Black walnut trees contain a chemical, juglone, that inhibits the growth and survival of surrounding plants. Juglone is concentrated mostly in the Flowers, nuts, and roots. As the tree’s roots can extend 2-3 times the diameter of the tree’s canopy, the affected area can be large.
Symptoms of severely affected plants
- Stunted growth
- Rapid yellowing and wilting that cannot be reversed with adequate water
How to determine if you have a black walnut tree
If you think the problems you’re having with your plants match the symptoms of plants suffering from juglone toxicity, determine if you have a black walnut tree.
Black walnut trees have dark bark, long, compound leaves, and when they are mature they produce walnuts surrounded by a thick casing that is a bit bigger than a baseball and green in color. This will turn black and decompose, leaving only the round walnut behind.
If you’re still not sure, you can check out the Arbor Day Foundation’s “What Tree is That?” online tree identification field guide at https://www.arborday.org/trees/whattree/ or you can call or come in with pictures and samples to speak with one of our certified arborists.
What to do if you have a black walnut tree
The sensitivity of your plants to the walnut tree will depend on the type of plant it is and on your soil type. Poorly draining soil will intensify the toxicity. As removal of the tree will not solve the issue of the toxic root system for several years, plant as far from the tree as possible. If this is not an option:
- Plant beyond the canopy line of your black walnut.
- Build raised beds with a barrier at the bottom (e.g. weed barrier or screening) to prevent walnut roots from growing into the bed.
- Remember to clear away leaves and nut husks when necessary, as these are also toxic.
- DO NOT compost with black walnut wood chips, bark, nuts, hulls, or leaves if you have sensitive plants.
The most reliable option is to plant species that are black walnut tolerant.
For a list of plant species that are black walnut tolerant please click here to download a PDF.