Will black walnut leaves cause compost to be allelopathic? Should they be kept out of compost? Or is this folklore? The specific compost is made with chicken manure (fresh), grass clippings and walnut leaves. Are there plants that tolerate the toxin in black walnut?
It seems that the main source of toxicity is the roots of the walnut
tree, rather than the leaves or shells. However, there are still those
who believe that there is enough juglone in the leaves that they should
be fully composted before use in the garden. Below are excerpts from information published online in various university extension websites, by various authors, and now unavailable:
“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- Frank Robinson 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.” [Michigan State University]
“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. Abraham 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.”[Katy Abraham]
“Robinson doesn’t agree on the use of walnut residue in composting. He
has this to say about black walnut saw dust, husks and leaves affecting
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 State University]
“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.” [Abraham]
This may also be of interest: The Walnut Tree: Allelopathic Effects and Tolerant Plants from the Virginia Cooperative Extension.
Frank Robinson’s article “Under the Black Walnut Tree,” Horticulture magazine, October 1986, pp. 30-33 concludes that many plants are indeed able to tolerate juglone’s toxicity. Some of the juglone-tolerant plants listed in the article and in other sources are included on Morton Arboretum website.