Bagged mulch is a popular gardening material, offering benefits like moisture retention and weed suppression. While organic mulch naturally decomposes, material sealed inside plastic bags can undergo a chemical change that renders it temporarily toxic to plants. This process, known as “souring” or “wood alcohol syndrome,” occurs before the product is even applied to the landscape.
Why Bagged Mulch Can “Go Bad”
The cause of mulch souring is a shift in the decomposition process from aerobic to anaerobic activity. This occurs when wood chips or bark pieces are packed tightly in sealed bags, leading to an oxygen deficiency within the material. Microorganisms in this oxygen-deprived environment break down the organic matter without air, a fermentation process that produces toxic byproducts.
This anaerobic fermentation generates volatile organic compounds and organic acids that accumulate within the mulch. Specific toxic substances created include methanol (wood alcohol), acetic acid (the main component of vinegar), ammonia gas, and hydrogen sulfide gas. These chemicals are responsible for the mulch’s toxicity and foul smell. The presence of these acids can drastically lower the mulch’s pH, sometimes to an extremely acidic range of 1.8 to 3.6.
Signs That Mulch Has Soured
The most reliable indicator that mulch has soured is a pungent, off-putting odor upon opening the bag. Healthy, properly stored mulch should smell pleasantly earthy, like fresh-cut wood or forest soil. Sour mulch often emits a strong smell resembling vinegar or alcohol due to the buildup of acetic acid and methanol. A rotten egg or sulfur smell indicates the presence of hydrogen sulfide gas, while an ammonia odor may also be detectable.
Applying soured mulch directly around plants can cause swift damage, particularly to young or herbaceous varieties. Within 24 hours of application, sensitive plants may exhibit symptoms that mimic fertilizer burn or drought stress. These signs include leaf scorch, yellowing or bleaching of foliage, defoliation, or a general wilted appearance, especially on the lower leaves closest to the volatile toxins.
Storing and Restoring Mulch
To prevent souring, avoid storing newly purchased bags in a damp, unventilated area for long periods. If storing bagged mulch for more than a few weeks, keep the bags elevated off the ground to prevent moisture buildup. Poking several small holes in the plastic allows for minimal air exchange, encouraging the beneficial aerobic decomposition process.
If you open a bag and detect the telltale pungent odor, the mulch can often be restored or “cured” before use. The most effective method is to spread the mulch in a thin layer, only a few inches deep, onto a tarp or paved surface. Aerating the material by turning it frequently with a shovel or pitchfork for a few days allows the trapped toxic gases to dissipate. Watering the spread-out mulch heavily also helps leach the soluble organic acids and harmful compounds out of the material, making it safe for your garden.