Can mulch go bad?
Yes, mulch may be unfit to use. If organic mulch such as wood chips decomposes without enough oxygen, anaerobic decomposition sets in. When this happens, the mulch produces alcohol and organic acids that can be toxic to plants. In addition, the pH of such mulch can be extremely low. Anaerobic conditions usually develop in large, wet mulch piles, or in bagged mulch with water in the bag. It's easy to tell if the mulch you purchased is toxic. Good mulch has a pleasant, humusy smell; acid (anaerobic) mulch has a strong, sour smell. Fortunately, you don't need to throw out the sour mulch. Simply spread it out in a thin layer for a few days on a hard surface such as your driveway or patio. Once the smell subsides, the mulch should be safe to use around your plants. Symptoms of damage from sour mulch include chlorosis (yellowing of foliage), defoliation, scorching, and even plant death from toxic fumes or from direct contact with the mulch.
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