When you open a fresh bag or receive a bulk delivery of wood mulch, an overpowering smell reminiscent of manure or rotten eggs may indicate “sour mulch.” This foul odor is a direct symptom of anaerobic decomposition. It occurs because the organic material was stored without sufficient oxygen, forcing microbial breakdown to proceed on a different chemical pathway. Recognizing this smell signals that the mulch contains compounds potentially harmful to garden plants.
The Science Behind the Foul Odor
The unpleasant smell results from conditions where mulch is piled too high, packed too tightly, or becomes waterlogged. In these environments, aerobic microorganisms quickly deplete the oxygen supply, leading to a shift toward anaerobic bacteria. These bacteria ferment the wood material, producing volatile organic compounds (VOCs).
The specific odors are linked to these gaseous compounds. The pungent, ammonia-like or manure smell comes from nitrogen-containing compounds. The distinct rotten egg odor is caused by hydrogen sulfide, a gas created when sulfur compounds break down without oxygen. Other common byproducts include organic acids, such as acetic acid, and various alcohols like methanol and ethanol. These compounds accumulate because they cannot evaporate from the dense, oxygen-deprived core of the mulch pile.
Potential Harm to Plants
Applying “sour” mulch directly introduces phytotoxic compounds into the plant environment, sometimes called “wood alcohol syndrome.” The volatile organic compounds, especially alcohols and organic acids, are released as toxic gases when the mulch is spread and exposed to air. These vapors can damage the leaves and stems of nearby plants.
Damage often appears rapidly, sometimes within 24 hours of application, typically on the lowest leaves closest to the mulch layer. Symptoms include bleaching or yellowing of the foliage, scorched or brown leaf edges, and wilting that mimics drought stress or fertilizer burn. Newly installed or small herbaceous plants are particularly susceptible, and in severe cases, the toxic compounds can stunt growth or cause death.
Fixing the Smell
Mulch suffering from anaerobic decomposition can be safely remediated before use by introducing oxygen. The most effective action is to spread the entire pile into a loose, shallow layer on a driveway or tarp. The layer should be no more than a few inches deep to ensure maximum air exposure.
Once spread, the mulch must be “turned” or aerated thoroughly using a pitchfork or shovel. This allows oxygen to penetrate the material, encouraging beneficial aerobic bacteria and helping the toxic VOCs off-gas safely. Depending on the smell’s severity, repeating this turning process every few hours or days for about a week should eliminate the foul odor. If the smell is intense, heavy watering or rinsing the spread-out mulch can help leach away acidic compounds, accelerating remediation.
Preventing Anaerobic Breakdown
The best way to avoid this problem is to prevent the conditions that lead to oxygen depletion. When purchasing bulk mulch, inspect and smell the material before delivery, ensuring it has a pleasant, earthy, or fresh wood scent rather than a sour one. If storing mulch, avoid creating deep, dense mounds.
Mulch piles should be kept loose and relatively shallow, ideally no more than four to six feet high, to allow for natural air circulation. Storing the material on a hard surface or a pallet, rather than directly on the soil, also helps ensure airflow underneath the pile. Protecting the stored mulch from excessive rain or waterlogging maintains the optimal moisture level needed for healthy, aerobic decomposition.