Tanbark or mulch in front of a San Jose home

Tanbark wood mulch separated by a dry creek bed, cement path, & brick retaining wall in a low-water San Jose yard.

May Day is a celebration of spring, but “April showers” were few and far between and it’s already starting to feel like summer! With another record-breaking hot year and the Bay Area in severe to extreme drought conditions homeowners concerned about water use and fire prevention are turning to gardening and landscaping for the solution. But a word of warning! Since updating my surprisingly popular post on mulch vs tanbark and the risk of termite infestation, I came across another reason to be cautious when applying it to your perimeter: fire.

Organic Mulches and Fire Hazard

Mulch can work wonders in a garden – it helps soil retain moisture, protects roots, reduces weeds, insulates the ground, can add nutrients and enrich the earth, adds visual appeal, and it’s affordable. It’s on every guide for landscaping water conservation (including Valley Water’s recommendations and San Jose Water’s tips)! Do a search and you’ll find it comes in a broad variety of materials. These can be divided into two groups: organic and inorganic. And organic matter can burn.

The University of Nevada Cooperative Extension has published their (easy to read) findings from a study comparing the combustibility of various organic landscape mulches. I recommend reading the booklet, but here are some of the key points I found most interesting:

  • Mulch types tested gave wide-ranging levels of combustibility
  • Shredded rubber mulch (yes, rubber is organic) was the most flammable with the hottest temperature and the highest flame (averaging over 3 feet tall!)
  • Pine needles and gorilla hair-types spread the fastest
  • Composted wood chips burned mostly as a smolder with few low flames and displayed the slowest rate of spread
  • Most mulch beds were layered 2-3 inches deep (what most garden centers seem to recommend in home landscapes), but one that was layered much thinner (about 1-inch) had a slower, cooler, smaller burn than its more generously layered counterpart
  • Fire retardant applied did not have a major impact on the burn behavior
  • The highest fire-hazard mediums were the shredded rubber followed by the pine needles and the shredded bark / gorilla hair
  • The lowest fire-hazard mulch in this study was demonstrated by the composted wood chips followed by the thinly layered chipped wood

The study ends with recommendations to property owners.

Within five feet of structures, don’t use any organic mulch. Stick with inorganic, non-combustible materials like concrete and pavers, or spread rocks, gravel, or decomposed granite instead. From 5’ – 30’ around buildings choose less-combustible organic mulches, but only in pockets with beds separated by fire-breaks of non-combustible or fire-resistant (a lush green lawn) areas. The most hazardous, highly flammable material should only be used more than 30 feet from a structure.

Mulch can improve moisture retention in the soil, but don’t let that fool you into thinking it stays damp! Even mulch that is watered by overhead spray irrigation loses water quickly baked by the sun and dried by the wind, especially in high fire season!

The University of California Cooperative Extension uses the findings in this study and adds a few recommendations of their own on the Mulch information page of their Wildfire Preparation for Home Landscaping guide. The University of Idaho also conducted a study to find the ignition and flammability of common mulches, including lawn and inorganic mulches in tests.

Mulch is an extremely valuable tool for your garden, whether you want to reduce your water use this summer, have a low-maintenance landscape, or improve the look of a yard before listing your home for sale. Like any tool, you just need to follow a few safety tips and learn how to use it. The type, location, and quantity of mulch used should all be carefully considered when planning your garden. So, whether it’s for termites or fire safety, keep the wood chips away from the foundation!


UNCE Combustibility of Landscape Mulches