Invasive Species
Invasive plants change how fire behaves. Several of the most widespread invasives in San Mateo County ignite easily, burn intensely, and extend the fire season, and controlling them is one of the most cost-effective fuel reduction actions a community can take.
How Invasive Plants Increase Fire Hazard
Plant invasions alter fuel properties, and fuel properties drive fire behavior. Annual grasses that invade shrublands increase the frequency of fire and lengthen the fire season. Woody invaders like the brooms and gorse add fuel load and burn intensity. Species with highly flammable tissue, such as eucalyptus, ignite more easily and burn hotter than the vegetation they displace.
Once an invasive plant establishes a new fire pattern, restoring the original condition gets harder and more expensive every year. The invasion spreads, the fire regime shifts to favor the invader, and land managers end up managing fuels, fire, and native plant recovery all at once. The most cost-effective intervention is early: remove small infestations before they change the system.
The Worst Offenders in San Mateo County
The San Mateo County Weed Management Area, a partnership of public agencies, private landowners, agriculture, and environmental organizations, identifies eight invasive species of top concern in the county:
- Yellow star thistle
- Jubata grass
- Pampas grass
- French broom
- Scotch broom
- Cape ivy
- Gorse
- Fennel
Most of these are also fire-prone. The brooms, gorse, and the pampas grasses appear on our fire-prone plant list and should be removed from defensible space zones wherever they occur. Broom and gorse in particular produce dense stands of fine, oily fuel that carry fire quickly upslope toward homes.
Invasive Plants and Fuel Reduction Work
Fuel reduction projects can backfire when they create openings that invasive plants colonize. Chipping, mowing, grazing, and prescribed fire all disturb soil and canopy, and invasives establish faster than natives in disturbed ground. Fuel reduction in San Mateo County should include follow-up monitoring and treatment for invasive establishment, or the project can trade one fuel problem for a worse one.
The U.S. Fish and Wildlife Service handbook on this topic provides practical guidelines for managers planning fuel treatments in areas with invasive plant pressure: Fire Management and Invasive Plants: A Handbook (PDF).
Learn More
- California Invasive Plant Council — identification, mapping, and control guidance for California invasive plants
- San Mateo County Weed Management Area — local coordination and priority species
- Fire-safe landscaping — what to plant instead