Identification

• Allelopathy is a phenomenon by which one species affects another species through chemical interactions. The allelopathic substance may be found in the stems, leaves, flowers, fruits and roots of the host plant, as well as in the soil around the host plant.
  
  

Features

• All parts of the black walnut tree contain a non-toxic substance called hydrojuglone. The buds, roots, nuts and hulls contain concentrated amounts. When hydrojuglone is exposed to air or soil, it undergoes a chemical reaction that creates juglone, a toxic substance.
  
  

Function

• Plants become poisoned by contact with the roots and decaying foliage of a black walnut or by water dripping from the tree. Juglone moves slowly through the soil, but is present as far out as the roots grow, which can be up to three times the size of the canopy of the tree.
  
  

Effects

• Juglone inhibits the affected plant's respiratory system and deprives the plant of the energy it needs for metabolic activity. The foliage of affected plants turns yellow, and the plants wilt and eventually die.
  
  

History

• Pliny noted during the Roman Era that walnut trees poisoned all plants. Further scientific research disproved this statement.