Contributor: California’s proposed ban on plants near homes could be dangerously bad advice

One of the most striking patterns in the aftermath of many urban fires is how much unburned green vegetation remains amid the wreckage of burned neighborhoods.

In some cases, a row of shrubs may be all that separates a surviving house from one that burned just a few feet away.

As scientists who study how vegetation ignites and burns, we aren’t surprised by these images: We recognize that well-maintained plants and trees can help protect homes from wind-blown embers and slow the spread of fire in some cases. So we are concerned about new wildfire protection regulations being developed by California that would prohibit almost all plants and other combustible material within 5 feet of homes, an area known as “Zone 0.”

Wildfire safety guidelines have long encouraged homeowners to avoid having flammable materials next to their homes. But the state’s plan for an “ember-resistant zone,” being expedited under an executive order from Gov. Gavin Newsom, goes further by also prohibiting grass, shrubs and many trees in that area.

If that prohibition remains in the final regulation, it’s likely to be met with public resistance. Getting these rules right also matters beyond California, because regulations that originate here often ripple outward to other fire-prone regions.

Research into how vegetation can reduce fire risk is a relatively new area of study. However, the findings from plant flammability studies, as well as examination of sites where vegetation and homes survive large urban fires, highlight its importance.

When surviving plants appear scorched after these fires, it is often on the side of the plant facing a nearby structure that burned. That suggests that wind-blown embers ignited houses first: The houses were then the fuel as the fire spread through the neighborhood.

We saw this repeatedly in the Los Angeles area after wildfires destroyed thousands of homes in January. The pattern suggests a need to focus on the many factors that can influence home losses.

Several guides are available that explain steps homeowners can take to help protect houses, particularly from wind-blown embers, known as home hardening. Some examples include installing rain gutter covers to keep dead leaves from accumulating, avoiding flammable siding and ensuring that vents have screens to prevent embers from getting into the attic or crawl space.

However, guidance related to landscaping plants varies greatly, and some of it is bad advice.

For example, some “fire-safe” plant lists contain species that are drought tolerant but not necessarily fire resistant. When it comes to keeping plants from becoming fuel for fires, what matters more than species selection is how well vegetation is maintained and whether it’s properly watered. Location matters too: Dry, unmanaged plants under windows or near fences may ignite rapidly and make it more likely that the house itself will catch fire.

When well-watered, living plant material is heated by a nearby energy source, such as a fire, the moisture inside it must be driven off before it can ignite. That evaporation cools the surrounding area and lowers the plant’s flammability.

In many cases, high moisture keeps a plant from igniting. We’ve seen this in some of our experimental work and in other studies that test the flammability of ornamental landscaping.

With enough heat, dried leaves and stems can break down and release volatile gases. At that point, a nearby spark or flame can ignite these gases and set the plant on fire.

Even when the plant does burn, however, its moisture content can limit other aspects of flammability, such as how hot it burns.

Green, well-maintained plants can slow the spread of a fire by serving as heat sinks, absorbing energy and even blocking embers. This apparent protective role has been observed in both Australia and California studies of home losses.

How often vegetation buffers homes from igniting during urban conflagrations is still unclear, but this capacity has implications for regulations.

Many of the latest Zone 0 recommendations, such as prohibiting mulch and attached fences made of materials that can burn, stem from large-scale tests conducted by the National Institute of Standards and Technology and the Insurance Institute for Business and Home Safety. These features can be systematically analyzed.

But vegetation is far harder to model. The state’s proposed Zone 0 regulations oversimplify complex conditions in real neighborhoods and go beyond what is currently known from scientific research regarding plant flammability.

Vegetation is not monolithic. A mature, well-pruned shrub or tree with a high crown may pose little risk of burning and can even reduce exposure to fires by blocking wind and heat and intercepting embers. Aspen trees, for example, have been recommended to reduce fire risk near structures or other high-value assets.

As California and other states develop new wildfire regulations, they need to recognize the protective role that well-managed plants can play, along with many other benefits of urban vegetation.

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 themselves, widespread clearing of landscaping immediately around homes could do little to reduce risk and could even aggravate the danger.

Max Moritz is a Cooperative Extension wildfire specialist and an adjunct professor of environmental science at UC Santa Barbara. Luca Carmignani is an assistant professor of mechanical engineering at San Diego State University. This article was produced in partnership with the Conversation.