When it comes to mulch, certain types have the potential to be more combustible than others and additional factors can contribute to this risk. Residents located in or around our Wildland Urban Interface (WUI) should learn about the best possible mulch types to use and where it should be placed around your home. Here are a few considerations:
- Dyed or colored mulch: Mulch that has been dyed or treated with artificial colors may contain flammable additives or chemicals.
- Fine textured or shredded mulch: Sometimes referred to as ‘Gorilla Hair’ mulch which consists of fine particles or shredded materials can be more prone to rapid ignition. These mulches contributed to fire spread and damage during the 2017 fires and the most recent Glass Fire in 2020.
- Dry or improperly maintained mulch: Mulch that is excessively dry can easily ignite and spread fire.
- Mulch made of Rubber: Mulch made of rubber material should be avoided. During ignition testing, rubber mulch has shown to put out significant flame height and heat.
- Accumulation of debris: If mulch is surrounded by accumulated dry leaves, twigs, or other combustible materials, it adds to the risk and combustibility of ground mulch in place.
Within 0-5 ft of the house and other structures:
Maintaining a noncombustible, ignition–resistant area immediately adjacent to the house and other structures is particularly important. Within five feet of the home, use noncombustible rock, gravel, concrete, and pavers. During a wildfire, embers may accumulate in this area, providing an ample source of ignition for combustible materials. While some mulch ground covers are less hazardous than others, they are all considered combustible materials. Mulch types that have a smoldering combustion risk may not be readily noticeable during a wildfire event and may go undetected by firefighters.
Within 5 ft – 30 ft feet of the house and other structures:
Mulches fall into two main categories, organic and inorganic. Organic mulches usually consist of pine needles, wheat straw, pine bark nuggets, shredded cedar and redwood bark, and other recycled wood chips. Inorganic mulch consists of non-plant material such as rock, gravel, and brick chips, which can be categorized as hardscape. Hardscape can also consist of rocks, stones, bricks, and concrete pathways. The main benefit of mulch is that it keeps the soil temperature cool and moist, which in the long run conserves water. Hardscape material increases the temperature making your soil, plants, and trees hotter than normal which, in turn, requires more water to keep them healthy. Using the appropriate mulch is key and planning out locations of plants or garden beds that will need the mulch is equally important. We recommend at least 30’ of clearance from your home if they are in use. Compost or large bark chips are usually the best option for fire resistance.