The black walnut tree manufactures a substance that is a natural insecticide according to experts at the Texas State University in Austin.
American black walnut tress contain a tannic acid chemists call juglone. The reddish yellow substance leaches from leaves, and some believe exudes from roots, or transfers from branches and foliage to the roots. Tree physiologists agree that roots of other plants that come in contact with those black walnut tree roots die--even other black walnut seedlings.
Juglone is sometimes washed out of the still green walnuts during late summer or even autumn rains. The growth of plants 60 to 80 feet away are inhibited by the juglone. The substance affects plants of various families. Studies have found that tomatoes, alfalfa, potatoes, apples, blackberries, rhododendron, mountain laurel and pine trees are all affected.
According to a professor and extension forester at Iowa State University, juglone is known to repel various garden insects. Just by placing branches of the tree around the house and under furniture dog and cat owners are amazed to find that their pets have fewer fleas.
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