DIGGING DEEPER: Are Black Walnut Trees Toxic to Other Plants?

Black walnut (Juglans nigra) is a common tree in our area that is valued for its many qualities – edible hard mast (nuts) for wildlife and humans, leaves as food for over 100 species of moth and butterfly larvae, attracting songbirds to the landscape, valuable wood for furniture and cabinet making, and high tannin content for dying. While black walnuts are prized for many attributes, they also cause gardeners some challenges when planting nearby. Black walnut produces an allelopathic chemical called juglone in all parts of the tree with higher concentrations in the roots, buds, and nut hulls. “Allelopathy” refers to the chemical inhibition of a plant/organism to another, due to the release into the environment of substances acting as germination, growth, survival, or reproduction inhibitors. Allelon which means “of each other”, and pathos which means “to suffer.” Black walnut trees use juglone as a defense mechanism - hindering insect and animal herbivory while also preventing the competitive growth of neighboring plants. What an evolved super-power!!

The roots, branches and leaves of the black walnut tree release juglone into the soil, discouraging the growth of other plants nearby and thereby reserving more of the area's natural resources for the tree itself. For many plants, this toxin leads to yellowing leaves, leaf drop, wilting and eventual death. It's very common to see a bare patch around the base of a black walnut tree.

What does this mean for gardeners?

• Be mindful of putting black walnut leaves, twigs, and nuts in your compost – if you do – make sure the compost is aged before applying it to your garden.

• Be aware there are some plants that can tolerate juglone and others that are highly susceptible to juglone. Sensitive plants would need to be planted outside of the root zone of a black walnut.

• Plants sensitive to juglone include:

  • some annuals include nightshades (tomatoes, potatoes, tobacco, peppers, petunias), cabbages, asparagus

  • sensitive shrubs include blueberry, chokeberry, cotoneaster, hydrangea, lilac, privet, rhododendron, yew, and some viburnum species

  • sensitive trees include linden, white pine, Norway spruce, silver maple, crabapple/apple, willow, red maple

Research is fairly limited on which plants can tolerate juglone in their root zones, but here are a couple resources to check out:

https://mortonarb.org/plant-and-protect/tree-plant-care/plant-care-resources/black-walnut-toxicity/

https: //extension.psu.edu/landscaping-and-gardening-around-walnuts-and-other-juglone-producing-plants

-Note that even if a black walnut tree is removed from your landscape, it could take years for the juglone to leave the soil – this is especially true for wet sites or poor draining sites with heavy clay.

-Some people can be sensitive to juglone. Skin contact with fresh saw dust and nut hull juices may be irritating. Wear proper protection if cutting down a black walnut – breathing in fresh cut sap/saw dust may upset your respiratory system. Ingesting fresh leaves and nut hulls is not advised.

While “digging deeper” on this topic, I found that there are a lot of conflicting information. It’s apparent that not much recent research has been published and a lot of the plants that are listed as tolerant of juglone are found by trial and error among gardeners. We’d love for you to share any of your observations here in the comment box of the blog post! One of the most enjoyable things about gardening is sharing amongst others!

Note: There are other trees in our area that produce juglone, but to a lesser degree. Included are:

butternut, English and Persian walnut, and hickories.