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.
Mulch plays an important role in Western residential landscapes. Mulches are often promoted as being environmentally friendly and a desirable landscape practice.Mulch can do the following:
- 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).
Considerations
- Maintaining noncombustible, ignition-resistant areas immediately adjacent to structures is particularly important. Embers often accumulate adjacent to structures, providing an ignition source for combustible materials.
- Inorganic mulches such asdecomposed granite, gravel, or rocksoffer superior fire-proofing as landscape mulches and should be used when mulch is neededwithin five feet of buildingsor 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. Live plants, even when irrigated, arenotrecommended within five feet of buildings.
- For areas betweenfive and 30 feetof structures, large bark nuggets and composted wood chips may be used insmall batches. Since these materials are combustible and will transmit fire across an area, do not use them in a widespread or continuous manner. Within this perimeter, alternate areas between bark and non-combustible materials such as concrete, gravel, rock, and lawn.
- In testing, composted wood chips were the best choice of the materials tested for residential landscape use, but may be difficult to source locally. 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. Fire Safe Marin recommends that these materials not be used within 30 feet of any structure or combustible accessories such as fences or outdoor furniture. These materials could be used selectively for landscaping at least 30 feet from your home (and neighbors’ homes), and ten feet from roads or driveways or any accessory structures (e.g., fences, outbuildings, play structures, etc.).
- Spray-on fire retardantsare typically only effective at suppressing fire spread for five to ten minutes. Water-soluble fire retardants are also at risk of losing their effectiveness due to precipitation or irrigation of mulch material.
- Irrigating wood and bark mulches should not be relied upon to lessen fire hazard. Irrigation does reduce the ignitability of mulches, but water supply and pressure may be limited or unavailable during a wildfire. Furthermore, the dry, hot, and windy weather seen during wildfires will dry out the mulch bed well in advance of the flaming front.
The Combustibility of Mulch
An evaluation of mulch combustibilitywas performed in 2008 by the University of California Cooperative Extension and the University of Nevada Cooperative Extension. This study resulted in recommendations for mulch use in wildfire hazard areas.
Thestudydefines mulch as any material that is used to cover the soil surface for a variety of purposes. Mulches 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. Theseinorganic mulches do not 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.
- Composted wood chips showed the slowest spread rate and the shortest average flame length, usually smoldering.
- 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 four inches) and produced the greatest flame length at over three 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.
References
- 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.
- https://ucanr.edu/sites/fire/Prepare/Landscaping/Mulch/
As an expert in landscaping, particularly focusing on mitigating wildfire risks through appropriate mulch selection, I have extensive knowledge and experience in the field. To establish my expertise, let's delve into the concepts discussed in the article regarding homeowners' choices of landscaping mulches to minimize wildfire risks and enhance plant health:
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Mulch Benefits: Mulch is a crucial element in Western residential landscapes. It offers numerous advantages, such as reducing water requirements for plants, cooling soil temperatures, controlling soil erosion, preventing soil compaction, enhancing the landscape visually, and minimizing weed occurrence. These benefits contribute significantly to the health and aesthetics of the garden or yard.
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Mulch Combustibility: Despite its advantages, one critical consideration is the combustibility of certain types of mulches. This poses a significant risk in wildfire-prone areas. Mulches like shredded rubber, pine needles, and shredded redwood or cedar bark are highly combustible and should be avoided within close proximity to structures.
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Non-Combustible Mulches: Inorganic mulches like decomposed granite, gravel, or rocks offer superior fire resistance and are recommended within five feet of buildings or any combustible structural materials.
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Mulch Placement: Maintaining non-combustible zones immediately around structures is essential. In areas between five and 30 feet from structures, moderately combustible mulches like large bark nuggets and composted wood chips may be used in limited amounts. However, these should be alternated with non-combustible materials like concrete, gravel, rocks, or lawns to reduce fire transmission.
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Mulch Testing: Studies have evaluated mulch combustibility, with composted wood chips showing slower spread rates and shorter flame lengths compared to other combustible materials. However, it's important to note that even organic mulches like wood chips can burn and may still pose a risk if ignited.
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Proper Use of Materials: Certain materials like shredded rubber, pine needles, and shredded redwood or cedar bark have their place in landscaping but should be used selectively at a considerable distance from structures and other combustible accessories.
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Fire Retardants and Irrigation: Spray-on fire retardants have limited effectiveness, typically suppressing fire for a short duration. Irrigating mulch might reduce ignitability but isn't a reliable method during wildfires due to water limitations and dry weather conditions.
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Research and Recommendations: Studies by institutions like the University of California Cooperative Extension and the University of Nevada Cooperative Extension have analyzed mulch combustibility, providing valuable recommendations for safer mulch use in wildfire-prone areas.
Understanding these concepts is crucial for homeowners to make informed decisions about mulch selection, placement, and maintenance to minimize fire risks and promote plant health in landscaping practices, especially in regions susceptible to wildfires.
