The Effects of Black Walnut Allelopathy

Allelopathy refers to one plant chemically influencing another plant or organism in a mostly negative way. Black walnut (Juglans nigra) is an allelopathic tree that releases a compound known as hydrojuglone. This compound oxidizes upon contact with the air and forms the highly toxic chemical juglone. Humans have known about the black walnut's allelopathic effects on people, animals and plants since the first century, according to the journal Plant and Soil.

Trees

• The black walnut tree uses allelopathy to increase its likelihood of reproducing. It wipes out or inhibits the growth of neighboring trees, which compete with the black walnut for sunlight, water and fertile soils. For example, a 1983 study in the Journal of Chemical Ecology showed black alder trees grown in proximity to black walnut trees developed trunks with stunted diameter growth. In addition, all of the black alder trees in the study died within 8 to 13 years of exposure to the black walnut.

Crops

• Crop plants, such as potatoes, tomatoes and peppers, can also suffer from black walnut allelopathy. While some crops experience stunted growth, others become discolored, dried out and eventually died. Soybeans and corn are also susceptible to black walnut allelopathy, according to a 1998 study in Plant and Soil. The study involved applying different concentrations of juglone to soybeans and corn samples. After three days, all sample plants, regardless of the concentration of the toxin, showed signs of decline, including stunted shoot and root growth.

Animals

• Black walnut allelopathy also effects animals. According to the Ontario Ministry of Agriculture and Rural Affairs, horses are particularly susceptible to juglone. Eating fallen nuts from the black walnut tree is potentially deadly to horses and may cause over-eating and swelling in the legs. Horses that inhale the tree pollen may experience breathing difficulties and discomfort. For this reason, it is important to keep twigs and leaves out of a horse's bedding.

People

• According to Livestrong.com, many people, like horses, are allergic to black walnut pollen and sawdust. Sawdust can cause runny noses or, in some cases, asthmatic reactions. In spite of its drawbacks, black walnut is popular in folk and herbal medicine. Health supplement retailers such as HerbalRemedies.com recommend black walnut as a homeopathic cure for ringworm, athlete's foot, jock itch and other parasitic, fungal or bacterial conditions.