The Q&A Archives: Can I compost black walnut leaves?

Question: I have two large black walnut trees on my property, and I want to compost their leaves this fall. However, my neighbors say the leaves contain a toxin that will harm my plants. What should I do? Gloria Charles Hurricane, UT

Answer: You can compost walnut leaves. The toxin in black walnut leaves begins to break down very quickly once exposed to air, water and bacteria, as in composting, and could be gone in two weeks. In the soil, breakdown will take longer, probably two months orso, says William Beineke, a walnut breeder at Purdue University in West Lafayette, Indiana. The toxin is called juglone, and it stunts the growth of certain plants. Tomatoes and other solanaceous plants are severely affected by this substance and are good indicators of a juglone problem. The toxin is mainly exuded into the soil by the roots, though the leaves, bark and wood contain smaller concentrations. Make sure a compost with walnut leaves is fully decomposed before applying it in the garden. If you are extremely concerned about the juglone in compost, restrict the walnut leaves to a separate compost heap, and test the mature compost on tomato seedlings raised for that purpose.

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