How Invasive Plants Are Exacerbating California’s Wildfire Crisis

Fire is always there It shaped the landscape in California. But today it’s hotter, burning more often, and more widespread than ever before—a change related to human development, climate changeand the spread of invasive species, which are non-native plants that negatively impact local ecosystems. Herbs and trees were brought California for agriculturedevelopment or by accident changed the state’s fire dynamics.

“Many non-native species can spread fire faster than native plants,” says David Acuña, battalion chief for Cal Fire at the state’s department of forestry and fire protection. This transformation is an overlooked driver for increasingly disruptive ones Wildfires in California and all over the world.

Southern California is dominated by thickets known as chaparral. This landscape was historically characterized by short, bushy vegetation, and any native grasses were perennial, conserving moisture and keeping it green most of the year. Fires, when they did occur, were rare because lightning was rare. When fires started, they burned hot, but could not spread far because the open spaces between plants acted as natural fire extinguishers.

The introduction of non-native grasses in the 1700s fundamentally changed this balance. Brought by European settlers, these grasses have evolved with heavy livestock grazing and regular burning, making them highly resistant to disturbance. They outcompeted native species and filled gaps in the undergrowth, creating a durable carpet of combustible material, especially in modified areas such as roads – frequent starting points for fires.

Unlike perennial native grasses, these non-native grasses are annuals, meaning they die back and grow from seed every year. Their short life cycle leaves behind a dense layer of dry, dead vegetation in late spring. “They have such a high surface area by volume, and they’re very flat and thin, so they hold a lot of dead material almost year-round,” says Carla D’Antonio, a plant community researcher and professor at the University of California, Santa Barbara. By May, dead grass covers the ground. “It ignites so quickly that it takes any kind of ignition—a cigarette, a spark from someone dragging a chain on the highway, or lightning,” says Hugh Safford, a vegetation and fire ecology researcher at the University of California, Berkeley.

Grasses fill every available space—a phenomenon called fuel persistence. When a fire sparks, the continuous line of dry vegetation acts as a wick, driving the flames into the undergrowth. “People underestimate the destructiveness of grasses because you can knock them down quickly with a hoe, whereas cutting down bushes is quite difficult,” says D’Antonio. “But if sparks and embers fly in the middle of a bunch of grass, then—boom—everything around you goes up like gasoline. It spreads so fast and it’s so persistent. It’s like throwing tissue paper on a fire.”

Eucalyptus trees brought to California from Australia in the mid-19th century add another layer of fire risk. Known for their aromatic scent, these trees have incredibly flammable, oily leaves. Their papery shells deflate and fall in the wind, carrying embers half a mile away. Acuña says the problem arises when people plant them near their homes. “You put a very hot, very strong burning plant like a eucalyptus tree near the house, which is mostly made of petroleum materials. It’s a very strong fire,” he explains.