Marin Master Gardeners
University of California
Marin Master Gardeners

Fire-smart Landscaping - Mulch

(Adapted from content developed by Sonoma Master Gardeners)

Mulch Quick Check List

  • 0’ – 5’ from house: ideally use non-flammable mulch
  • 5’ – 30’ from house: separate composted wood chip areas with non-flammable materials such as gravel, rocks, decomposed granite or stone
  • 30’+ from house: choose larger, composted wood chips.  Fine, stringy mulches burn faster than larger chunks
  • In general, fine (less than ¼” particles) or stringy mulches like gorilla hair ignite and burn more rapidly than larger chunks.
  • When apply mulch, limit depth of mulch to 2” deep. Note: it is common to see depth recommendations of up to 3” deep which is fine in zone two and beyond
  • When exposed to fire, thick mulch layers (greater than 2” deep) tend to smolder and are difficult to extinguish.

What is Mulch? Mulch is a layer of material, commonly organic, covering the soil surface to exclude sunlight, retain water, and replenish soil nutrients. Organic materials frequently used as mulch include wood chips, leaves, grass clippings, straw, and compost. Mulches are beneficial in the garden for reducing the water requirements of plants, cooling soil temperatures, suppressing weeds, reducing erosion and dust, maintaining soil organic matter content, and preventing soil compaction. Maintaining organic matter content is important because it aids in nutrient exchange, increasing or maintaining soil water-holding capacity, and augmenting drainage.  However, mulch, compost, and other organic materials can be flammable, and therefore, can pose a risk to your home if ignited.

Which mulch is slowest to burn? A study performed in Carson City, Nev., in 2008 tested the combustibility of several different mulches. The mulch that ignited most easily, and with the fire spreading most rapidly, was shredded Western Red Cedar bark. Composted wood chips spread 2”-3” deep showed the slowest fire-spread rate of the 8 mulches tested. However, the wood chips burned primarily through smoldering combustion, which might not be noticed by firefighters during a wildfire. Nevertheless, wood chips were still considered the best mulch choice for residential landscapes.

Reference Materials for further reading

Compost and Mulch: http://marinmg.ucanr.edu/Manage_A_Garden/Composting/

Flammability of Mulch Types: https://ucanr.edu/sites/MarinMG/files/321642.pdf

Webmaster Email: banielsen@ucanr.edu