How Black Walnut Trees Affect Nearby Garden Plants

By Blake Jackson

Black walnut trees are valued for their impressive size, attractive appearance, and the food they provide for wildlife. However, gardeners should be aware that these trees can negatively affect certain plants growing nearby due to a naturally occurring chemical called juglone.

According to Bruce Spangenberg, Horticulture Outreach Specialist with the UW-Madison Division of Extension, black walnut trees (Juglans nigra) produce juglone throughout the entire tree, with the highest concentrations found in the buds, roots, and nut hulls.

The chemical is most abundant beneath the tree canopy, where roots spread and leaves and nut husks decompose. Butternut trees (Juglans cinerea) also produce juglone, although in lower amounts.

Juglone can cause a condition known as allelopathy, in which one plant releases substances that interfere with the growth of another. Sensitive plants growing within the root zone of black walnut trees may experience stunted growth, yellowing leaves, wilting, or even complete death.

The damage is most likely to occur when plant roots come into direct contact with walnut roots rather than from juglone moving freely through the soil. Well-drained soils generally reduce the severity of these problems compared with poorly drained sites.

Experts do not recommend removing black walnut trees to solve the issue because their roots can remain in the ground for years, continuing to affect nearby vegetation.

Instead, gardeners should avoid planting susceptible species within the tree's root zone, refrain from using walnut leaves or nut husks as mulch and consider raised garden beds or root barriers to limit root contact.

Several vegetables, including tomatoes, peppers, and potatoes, are especially vulnerable to juglone. Many ornamental plants, fruit trees, and evergreens can also be affected.

Fortunately, many species tolerate juglone well. Vegetables such as beans, beets, carrots, corn, melons, and squash typically grow successfully near black walnuts.

Popular ornamentals like asters, begonias, daylilies, ferns, impatiens, zinnias, and clematis, along with lawn grasses such as Kentucky bluegrass and fescues, are also considered good choices for landscapes near these trees.

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