Answer: I'm afraid the jury is still out on that one. There have been numerous studies about where the juglone is concentrated and whether or not the toxins degrade during the composting process. Because the controversy continues, I'd opt for building the new compost pile away from the walnut tree. Here's some of the research I've uncovered - you can draw your own conclusions:
Our research produced several articles concerning walnut toxicity from juglone:
UNDER THE BLACK WALNUT TREE by Frank Robinson Horticulture magazine October 1986. Mr. Robertson at that time was estate manager at Albemarle Farms, Charlottesville, VA. He is now executive director of the American Horticultural Society, Mt. Vernon, VA. Most sources quote his work, ofttimes without giving him credit.
Michigan State University's web site for black walnuts (no longer available)
The Green Thumb Garden Handbook by George (Doc) and Katy Abraham, 1971, Prentice-Hall Inc., Englewood Cliffs. NJ., page 85.
Ohio State University Extension's web site for Black Walnut Toxicity
The Dawes Arboretum, Newark, OH web site. List of trees and shrubs not affected by Black Walnut Juglone Toxicity
What appears below is an attempt at combining this research into a single meaningful one with reference to the source. The sources quoted do not always agree on the toxic effects of walnuts.
"This toxic affect on surrounding plants appears to be related to root contact, as walnut hulls and leaves used as mulch have not shown toxic effects on plant growth. [Warning- Robertson disagrees.] Because Walnut roots do not occupy the surface layers in most soil, many shallow rooted plants growing under walnut trees don't come in contact with the roots and are not affected by them." [Mich]
You've probably always heard that you should never add black walnut sawdust [or wood chips] to the compost pile because the juglone will kill everything that grows in the compost. Abrahams says that's not necessarily true; that juglone is not found in walnut saw dust or wood chips. Nor do dead walnut trees exude juglone. Juglone is harmless to humans so you can go right ahead and safely eat fruit and vegetables grown near walnuts. [Abraham]
Robertson doesn't agree on the use of walnut residue in composting. He has this to say about black walnut saw dust, husks and leaves effecting plants. "Tomatoes growing in clean soil in pots were severely stunted when leaves and nuts fell into the pots while we were on vacation. I know what juglone can do. I have seen a 15-year-old rhododendron killed a few weeks after its owner mulched it with black-walnut husks, and roses injured by an application of compost containing black-walnut sawdust." [Robinson]
"The juglone toxin occurs in the leaves, bark, and wood of the walnut but these contain lower concentrations than the roots. Juglone is poorly soluble in water and does not move very far in the soil. Walnut leaves can be composted because the toxin breaks down when exposed to air, water and bacteria. The toxic effect can be degraded in two to four weeks. In soil, breakdown may take up to two months. Black walnut leaves may be composted separately, and the finished compost tested for toxicity by planting tomato seedlings in it. Sawdust mulch, fresh sawdust or chips from street trees prunings are not suggested for plants sensitive to juglone, such as blueberry. However, composting of bark for a minimum of six months provides a safe mulch even for plants sensitive to juglone" [Ohio]
To be on the safe side, composted material containing juglone should be allowed to breakdown over a period of time before use. This composted material can be used with plants that are not susceptible to juglone damage. If it is important to use it for general composting purposes, testing it first with a few tomato plants for a few weeks should reveal its level of toxicity.
Best wishes with your new compost bin!
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