The Garden Corner by Sharon Allen Black Walnut Trees
According to an article (HYG-1148-93) by authors Richard C. Funt and Jane Martin of Ohio State University, black walnut trees can be toxic to plants, humans, and horses. The roots of the black walnut produce a toxic substance known as juglone.
The toxic zone extends about 50-60 feet from a mature tree and enlarges each year as the tree ages.
Plants sensitive to this zone include herbaceous perennials (Colorado Columbine, Wild Columbine, asparagus, mums, Baptisia Australis, Hydrangea species, lilies, alfalfa, some peonies species, and rhubarb); trees (silver maple, European Alder, White Birches, Northern Hackberry, apple and crabapples, Norway Spruce, Mugo Pine, Red Pine, Eastern White Pine, and basswood); shrubs (Red Chokeberry, Hydrangea species, Mountain Laurel, Privet, Amur Honeysuckle, brush cinquefoil, rhododendrons, and azaleas, blackberry, lilac, yews, blueberry and Viburnum); and annual and vegetable transplants (cabbage, peppers, tomatoes, petunia, eggplants, potatoes, and double- flowered cole vegetables).
Bedding materials of either black walnut chips or sawdust may negatively affect horses. Horses and humans may also have allergic reactions to walnut tree pollen.
Squirrels are notorious black walnut tree planters, and believe me, they plant trees EVERYWHERE! This is a good time of the year to search for offending seedlings. I usually use a dandelion weeder to reach under the walnut, usually buried about 4-6 inches deep, and remove both walnut and tree seedling.
It is very important to get the seedlings as early as possible. The older they get, the more difficult they are to remove. Young walnut trees 2-8 feet high can have a root diameter of twice the tree height.