The black walnut tree is grown primarily for its wood and nuts, yet the tree is used in landscaped areas for its shade. The tree produces a non-toxic chemical called hydrojuglone from its leaves, bark and roots. Once this hits the air or soil, it oxidizes into a toxin called allelochemical juglone. At this point it becomes a threat to other trees and plants through a process called allelopathy, notes North Carolina State University.
Poor Health of Surrounding Trees
Any plants that fall under the canopy of a black walnut tree are at risk of exposure to wilting, stunting or death from the toxins exuded by the tree. Black walnut roots have been known to spread out farther than the drip-line, or farthest point rain will drip from the leaves. North Carolina State University explains that any other tree root that comes within 1/4 to 1/2 inch of a black walnut root will be exposed to jugalone and if susceptible, may wilt or die. Even rain dripping from leaves will carry jugalone, as will decaying leaves blowing about the yard, exposing sensitive plants to allelopathy.
Mycorrhizal Fungi
Toxins from the black walnut tree are not absorbed by plants, but adhere to the organic matter. This organic matter is necessary for healthy microbial growth essential in vigorous gardens. Part of this includes a fungus called mycorrhizal fungi. Allelopathy interferes with the process of this fungus and interrupts good soil development.
Effects on Livestock
The Ontario Ministry of Agriculture Food and Rural Affairs notes that some horses and ponies may react negatively to bedding from black walnut sawdust or chips. Inflammation of the hoof, called laminitis, and increased respiratory rates may be signs that your horse or pony is having an adverse reaction caused by allelopathy. Allergic reactions in people and horses may occur from the pollen of the black walnut tree, as well.
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