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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

Garden writer and blogger, author of more than 60 gardening books, the laidback gardener, Larry Hodgson, lives and gardens in Quebec City, Canada. The Laidback Gardener blog offers more than 2,500 articles to passionate home gardeners, always with the goal of demystifying gardening and making it easier for even novice gardeners. If you have a gardening question, enter it in Search: the answer is probably already there!

3 comments on “Can You Compost Black Walnut Leaves?

  1. 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.

  2. 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.

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