QBARS - v27n3 Effect of Black Walnuts on Rhododendrons

Effect of Black Walnuts on Rhododendrons
August E. Kehr, Potomac Chapter
Staff Scientist, Plant and Entomological Sciences
National Program Staff, Agricultural Research Service
U.S. Department of Agriculture, Beltsville, Maryland

During the recent meetings at Pittsburgh, the question was raised on a visit to one of the gardens whether black walnut trees would have an adverse effect on rhododendrons. Fortunately some reliable information is available on this matter.

In 1928 (1) the toxic principle from black walnut trees was identified, by comparison with the purified chemical, as juglone (5-hydroxy-1, 4-napthoquinone). Later, in 1938 (2), it was reliably reported that about 500 9-year -old nursery plants of R. catawbiense transplanted under black walnut died in late August of the season they were transplanted. Rhododendron plants whose roots were not in contact with black walnut roots remained in a healthy and thriving condition, even though as close as two feet from the tree trunk.

More recently (3), the relative concentrations of juglone in various parts of the tree were reported as follows:

Organ juglone (mg/gm dry tissue)
Leaves 1.23
Hulls 6.71
Roots 7.73

In these studies the concentration of juglone appeared to be highest in July. It would appear to be risky to plant rhododendrons and other ericaceous plants under black walnut trees. Interestingly, bluegrass is reported to thrive under these same conditions (2).


  1. Davis, E. F., 1928, The toxic principle of Juglans nigra as identified with synthetic juglone, and its toxic effects on tomato and alfalfa plants, Amer. J. But., 15:620.
  2. Brooks, M. G., 1951, 47, 31 pp.
  3. Lee, K. C., and R. W. Campbell, 1969, Nature and occurrence of juglone in Juglans nigra L. HortScience, 4:297-298.