Grass can catch on fire with surprising speed and ease, making it a significant hazard, particularly during dry seasons or in arid climates. The fire hazard is directly related to its physical structure and high responsiveness to weather conditions. Grass fires, often called grass-fueled wildfires, spread rapidly across open landscapes, posing a serious threat to homes and communities.
Conditions That Make Grass Burn
The transition of grass to a highly flammable fuel is primarily governed by its moisture content. Green, actively growing grass contains a high percentage of water, making it resistant to ignition because heat must first vaporize that water. Once the grass cures or dries out, its moisture content drops significantly, often falling below the 30% threshold where it is considered highly flammable dead fuel.
The physical characteristics of grass also contribute to its extreme flammability, especially its high surface-to-volume ratio. This means a thin blade of grass has a large surface area relative to its small volume, allowing it to lose moisture quickly and ignite almost instantly when exposed to heat. This rapid ignition is characteristic of “flashy” fuels, which spread fire at a very high rate.
The density and height of the grass, known as the fuel load, affect how intensely a fire will burn. A dense stand of tall, cured grass provides continuous fuel, leading to higher fire energy output and faster spread rates. High ambient temperatures and strong winds further increase the danger by pre-heating the grass and supplying abundant oxygen to the flames.
Ignition Sources
Grass fires require an external heat source to raise the fuel temperature to its ignition point, and human activities are responsible for the vast majority of these sparks. Discarded smoking materials, such as a carelessly tossed cigarette butt, provide enough heat to ignite dry grass. Human activity accounts for approximately 85% of wildfires in the United States.
Sparks generated by machinery are another common cause of ignition in dry grass. This includes sparks from all-terrain vehicle exhaust systems, the metal blades of lawnmowers hitting a rock, or chains dragging from trailers on pavement. These mechanical sparks can easily land in dry grass and start a fire that spreads quickly, especially during hot, dry, and windy conditions.
Natural causes, though less frequent, also contribute to grass fires, with lightning being the most common non-human ignition source. Lightning strikes can generate enough heat to ignite dry fuel sources, though these fires often coincide with storms that bring rain or higher humidity. Improperly used outdoor heat sources, such as fireworks, debris burn piles, or portable fire pits, are also frequent human-related catalysts.
Prevention and Safety Measures
One effective safety measure is creating a defensible space, a buffer zone around a structure where vegetation is modified to reduce the wildfire threat. The area immediately surrounding the home (the first 30 feet) should be kept “Lean, Clean, and Green” by removing all dead plants, leaves, and weeds. Within this zone, grass should be mowed to a maximum height of four inches to reduce the available fuel load.
Beyond the immediate perimeter, the area extending up to 100 feet should have well-spaced and maintained vegetation to prevent fire from easily reaching the structure. This involves trimming low-hanging tree branches and ensuring vertical space between the ground, shrubs, and trees. This eliminates “ladder fuels” that allow fire to climb into the canopy.
Proper maintenance of machinery and safe disposal of heat sources are effective prevention strategies. Equipment that can generate a spark, such as mowers or trimmers, should be used before 10 a.m. when humidity is generally higher and temperatures are cooler. All smoking materials must be fully extinguished and disposed of in appropriate, non-combustible receptacles, never tossed onto the ground.