Fruit trees and small fruits Poisonous Plants Trees

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. After studies at the University of Toronto and Laval University where he obtained his B.A. in modern languages in 1978, he succeeded in combining his language skills with his passion for gardening in a novel career as a garden writer and lecturer. 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 is a regular contributor to and horticultural consultant for Fleurs, Plantes, Jardins garden magazine and has written for many other garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening, Rebecca’s Garden 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 50 other titles in English and French. He can be seen in Quebec on French-language television and was notably a regular collaborator for 7 years on the TV shows Fleurs et Jardins and Salut Bonjour Weekend. He is the President of the Garden Writers Association Foundation and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. An avid proponent of garden tourism, he has lead garden tours throughout Canada and to the gardens of over 30 countries over the last 30 years. He presently resides in Quebec City, Quebec.

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. https://pubs.extension.wsu.edu/do-black-walnut-trees-have-allelopathic-effects-on-other-plants-home-garden-series

  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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