How Invasive Plants Are Fueling California’s Wildfire Crisis

Non-native grasses and eucalyptus trees were brought to California centuries ago for agriculture and landscaping, but they’ve changed the state’s natural fire dynamics.

Fire has always shaped the landscape in California. But today it burns hotter, more frequently, and spreads further than ever before—a shift driven by human development, climate change, and the prevalence of invasive species, which are non-native plants that have negative effects on local ecosystems. Grasses and trees brought to California for agriculture, landscaping, or by accident, have transformed the state’s fire dynamics.

“Many non-native species can propagate fire faster than native plants,” says David Acuña, battalion chief for Cal Fire, the state’s department of forestry and fire protection. This transformation is an overlooked driver for the increasingly destructive wildfires in California and around the world.

Southern California is dominated by shrublands known as chaparral. This landscape was historically characterized by short, shrubby plants, and any native grasses were perennial, maintaining moisture and staying green for most of the year. Fires, when they occurred, were rare because lightning strikes were infrequent. When fires did ignite, they burned hot but wouldn’t spread far because the open gaps between plants acted as natural firebreaks.

The introduction of non-native grasses in the 1700s fundamentally altered this balance. Brought by European settlers, these grasses evolved alongside heavy livestock grazing and routine burning, making them highly resilient to disturbance. They outcompeted native species and filled the gaps in shrublands, creating a continuous carpet of flammable material, especially along altered areas like roadways—frequent starting points for fires.

Unlike perennial native grasses, these non-native grasses are annuals, meaning they die each year and regrow from seeds. Their short life cycle leaves behind a dense layer of dry, dead vegetation by late spring. “They have such a high surface area to volume and are very flat and thin, so they maintain a lot of dead standing material, almost all year round,” says Carla D’Antonio, a plant community researcher and professor at the University of California, Santa Barbara. By May, dead grass blankets the ground. “It’s so flammable that it takes any ignition—cigarette, spark from someone dragging a chain on the highway, or lightning,” says Hugh Safford, a vegetation and fire ecology researcher at University of California, Davis.

The grasses fill every available space—a phenomenon called fuel continuity. When fires spark, the uninterrupted line of dry vegetation acts like a wick, carrying the flames into the shrublands. “People underestimate the destructiveness of grasses because you can go hack them down with a hoe quickly, whereas a shrub is pretty hard to cut down,” says D’Antonio. “But if the sparks and embers fly in the middle of a bunch of introduced grasses, then—boom—everything around you just goes up like gasoline. It spreads so fast and it’s so continuous. It’s like throwing tissue paper onto a fire.”

Eucalyptus trees, introduced to California in the mid-19th century from Australia, add another layer of fire risk. Known for their aromatic scent, these trees have incredibly flammable, oily leaves. Their papery bark sloughs off and catches in the wind, transporting embers up to half a mile away. The problem comes when people plant them right next to their home, says Acuña. “You put a very hot, very vigorous burning plant like a eucalyptus tree next to a house, which is primarily composed of petroleum materials. That’s a very strong fire,” he explains.

The 1991 Tunnel Fire in Oakland, California, ignited debates—and multiple lawsuits—about whether to remove the widespread eucalyptus. “People want to keep them because they’re iconic, but wow are they are so freaking flammable,” says Safford. Yet in terms of a landscape-scale issue, Safford emphasizes that grasses remain a bigger concern.

But it’s not just California; invasive species have created fire hazards worldwide. Eucalyptus plantations have contributed to massive wildfires in Portugal, and grass fires have spread everywhere from the Great Basin in the American West to tropical forests in Chile. “It’s a major threat to native ecosystems around the world,” says Safford. “There’s a lot of international interest, so if someone could figure out just how to control grass, I think that would be quite a find.”

Vegetation management plays a critical role for fire mitigation: “The best way people are managing grass right now is just by hand-cutting roadsides and having crews out there every spring,” says D’Antonio. “I live in the mountains and our community has a roadside group that goes out with volunteers and clears the roadsides and rakes off all that dead grass every year.” However, this shouldn’t be conflated with President Trump’s misleading claims from 2019 about “raking the forest floor” that resurfaced on social media during the LA wildfires. (It may work for tracts of grass along roadways, but not woody debris on forest floors.) And while raking is one method of vegetation management, effective fire prevention requires a broader approach, including controlled burns, strategic grazing, and clearing dead vegetation from key areas.

In some areas, sheep grazing is being tested as a low-impact way to manage grass growth. Some areas in Southern California are even testing a “BurnBot”, a machine that travels over the ground, performing controlled burns by torching anything directly underneath it—clearing both existing vegetation and plant seeds.

D’Antonio and her graduate students are researching ways to replace these fire-prone areas with native grasses that are more fire-resilient. “Once they’re established, they’re fairly deep-rooted and they can access soil moisture deeper, so they stay more moist during the summer,” she explains. “Our goal is to create a community of native perennial grasses that can maintain itself so we don’t have to do constant maintenance.”

Homeowners also have an important role to play. “Everybody wants to blame the Forest Service when a fire rips through, right? But we chose to live in this landscape,” says D’Antonio. “So start with your home.” Acuña says that Cal Fire provides tips for home- and landowners looking to fire-proof their property through landscaping and creating defensible space.

Ultimately, invasive plants are a problem that requires collective action—community involvement, responsible land-management practices, and forward-thinking research to restore native ecosystems. Without these efforts, California’s wildfire crisis will only intensify, fueled by plants never meant to be here in the first place.

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