FORT COLLINS, Colo. — Why does a walnut produce a chemical, juglone or hydrox juglone? This is a naturally occurring chemical that is a defense mechanism protecting the walnut tree from surrounding plants. What happens to the surrounding plants? It stunts their growth. Juglone inhibits plant respiration. If there are sensitive plants under a walnut, the juglone will deprive these plants of water and nutrient uptake. Symptoms such as wilting or yellowing leaves occur.
Juglone is not soluble in water. Consequently, it does not move very far in the soil. If sensitive plants are under the walnut and the ground is compact, it will succumb to this juglone toxin. If the soil were well-drained and had good organic matter, the sensitive plant would have fared better.
The juglone can persist for years after a walnut is removed. And if you find a lot of plants cannot grow in a particular location, then ask the previous homeowners if there was a walnut tree there. If there were no walnut trees, then test the soil for nutrient value and pH.
Allelopathy is a term meaning one plant inhibits another from growing. This happens from the roots of the walnut touching the roots of other plants. This toxin juglone can also stop any germination of seeds.
Walnuts are not alone in the production of juglone. According to Penn State Extension, English or Persian walnut (J. regia) and hickories (Carya) produce juglone to a lesser degree. So, what plants can survive under a walnut? Below is a brief list:
Herbaceous Perennials: Yarrow, Hosta, Bugleweed, Hollyhock, Iris spp., Aster, Mums & Daffodils
Vines: Clematis, Virginia Creeper, Wisteria
Shrubs: Barberry, Forsythia, Rose of Sharon, Juniper, Elderberry
Vegetables: Onion, Beets, Squash and Melons, Carrots, Beans, Parsnips and Corn.
Reference and full list of Juglone Tolerant Plants at the following link: https://extension.psu.edu/landscaping-and-gardening-around-walnuts-and-other-juglone-producing-plants
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–Linda Langelo
CSU Horticulture Agent
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