Toxic mulch

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

It seems that every year we hear from homeowners having problems with recently spread mulch. The problem is rapid desiccation of foliage of all plants in landscape beds and grass along these beds within a day or two of spreading the mulch. This problem has been termed toxic mulch, sour mulch or hot mulch. Toxic mulch typically has a very sour, pungent, vinegary odor. A mulch sample submitted recently to the lab had a pH of 4.1.  

Toxic mulch results from poorly processed, managed, stored or “green” mulch. Chemicals produced by fermenting organisms in the mulch pile under oxygen-deficient conditions can cause significant injury on annual and perennial landscape plants. These toxic substances can include ammonia, alcohol, methanol, acetic acid or hydrogen sulfide. These chemicals often escape, or volatilize, soon after spreading the mulch.

Desiccation or dying of leaf tissue will first be observed on the bottom leaves of plants and then progress upward. Large perennial plants may have injury only on the lower leaves and will most likely recover over time. The volatizing chemicals may be too much for smaller annual plants. Mulch with a pungent odor should not be spread in landscape beds. If lower leaf injury is observed soon after spreading mulch, quickly remove it in hopes of preventing further damage. Toxic mulch can be spread out in a driveway and watered in for a few days to leach out the toxic chemicals.

Injury around tree
Toxic mulch injury to grass around a tree.

Clombine
Toxic mulch injury on columbine.

Primrose
Toxic mulch injury on evening primrose.

lemon
Toxic mulch injury on lemon balm.

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