Should I Mulch in Southern California?

Mulch and Water Conservation

• California's Water Conservation in Landscaping Act of 1990 encourages using mulch to reduce the amount of water used for landscaping purposes. Applying mulch over the soil reduces water evaporation by as much as 50 percent. Organic mulches do a better job of conserving water than inorganic mulches such as ground rubber and stones; organic mulches retain moisture, transferring this later to the soil and preventing runoff.

Other Advantages of Mulch

• According to the University of California Cooperative Extension, plants are healthier and grow better when mulched. Mulch reduces the number of weeds so that plants have less competition for available moisture and nutrients. It also reduces the incidence of disease by preventing soil pathogens from splashing onto the plants during rain or irrigation. A thick layer of mulch moderates soil temperature so that the roots aren't exposed to extreme highs and lows as experienced in the course of 24 hours in Southern California's desert and mountains. Another important function of mulch is that it helps prevent soil erosion and compaction. Plants also look better when surrounded by a uniform mulch rather than bare soil.

Disadvantages of Mulch

• Mulch prepared from diseased plant material can transfer diseases to healthy plants. If you make your own mulch from garden waste, be careful to exclude diseased material. Many plant diseases and weed seeds survive composting. The biggest danger from organic mulch comes from its flammability. Wildfires, common in Southern California, spread quickly when they come in contact with organic or rubber mulch. You don't want that material near your house.

Mulch and Fire Safety

• In 2008, the University of California Cooperative Extension, University of Nevada Cooperative Extension and the Carson City Fire Department evaluated the combustibility of a number of mulches. They found the most hazardous to be rubber, due its high burning temperature, and shredded western red cedar and pine needles. The slowest burning were Tahoe chips and composted chips, each tested in a single layer. Tahoe chips treated with flame retardant delayed the spread by an additional 10 minutes.

  Their report recommends using a maintained lawn or other fire-resistant plants, pavement, gravel or noncombustible rocks within 5 feet of the home. Tahoe chips, with or without fire retardant, pine bark nuggets and composted chips are better used farther away in the 5- to 30-foot zone and only when mulched areas are separated by noncombustible and ignition-resistant lawn, gravel and rock.