When is mulch not a good thing for the environment?
Usually, mulch is great for your garden and great for the environment. A layer of chipped wood, the most popular type of mulch, can reduce erosion, moderate soil temperature, retain moisture, and eventually degrade into soil amendment.
Storms, however, can cause two types of problems with mulch. First, if winds and water push your mulch up against the trunk of a tree or against the woody stems of a bush, the moist points of contact can promote fungal growth. Second, if you allow your mulch to run off your yard during heavy storms or strong winds, it can become a pollutant.
Wood chips and other forms of organic mulch are “natural,” so they may seem harmless as they tumble down gutters into storm drains. Decomposition in areas of storm drain discharge, however, can be a problem. Excessive decomposing plant material removes oxygen from water, leaving insufficient amounts for fish and other aquatic life to breathe. Also, the nutrients produced during decomposition promote algal blooms and remove even more oxygen from the water. In a related potential problem, rotting plant material can block small portions of a waterway, forming shallow pools where the water stagnates and may support the breeding of mosquito larvae.
Of course, these are worst-case scenarios, and the people who illegally dump yard debris into waterways or sweep grass clippings into storm drains are far more guilty than is the diligent gardener who lets a little mulch escape from a well-maintained landscape.
The best way to keep mulch from leaving your yard and ending up in the storm drain is to put up a barrier that will keep it on your property. Many types and styles of edging are effective as barriers to keep landscape material in its place. These include decorative stones, a low fence, a hedge row, or wattles.
If you don’t already have edging around your rock gardens, wood chips or other materials, now is the time to add it to your landscape. Also, remind your gardener, if you have one, not to blow landscape debris or mulch into the street and storm drains.
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Also contributing to this article were Kelly Hahs of the PWA Watershed Protection District, and Kay Allen, Environmental Compliance Program Coordinator, city of Simi Valley