Juglone (Walnut) Tolerance
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Many members of the walnut family (Juglandaceae) produce in their tissues a chemical called Juglone. Production is highest in black walnut (Juglans nigra) and butternut (Juglans cinerea), with members of the genus Carya producing minimal amounts. Juglone is an allelopathic substance, meaning it affects the growth of other plants. Susceptible plants growing in close proximity to black walnut or butternut may suffer growth stunting, wilt ("walnut wilt") and death. At the physiological level, juglone interrupts the metabolic processes of susceptible plants and causes their demise. This allelopathic reaction can make gardening in the midst of native or cultivated walnut trees very difficult. As such, the need to identify ornamental plants tolerant of juglone is of primary importance. While few formal studies of juglone resistance have been undertaken, one can find many reports of resistant plants. This category attempts to compile such observations from numerous university and other sources. As with any claim of resistance, the relative degree of susceptibility may vary between species and under different growing conditions. Good soil drainage, for example, can reduce juglone toxicity. For the most accurate information for your area, it may be advisable to observe and emulate successful plantings around local walnut trees.

 

Primary Reference(s) for This Information:

 

"Black Walnut Toxicity", Green Tips, Department of Horticulture, Michigan State University, http://www.msue.msu.edu/msue/iac/greentip/blackwal.htm

"Landscape: Juglone Tolerant Plants", University of Wisconsin Urban Horticulture, http://www.uwex.edu/ces/wihort/landscape/juglone.htm

"Walnut Toxicity", Todd Leuty, Ontario Ministry of Agriculture, http://www.gov.on.ca/OMAFRA/english/crops/facts/info_walnut_toxicity.htm