceptible to juglone toxin.
Because Michigan lies on the natural range of black walnut, it is not unusual for homeowners to have this plant in their backyard. This tree is relatively fast growing, makes a satisfactory landscape tree and produces edible nuts. Faced with choosing between a tree or a garden, homeowners should remember that chopping down a tree is not the immediate solution, unless the root pieces are painstakingly removed.
Barriers to root growth growth may me an alternative to tree removal. Where susceptible species survived alongside wilted ones, it was observed that a rock or old concrete foundation kept the roots from contacting the susceptible plants. Placing a wood or concrete barrier in the soil between your tree and the garden may separate the roots.
If toxicity is a problem because of present or past tree plantings, limit your garden selections to relatively shallow rooted species. Tomatoes and potatoes are deep rooted and very susceptible to juglone. Frequent light watering will promote shallow rooting and may help alleviate some problems.
When landscaping a new homesite and you want both a walnut tree and a garden, plant the tree in the front yard and the garden in the rear. Do not use the leaves and husks as mulch for your vegetables.
Text Prepared by: Mary A. Wilson, Genesee County Extension Horticultural Agent Sources: “Black Walnut Toxicity in Plants”, MH-10; Horticulture Newsletter, Vol. II, No. II; “Allelopathy: Chemical Interactions Between Plants” American Nurseryman, January 15, 1986; “Under the Black Walnut Tree”, Horticulture, October 1986