Why Does Mulch Stink?

Decomposition

• From time to time, composting mulches turn sour. The smell is a signal to correct a problem, and the sooner the better. A rotting smell indicates too little air or too much water in the compost pile --- add straw, wood chips or another dry material, and turn the pile. An ammonia or vinegar smell signals too much nitrogen, such as grass clippings or other green waste. Again, add carbon-rich wood chips, straw or sawdust to balance the composting process. Don't spread sour mulch in the garden, or the toxic chemical buildup may harm your plants.

Animal Waste

• Your lush garden bed and soft soil may be an open invitation to neighborhood cats, dogs or other animals. Cats bury their waste in fine mulch, where it remains until it either decomposes or reveals itself under your trowel. If a talk with the owner doesn't solve the problem, try an animal-safe repellent or, if necessary, a live trap. Place chicken wire under your mulch to foil digging paws. Alternatively, if you have roses or other thorny shrubs, lay the cuttings over the mulch to make the site unappealing. Never use mothballs or cayenne pepper, as these products may severely injure an animal.

Stinkhorn Fungus

• The stinkhorn fungus lives up to its name. The pungent, rotten-meat smell announces the organism's presence long before you'll notice the fungus itself. Stinkhorns grow in decomposing, organic matter --- including the wood mulch around your flowerbeds --- and appear with cool, rainy weather. The dormant, underground fungus pops up rapidly and ranges from 2 to 6 inches tall. It resembles a common mushroom and may be thin, fat, tall, round, pink or orange. Don't fret; the stinkhorns will disappear with the return of dry weather.

Rubber Mulches

• Summer heatwaves reveal one of rubber mulches' least attractive sides --- the chemical stink. Rubber mulch, which manufacturers tout as a green, environmentally friendly alternative to wood mulch, consists of shredded tires and other rubber products. The sun's radiation breaks down rubber mulch just as it would a tire, and the mulch off-gasses its chemical components. The petroleum-based rubber may be pleasing to the eye, but the nose isn't fooled.