COLUMBIA, Mo. - Fermented or "sour" mulch from large batches hit with too much rain can damage the leaves and stems of annuals, perennials and small woody plants, said a University of Missouri Extension horticulturist.

"The excessive rain across the state can cause large stockpiles of mulch to ferment," said Chris Starbuck. "With the onset of warmer weather, the compounds produced by fermentation volatize quickly after the mulch is spread."

Plant damage shows up as bleaching or silvering of foliage and can appear within a few hours of mulch application.

"Established plants can often recover by growing new leaves once the toxic volatile compounds have dissipated," Starbuck said. In extreme cases, leaf burning and leaching of toxic compounds into the root zone can kill small, newly planted plants.

"To avoid problems associated with sour mulch, it's best to use your nose and a little common sense when mulching plants," he said. "If the material has an unpleasant, sour smell or feels hot to the touch, do not use it immediately on delivery."

Instead, spread the material out to allow potentially toxic compounds to volatize for a day or two, and then try it on a few plants before mulching an entire bed.

Fermentation occurs when excess moisture and the resulting lack of oxygen in the center of large mulch piles lead to high populations of anaerobic microorganisms. This can produce such chemicals as acetic acid, fatty acids, methanol, formaldehyde, ketones and phenols.

The pH of mulch is a good indicator of fermentation. Landscapers and mulch suppliers might take the pH of water extracted from a large mulch batch as a safety precaution before delivering to a job site.