Juglone (Jug)

Juglone occurs naturally in the leaves, roots, husks, fruit (the epicarp), and bark of plants in the Juglandaceae family, particularly the black walnut (Juglans nigra), and is toxic or growth-stunting to many types of plants. It is sometimes used as a herbicide, as a dye for cloth and inks, and as a coloring agent for foods and cosmetics. Juglone is an example of allelopathic compound, a substance that is synthesized by one type of plant and affects the growth of another. Landscapers have long known that gardening underneath or near black walnut trees can be difficult. Juglone exerts its effect by inhibiting certain enzymes needed for metabolic function. A number of plants and trees are resistant to juglone including some species of maple (Acer), birch (Betula), and beech (Fagus). It is highly toxic to many insect herbivores.