Recently, some South Carolina gardeners have reported their plants becoming injured or dying after mulch was placed around them. Most often, bedding and low-growing plants were involved. Factors such as lack of air, pile size, storage site and moisture and particle size can create toxic mulch. Unless mulch is allowed to breathe, it can become toxic.
Toxic mulch smells like vinegar, ammonia and sulfur.
Good mulch has the earthy smell of a good garden soil or freshly cut wood. Without air, mulch begins fermenting. The larger the pile, the harder it is for fresh air to reach the center. The weight of the pile causes mulch particles, especially fine-textured ones, to compress together, reducing the air supply inside the pile. Covering mulch with plastic or getting it wet during storage can reduce air flow and promote toxicity.