{"id":2974,"date":"2019-09-18T00:08:22","date_gmt":"2019-09-18T07:08:22","guid":{"rendered":"https:\/\/depts.washington.edu\/hortlib\/pal\/pal-question-438\/"},"modified":"2023-08-09T09:53:43","modified_gmt":"2023-08-09T16:53:43","slug":"walnut-leaves-in-compost","status":"publish","type":"pal","link":"https:\/\/depts.washington.edu\/hortlib\/pal\/walnut-leaves-in-compost\/","title":{"rendered":"walnut leaves in compost"},"content":{"rendered":"

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?<\/strong><\/p>\n

 <\/p>\n

It seems that the main source of toxicity is the roots<\/em> of the walnut
\ntree, rather than the leaves or shells. However, there are still those
\nwho believe that there is enough juglone in the leaves that they should
\nbe 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:<\/p>\n

“This toxic affect on surrounding plants appears to be related to root
\ncontact, as walnut hulls and leaves used as mulch have not shown toxic
\neffects on plant growth. [Warning- Frank Robinson disagrees.] Because Walnut
\nroots do not occupy the surface layers in most soil, many shallow rooted
\nplants growing under walnut trees don’t come in contact with the roots
\nand are not affected by them.” [Michigan State University]<\/p>\n

“You’ve probably always heard that you should never add black walnut
\nsawdust [or wood chips] to the compost pile because the juglone will kill
\neverything that grows in the compost. Abraham says that’s not
\nnecessarily true; that juglone is not found in walnut saw dust or wood
\nchips. Nor do dead walnut trees exude juglone. Juglone is harmless to
\nhumans so you can go right ahead and safely eat fruit and vegetables
\ngrown near walnuts.”[Katy Abraham]<\/p>\n

“Robinson doesn’t agree on the use of walnut residue in composting. He
\nhas this to say about black walnut saw dust, husks and leaves affecting
\nplants. ‘Tomatoes growing in clean soil in pots were severely stunted
\nwhen leaves and nuts fell into the pots while we were on vacation. I know
\nwhat juglone can do. I have seen a 15-year-old rhododendron killed a few
\nweeks after its owner mulched it with black-walnut husks, and roses
\ninjured by an application of compost containing black-walnut sawdust.'”
\n[Robinson]<\/p>\n

“The juglone toxin occurs in the leaves, bark, and wood of the walnut but
\nthese contain lower concentrations than the roots. Juglone is poorly
\nsoluble in water and does not move very far in the soil. Walnut leaves
\ncan be composted because the toxin breaks down when exposed to air, water
\nand bacteria. The toxic effect can be degraded in two to four weeks. In
\nsoil, breakdown may take up to two months. Black walnut leaves may be
\ncomposted separately, and the finished compost tested for toxicity by
\nplanting tomato seedlings in it. Sawdust mulch, fresh sawdust or chips
\nfrom street trees prunings are not suggested for plants sensitive to
\njuglone, such as blueberry. However, composting of bark for a minimum of
\nsix months provides a safe mulch even for plants sensitive to juglone.”
\n[Ohio State University]<\/p>\n

“To be on the safe side, composted material containing juglone should be
\nallowed to breakdown over a period of time before use. This composted
\nmaterial can be used with plants that are not susceptible to juglone
\ndamage. If it is important to use it for general composting purposes,
\ntesting it first with a few tomato plants for a few weeks should reveal
\nits level of toxicity.” [Abraham]<\/p>\n

This may also be of interest: The Walnut Tree: Allelopathic Effects and Tolerant Plants<\/a> from the Virginia Cooperative Extension.<\/p>\n

Frank Robinson’s article “Under the Black Walnut Tree,” Horticulture<\/em> 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<\/a> website.<\/p>\n","protected":false},"excerpt":{"rendered":"

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…<\/p>\n

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