One of the most striking patterns in the aftermath of many urban fires is how much unburned green vegetation remains among the remains of burning areas.
In some cases, it could be all shrubs that separate homes born from burning homes a few feet away.
As scientists studying how vegetation can ignite and burn, we are not surprised by these images: we recognize that well-maintained plants and trees can help protect your home from wind-blown embers and in some cases slow the spread of fire. Therefore, we are concerned about new wildfire protection regulations developed by California that bans almost all plants and other flammable materials within five feet of a home, an area known as “Zone 0.”
Wildfire safety guidelines have long encouraged homeowners not to avoid flammable materials next to their homes. However, the state’s plan for the “Ember-Resistant Zone” which has been expedited under an executive order from Gov. Gavin Newsom is moving further by banning grass, shrubs and many trees in the area.
If that ban remains in the final regulations, it could be met with public resistance. Getting these rules right is also important beyond California. This is because the restrictions that occur here often ripple outwards into other fire-prone areas.
Research into ways vegetation reduces the risk of fire is a relatively new field of research. However, findings from plant flammability studies, as well as exploring where vegetation and homes survived large urban fires, underscore the importance of this.
When surviving plants appear to be burnt after these fires, they are often on the side of the plant, facing nearby burning structures. It suggests that the embers blew in the wind first set the house on fire. The house was then fueled when the fire spread across the neighborhood.
Photos before and after the 2025 Palisade fire show thick green vegetation between two closely spaced houses. The arrows indicate the direction of the fire spread. When surviving plants appear to be burnt after these fires, they are often on the side of the plant, facing nearby burning structures. It suggests that the embers blew in the wind first set the house on fire. The house was then fueled when the fire spread across the neighborhood.
(Max Moritz; Cal fire damage inspection photo)
We’ve seen this over and over in the Los Angeles area after wildfires destroyed thousands of homes in January. This pattern suggests that we need to focus on many factors that can affect household loss.
Several guides are available that explain the steps homeowners can take to protect their homes, particularly wind-blown embers known as home curing. Some examples include installing rain gutter covers to prevent dead leaves from accumulating, avoiding flammable paneling, and ensuring that there is a screen to prevent embers from entering the attic.
However, guidance related to landscaping plants is very different, some of which are bad advice.
For example, some “fire safety” plant lists contain species that are drought-resistant but not necessarily resistant. When it comes to preventing plants from becoming fuel due to fire, what is more important than the selection of species is how well the vegetation is maintained, and whether it is properly watered. Location is also important: dry, uncontrolled plants under windows or near fences can quickly catch fire, making the house itself more likely to set fire.
If water-rich plant materials are heated by nearby energy sources such as fire, the moisture inside them must be stolen before ignition. Its evaporation cools the surrounding area and reduces the flammability of the plant.
In many cases, the plants do not ignite when the humidity is high. We have seen this in some of our experimental work and other studies testing the flammability of ornamental landscaping.
With sufficient heat, dry leaves and stems can break down and release volatile gases. At that point, nearby sparks and flames can light these gases and burn the plants.
But even if a plant is burning, its moisture content can limit other aspects of fiery that can become burning hot.
Green, well-maintained plants can act as heat sinks, absorb energy and block embers, which can slow the spread of fire. This apparent protective role has been observed in both Australian and California household loss studies.
It is still unknown how often vegetation buffers the opening during urban fires, but this ability affects regulations.
Many of the latest Zone 0 recommendations, such as the ban on mulching, such as the sticking fence made from potentially burning materials, come from large-scale testing conducted by the National Institute of Standards and the Institute for Business and Housing Safety. These features can be systematically analyzed.
However, vegetation is much more difficult to model. The state’s proposed zone 0 regulations simplify complex conditions in real neighbourhoods and go beyond what is currently known from scientific research into plant flammability.
Vegetation is not monolithic. Mature, organized shrubs or high-crowned trees have little risk of burning and can even reduce exposure to fires by blocking wind and heat and intercepting embers. Aspen Trees, for example, are recommended to reduce the risk of fire near structures and 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.
California’s proposal considers the current emphasis on highly normative vegetation removal, rather than maintenance, is very simple. Without complementary requirements to harden the house itself, extensive settlement of landscaping around the house could do little to reduce the risk, and could even exacerbate the risk.
Max Moritz is a wildfire specialist and adjunct professor at UC Santa Barbara’s Co-op Expansion of Environmental Sciences. Luca Carmignani is an assistant professor of mechanical engineering at San Diego State University. This article was created in collaboration with the conversation.
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