Homeowners should choose the right landscaping mulches to reduce the likelihood of ignition from embers during a wildfire and improve the health of plants around their homes
- reduce the water requirements of plants
- cool soil temperatures
- reduce the occurrence of weeds
- control soil erosion and dust
- prevent soil compaction
- visually enhance the landscape
Unfortunately, despite the positive attributes, many mulches are combustible - a major drawback when used in home landscapes located in wildfire-prone areas (Quarles 2011).
An evaluation of mulch combustibility was performed in 2008 by the University of California Cooperative Extension and the University of Nevada Cooperative Extension. The results from this project offer recommendations for uses of mulches in wildfire hazard areas.
The evaluation defines as any material that is used to cover the soil surface for a variety of purposes. They can be classified as organic or inorganic. Organic mulches usually come from plant materials and include pine needles, pine bark nuggets, shredded western cedar and even ground or shredded rubber. Inorganic mulches consist of rock, gravel and brick chips. These inorganic mulches tend not to burn and are safe to use in any setting.
Eight mulch treatments were evaluated for three characteristics: flame height, rate of fire spread and temperature. On the test day, the National Fire Danger Rating System value was Extreme. All eight mulches were found to be combustible but varied considerably in the three areas measured.
- Shredded rubber, pine needles and shredded western red cedar showed the greatest potential for all three characteristics.
- Shredded rubber burned at the hottest average temperature (in excess of 630 degrees F at a height of 4 inches) and produced the greatest flame length at over 3 feet.
- Shredded western red cedar had the most rapid rate of spread, traveling at an average rate of 47.9 feet per minute. It also produced embers that moved beyond the plot perimeter and ignited adjacent mulch plots.
- Composted wood chips showed the slowest spread rate and the shortest average flame length, usually smoldering.
So what does all of this mean? We have a variety of mulch choices in our landscaping – and we need to know the best uses for each choice.
- Inorganic mulches such as decomposed granite, gravel, or rocks offer superior fire-proofing as landscape mulches and should be used when mulch is needed within 5 feet of buildings or any combustible structural materials such as siding or decking. Any fallen or windblown leaf litter or debris that has collected on the rocks must be regularly removed to prevent small debris fires from igniting structures.
- From 5' to 30' away from structures, composted wood chips are the best choice of the materials tested for residential landscape use. They are organic and will still burn, but tend to burn at the lowest speed and lowest flame length. If this material is ignited, it could still ignite siding, plant debris and other combustible materials. The smoldering of this product could also go undetected by firefighters during a wildfire.
- Shredded rubber, pine needles and shredded redwood or cedar bark can have their place in your landscaping design, just further from your home (FIRESafe MARIN recommends that these materials not be used within 30' of any structure or combustible accessories like fences or outdoor furniture). These materials could be used selectively for landscaping at least 30’ from your home (and neighbors' homes), and 10' from roads or driveways or any accessory structures (including fences, outbuildings, play structures, etc).
Quarles, S. and E. Smith. 2011. The Combustibility of Landscape Mulches, SP-11-04, University of Nevada Cooperative Extension, Reno, NV.
Rogstad, A., T. DeGomez, C. Hayes, J. Schalau, and J. Kelly. 2007. Comparing the ignitability of mulch materials for a firewise landscape. University of Arizona, College of Agriculture and Life Sciences Bulletin, AZ1440. 5p.