Why black walnut trees are toxic to other plants

September 28, 5:42 PMPittsburgh Gardening Scene ExaminerRose Field

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Black walnuts nestled in a tree

In the fall many Western Pennsylvanians enjoy collecting black walnuts for food. The large trees grow from 70' to more than 100' tall and are common in our area, although not much in favor nowadays as landscape plants. Years ago they were planted not only as the source of delicious nuts, but because they produce very expensive timber. If you happen to own a property with existing black walnut trees, you probably know about their other prominent characteristic: they produce a chemical which is toxic to many other plants.

Black walnut trees (Juglans nigra), and to a lesser extent the related butternut (Juglans cinerea), produce a substance called juglone which is an allelopathic chemical, meaning it is toxic to other plants. Allelopathy has a useful purpose in the plant world as a survival of the species mechanism. If too many seeds of any given plant were to flourish directly beside the parent, overcrowding would be harmful to both the parent and child plants. Allelopathy serves to poison a circle around the parent tree, while nuts which get carried farther away by animals thrive. Fortunately for the tree and unfortunately for gardeners, this keep-away toxicity poisons lots of other plants. Here's a list from the WVU Extension service of those which are and aren't sensitive. Another list from the Ohio State Extension Service duplicates it, but includes a few more; both sites have good information. Tomatoes, hydrangea, azaleas and rhododendron are especially vulnerable. Ohio State also notes that some people suffer allergic symptoms from black walnut pollen.

Although the juglone breaks down eventually when exposed to air, water and bacteria, take care when adding black walnut leaves to compost or using mulch from a tree on other crops. Ohio State recommends planting tomato seeds in compost or mulch to test for residual toxicity before using these substances on plants. 

"Common Trees of Pennsylvania" from the state forestry department shows good line drawings to help ID the black walnut. 

If you do gather black walnuts, be advised that the free nuts come at the cost of lots of work. Processing them is rather messy, time consuming and involves several steps. This University of Minnesota Yard and Garden Brief will take you through the steps.

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