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Can You Compost Black Walnut Leaves?

Black walnut leaves (Juglans nigra). Photo: Lucid, flickr

Black walnut (Juglans nigra) and, to a lesser extent, other walnuts (Juglans spp.), as well as their close relatives, the hickories (Carya spp.), release a product called juglone that is allelopathic, that is to say toxic to most other plants. Thus, walnut trees suffer little from competition, as few other plants can grow well at their base (here’s a list of plants that are resistant to juglone). Juglone is present in all parts of the walnut tree except the nut itself: leaves, branches, bark, wood, and especially its roots and nut husks.

The question is therefore the following: should the home gardener put “toxic” black walnut leaves into their compost bin? A study done at Ohio State University looked into the matter and the answer is yes: juglone decomposes within weeks in contact with air, water and bacteria. If the leaves are finely shredded, decomposition is even quicker. And once decomposed, walnut leaves apparently give a compost of excellent quality.

Walnut sawdust and wood chips as well as walnut husks, however, are slower to decompose: it’s best to compost them for 6 months before using them in the garden to be sure the juglone has thoroughly decomposed.

Adapted from an article originally published on October 15, 2015

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

3 comments on “Can You Compost Black Walnut Leaves?

  1. I have been gardening with large mature black walnuts for years. The allelopathic effects are overrated. If you want to garden near/under a walnut, just add soil to create the bed rather than digging in the soil and roots (which isn’t good for the tree anyhow). I have beautiful 20 year old perennial beds under my 130 year old walnut – the worst challenge is cleaning up the walnuts and stems and leaves every fall. Good list on your other link of plants that do just fine under walnuts. The latest science.

  2. Not only do they make good compost, but they can make a nice, although temporary, herbicidal mulch before they decompose. Some clients request chips from black walnut trees or Monterey cypress for the same purpose.

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