The following question was sent to the P&PDL diagnosticians here at Purdue University:

Question: I'm very curious as to the current standing on the allelotoxin, juglone, which can be found in many woody plants (hickories, pecan, etc.). Some debate has passed over current years as to juglone being the true causative factor in black walnut toxicity. I know that there are many factors which seem to regulate juglone toxicity in soil (moisture, micro-organisms, etc.). What is the current standing of juglone as being the allelotoxin in the black walnut, and if juglone is responsible, what is its breakdown pattern in soils? (biochemical degradation). A tough question that I do not know the answer to. Also, do you have suggestions for trees and shrubs that will grow in the shadow of the black walnut?

Answer: These are good questions! The largest concentration of juglone and hydrojuglone (converted to juglone by sensitive plants) occur in the walnut's buds, nut hulls, and roots. However, leaves and stems do contain a smaller quantity. Juglone is poorly soluble in water and thus does not move very far in the soil.

Small amounts of juglone are released by live roots. Therefore, some sensitive plants may tolerate the amount of juglone present in the soil near a black walnut tree, but may not survive direcly under its canopy. Also, decaying roots still relase juglone, toxicity can persist for some years after a tree is removed.

The following landscape plants have been observed to be tolerant to juglone: arborvitae, autumn olive, red cedar, catalpa, clematis, crabapple, daphne, elm, euonymous, forsythias, hawthorn, hemlock, hickory, honeysuckle, junipers, black locust, Japanese maple, maple (most), oak, pachysandra, pawpaw, persimmon, redbud, rose of sharon, wild rose, sycamore, viburnum (most), Virginia creeper.

The above information is described in more detail in Purdue's Black Walnut Toxicity Publicaton HO-193 (PDF 36K).

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