"Sour" Mulch Can Burn Your Plants

In years with above normal rainfall, it is fairly common for the leaves and stems of annuals, perennials, and small woody plants to be injured seriously by volatile chemical compounds released from fermented mulch. Damage of this type often shows up as bleaching or silvering of foliage within a few hours of mulch application and is intensified during hot weather. Damage of this type is generally transitory, lasting 24 to 48 hours. After the fermentation products have volatilized, large plants will recover. 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.

Chemicals produced during fermentation may include acetic acid, fatty acids, methanol, formaldehyde, ketones and phenols. When these compounds are present in significant amounts, there is an unmistakable acrid odor to the material, and the mulching material may cause severe injury to plants. In a study of samples of "sour" Douglas-fir sawdust, Bollen and Lu found that radish and sunflower seeds mulched with the material that did not germinate, while corn and onion seedlings were killed soon after emergence. Tomato, pepper and cabbage plants mulched with sour sawdust after transplanting, died within one week of mulch application, and looked bleached. Samples of fresh or weathered sawdust taken from nearby piles had no adverse effects on plants other than slight nitrogen deficiency symptoms. Testing revealed that the fermented sawdust contained essentially no microorganisms and had a pH of 2.2, requiring the equivalent of 300 pounds of lime to neutralize it to pH 8.5. Acetic acid (vinegar) was the primary acid detected by gas chromatography.

To avoid problems associated with sour mulch, one should use his or her nose and a little common sense when mulching plants. If the material has an unpleasant, sour smell or feels hot to the touch, do not use it immediately upon delivery. If possible, spread the material out to allow potentially toxic compounds to volatilize for a day or two and then try it on a few plants before mulching a whole bed. Place a few cell packs of tender annuals on top of the mulch. Damage from acetic acid fumes often shows up as bleaching, silvering or collapsing of foliage within a few hours on a warm, sunny day.

The pH of mulch is a fairly good indicator of fermentation products. A good safety precaution for landscapers and mulch suppliers might be to take the pH of a water extract of a large batch of mulch before delivering it to a job site. This is an especially good idea if the mulch seems soggy, has a vinegar-like odor, and tingles the nose. Soak a sample in water for ten minutes, and take the pH of the water with a pH meter or paper. A pH paper indicating in the 3 to 7 range is ideal because it is accurate, inexpensive, and maintenance free. (One source of pH paper is Southern Scientific at http://www.southernscientific.com/test_papers_meters.asp) Any mulching material with a pH lower than 3.5 or 4.0 should be used with caution on annual and perennial beds.

When planting a large bed of annuals or ground covers, mulch is commonly applied before planting. If so, delaying planting for a day after the mulch is spread will reduce the chances of damage from volatile compounds in the mulch. Watering may also leach out toxic compounds, but it may make matters worse unless the mulch is spread out before watering. Also, watering after planting may leach toxic materials into the root zone. In situations where damage has already occurred, it is best to wait a few days before replanting. Lime applied to the surface may reduce the problem somewhat by neutralizing some of the acetic acid. Do not use hydrated lime.

Reference:
Bollen, W.B. and K.C. Lu. 1970. Sour sawdust and bark-It’s origin, properties and effects on plants. USDA For. Ser. Res Pap; Pacific Northwest For and Range Exp Sta. Portland, OR

Chris Starbuck, Woody Ornamental Horticulture, UMC (573) 882-9630