April Fire Safe Landscaping Tip: MULCH

Mulch is any material spread over soil to retain moisture, suppress weeds, and prevent erosion. Inorganic mulches such as rock and pebbles are a non-combustible option. Organic mulch is composed of plant material. Common organic mulch includes composted mulch which contains woody material and may have soil mixed in and medium bark nuggets and wood chips. All organic mulch is combustible but different materials and their size have varying flammability. It is important to select the right mulch and apply it in the right manner for each of the Defensible Space Zones to decrease fire risk as such, the best choice is composted wood mulch or bark chips or bark mulch 1-1.5 inches in diameter.

Zone 0 (0-5 feet from structures)

Use only non-combustible mulch like stone, gravel, or decomposed granite in this critical area around your house and wooden structures. These materials reduce the risk of ember ignition, offering extra protection for your home. Apply a weed barrier under rock mulch to inhibit weed growth, synthetic weed barrier is acceptable, ensuring that the weed cloth is not exposed or touching combustible siding.

Zone 1 (5-30 feet)

Composted wood chips or medium bark mulch work well here, but follow these guidelines:

• Keep combustible mulch depth to 3 inches or less
• Use combustible mulch in plant groupings separated by non-combustible mulch or hardscaping pathways or dry creek bed features. Synthetic weed barrier is not recommended beyond 5’ from the home because they can inhibit water infiltration.
• Keep mulch away from the trunk of larger bushes and trees as exposed roots and bark can be killed by burning mulch. This is also better for the health of the tree.

Zone 2 (30-100 feet)3

Follow the same guidelines as in Zone 1.  Pine needles to 3” are acceptable in this zone as well.

Materials to Avoid 

The most hazardous mulches are shredded western red cedar ("gorilla hair") and shredded or ground rubber and should be avoided, as they ignite easily and burn faster than wood chips. Fire retardant-treated mulches have a limited effective life and have shown to be no less flammable than untreated mulch. 

• Mulch
• Defensible Space Zones
• UC ANR Fire Network’s Defensible Space Landscaping: Mulch 
• Back to Fire-Smart Landscaping Tips

Photo credit Becca Ryan

Tip provided by UC Marin Master Gardeners