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November 29th, 2011

Toxic Trees

We like to till in leaves with manure in the fall in our veggie beds. Are there any leaves that are toxic? In our county, we have cottonwood, aspen, oak, and maple.

blog-dougBlack walnut (Juglans nigra) and tree of heaven (Ailanthus altissima) are two trees to beware of as you gather leaves to use for soil improvement. All parts of black walnut trees contain juglone, a chemical that is toxic to certain other plants, including tomatoes and rhododendrons. The juglone breaks down after a year of composting; gardeners who compost the leaves of walnuts or related species (pecans, hickories, and butternuts) should make a separate pile from the rest of their yard waste. The growth-inhibiting chemicals produced by tree of heaven are found mostly in the roots but may also be present in the leaves.

This ability of some plants to influence the growth and survival of neighboring plants by producing a toxic substance is known to botanists as negative allelopathy. Plants such as black walnut use this trait as a way to reduce competition from other species. Some of the most aggressively spreading weeds—think garlic mustard—literally exterminate the competition with allelopathic chemicals.

Allelopathy can also work to the gardener’s benefit. Corn gluten meal, sometimes used as a pre-emergence weed killer in lawns, contains an allelopathic compound that stifles the germination of weed seeds.

But back to your original question: Cottonwood, aspen, oak, and maple leaves can all be tilled into garden soil with no fear of toxic repercussions.  —Doug Hall

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