1 - Proper composting and vermicomposting will probably detoxify the allelopathic chemical juglone produced by walnuts.
Yard and Garden March 15, 2014
Q.
Can we use Juglans leaves for vermicompost (for worm composting) or for traditional compost? I am concerned about allelopathy.
-Teyyub Babayev
Via internet
A.
All species of Juglans (walnut trees) produce the allelopathic chemical juglone, but it is most concentrated in black walnut (Juglans nigra). Juglone can have negative effects on the growth of many garden plants, especially tomato plants. There are other plants that produce chemicals which influence the growth of other plants. These chemicals often diminish or prevent the growth of other plants. However, most allelopathic chemicals are organic compounds that can be detoxified under conditions of proper composting. I found information from universities indicating that when leaves of black walnut are properly composted the chemical, juglone, is decomposed.
Once university publication stated, "Walnut leaves can be composted because the juglone toxin breaks down when exposed to air, water and bacteria. The toxic effect can be degraded in two to four weeks. In the soil, breakdown may take up to two months after the living walnut tree has been removed. Mulch or woodchips from black walnut are not recommended for plants sensitive to juglone. However, composting the woodchips for a minimum of six months allows the chemical to break down to a safe level even for plants sensitive to juglone." This is from: Iowa State University: Black Walnut.
This report did not discuss vermicomposting, but it is probable that worm composting will also detoxify the juglone. The article listed above also lists plants sensitive to juglone and those resistant to juglone. I also looked for information regarding the impact of juglone on the worms used for making vermicompost. Juglone is known to impact several types of worms, even being used as a "wormer" to help remove worms from animals. Other comments tell of earthworms quickly decomposing piles of black walnut leaves. Attempting vermicomposting the black walnut leaves is worth trying. Mixing the walnut leaves with other leaves may minimize negative effects of juglone on the worms. After composting or vermicomposting, a wise procedure would be to test your compost carefully on a few sensitive plant species to confirm that the composting process has sufficiently detoxified the juglone in walnut leaves. If there are still symptoms of toxicity, provide additional composting time with aeration and moisture after the vermicompost has formed to allow aerobic decomposition of the juglone.
Are you writing from the USA? My answer is based on the black walnut which is the Juglans species with greatest juglone content in the USA. Several Juglans species grow in other regions of the world. Species of Juglans from other regions may have levels of juglone that may require additional composting. In that case, composting (traditional composting and vermicomposting) should detoxify the juglone, but the time required may be different. Testing on sensitive plants after composting would be wise before extensive use of the vermicompost.
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.
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