Over the years, I’ve visited dozens of major wildfires to examine how they burned and what they didn’t burn. One noticeable pattern in urban settings is how houses burn to their foundation, while nearby trees and shrubs remain green.
Global climate change is exacerbating urban wildfire risks. For instance, between 2003 and 2023, the global frequency of extreme fire events escalated to 2.2 times its original rate, and the average intensity of major fires rose to 2.3 times its previous level.
High winds are one reason the usual “fuel reduction” prescriptions, such as livestock grazing, logging, or prescribed burns, often fail to protect homes. Embers are blown over, around, and through such “fuel breaks.”
Home hardening, such as non-burnable roofs, screened vents, cement siding, and sprinkler systems, is known to reduce homes’ flammability.
While most people believe a “wall of flames” destroys communities, in truth, in most instances, no 30-foot-high flames torch homes. Rather, it is surface fuels or wind-tossed embers that ignite most structures.
For instance, I visited Los Alamos, where 2000 the Cerro Grande Fire burned through understory grass to ignite the homes, leaving ponderosa pine adjacent to the structures green and uncharred. Since then, I have visited numerous other blazes where wildfires with flames often no more than 1-2 feet tall reached the buildings and ignited them.
Because of these observations, I have always thought one should create a minimum five-foot vegetation-free perimeter adjacent to the home. Putting gravel or cement could prevent a surface fire from igniting the home. Indeed, I have either gravel or bare dirt adjacent to my own home’s foundation.
California Governor Gavin Newsom recently issued an executive order prohibiting trees, shrubs, and grasses in the immediate 5-foot area adjacent to homes in the Very High Fire Hazard Severity Zones.
Ordinarily, I would agree with the Governor’s proclamation.
However, new research from California scientists is challenging this idea.
If you visit many of these urban wildfires, as I have, the presence of green vegetation adjacent to homes is significant. What appears to be occurring is that high winds, such as the 100 MPH Santa Anna gales that ignited the LA fires that destroyed 15,000 homes by carrying embers over streets and fire breaks to ignite homes.
This link will take photos of homes that survived the LA fires.
How does one explain burnt-out foundations with green vegetation next to the former home? One explanation is that most homes are more combustible than the adjacent vegetation, particularly watered trees and shrubs. The internal moisture of urban irrigated vegetation is often much higher than, say, a kiln-dried deck or wooden siding.
This is particularly true of deciduous vegetation. I’ve seen numerous wildfires that charred conifers (which have flammable resins) while deciduous aspen remained unscathed.
Wildfires will heat the internal moisture inside the plant, but in the process, they cool the immediate area adjacent to the plant and take longer to ignite, thus providing some safety to homes. Therefore, the moisture content of the vegetation influences flammability and fire spread.
For instance, during the 1988 extreme drought that contributed to a million acres in and around Yellowstone to burn, the conifers’ internal moisture content dropped as low as 1%, while kiln-dried lumber was often 12-15%.
Thus, well-watered urban vegetation can help reduce ignition by reducing heat and, at times, blocking wind-tossed embers from reaching a home. Of course, not all vegetation is easily resistant to wildfire. Some possess flammable resins that, when heated, can ignite. Nevertheless, banning all vegetation adjacent to homes may not provide additional fire resistance and could improve home survival.
The scientists conclude: “We believe the California proposal’s current emphasis on highly prescriptive vegetation removal, instead of on maintenance, is overly simplistic. Without complementary requirements for hardening the homes, widespread clearing of landscaping immediately around homes could do little to reduce risk and even aggravate the danger.”
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