The black walnut tree, Juglans nigra,is a member of the hickory family. The presence of a black walnut tree can have a negative impact on attempts to garden in proximity to it and to animals as well.
The black walnut is an attractive yet messy plant. The catkins, or male flowers are shed in the spring and its drupes, which contain the prized nut, will cover the ground at harvest time. In open areas it can achieve heights of upwards of 40 feet.
Juglone
Juglone occurs in parts of the plants in the Juglandaceae family of which the black walnut is a member. Juglone is toxic or growth-stunting to many types of plants. Juglone in the black walnut is concentrated in the drupe but is also found in the roots, leaves, and bark.
Chemical Effect of Juglone
The technical term for the effect of juglone produced by the black walnut on other plants is referred to as its allelopathy, the production a a biochemical which has a negative impact on another species. A simplistic way to define it is that the juglone has a strangulation impact on the intolerant species. Because of this impact on a wide variety of species, juglone has been used as a herbicide.
Certain Plants are Juglone Tolerant
Certain plants are juglone tolerant, that is they will grow in the vicinity of a black walnut tree. For the vegetable gardener these would include beans, carrots, corn, squash, melons, parsnips, and onions.
For the flower gardener marigolds, pansies, begonias, impatiens, and zinnias are some common species which are not impacted.
Several fruit tree species can share the habitat of the black walnut. They include peaches, cherries, plum, and pear.
Some common shrubs and trees include holly, forsythia, juniper, honeysuckle, poplars.
Juglone Intolerant Plants
For the vegetable gardener perhaps two of the most important garden staples the tomato and the potato are extremely jugone intolerant. Among flowering plants azaleas, peonies, chrysanthemum, forget-me-not, and lily-of-the-valley are intolerant. Fruits such as blue and black berries and grapes are intolerant as well. In instances where a black walnut tree was located the toxicity remains in the soil for years.
Other Harmful Effects
Although this article is focused on the impact on gardening it is worth noting that juglone has had an impact on farm animals including horses.
Benefits of the Black Walnut
Despite its disadvantages black walnut provides wonderful shade, fruit which can be used in baking particularly black walnut pie and fudge. The important point for the home gardener is to be aware of the plants which might be impacted by this tree. When in doubt, further information can be obtained through a state agricultural extension office.
Sources:
Thompson, Bruce (1978). Black Walnut for Profit: A Guide to Risks and Rewards. Portland, OR: Timber Press. ISBN 9780917304408.
Tony De Vita - My basic background was in mathematics. Having received a B.S. summa cum laude, from Wilkes College, Wilkes-Barre, Pa. My first job was ...
