Background on Invasive Plants

Water hyacinth (Eichhornia crassipes) and other aquatic weeds have significant impact on wildlife, recreation, and water conveyance, especially in the Sacramento River Delta (Photo: Bob Case)

Invasive species are widely recognized as a top threat to healthy ecosystems and economies. In California, invasive plants impact farming, ranching and timber operations; hunting, fishing, boating, and hiking opportunities; water supply and delivery; flooding and wildfire damage; and native plant and wildlife species.

What Makes a Plant “Invasive”?

When plants that evolved in one region of the globe are moved by humans to another region, a few of them flourish, crowding out native vegetation and the wildlife that feeds on it. Some invasives can even change ecosystem processes such as hydrology, fire regimes, and soil chemistry. These invasive plants have a competitive advantage because they are no longer controlled by their natural predators, and can quickly spread out of control. In California, approximately 3% of the plant species growing in the wild are considered invasive, but they inhabit a much greater proportion of the landscape.

Who Lists Invasive Plants?

Invasive plants are defined by the California Invasive Plant Council (Cal-IPC) as plants that grow wild and cause damage in a region where they are not native. These plants are listed in their Inventory.

The California Department of Food and Agriculture (CDFA) maintains a list of “noxious weeds” that are subject to regulation or quarantine by county agricultural departments. For more information, see CDFA’s Integrated Pest Control Branch. These weeds are typically agricultural pests, though many also have impacts on natural areas.


Invasive plants cause a variety of problems. Aquatic species can form dense mats that kill aquatic life, impede recreation, and keep migratory birds from landing. Some plants increase the danger of wildfires by creating greater fuel loads than native species. Others lower water tables by using significantly more water than native plants. Some alter soil chemistry by exuding salts, or fixing nitrogen. And most invasive plants displace native plants that support wildlife.

How Do They Get Here?

Invasive plants reach California by many pathways. Some are attractive and are introduced as ornamentals. Others came with early settlers as forage species for livestock. Still others were accidental introductions or contaminants in shipments of seed or hay.

What’s Being Done?

Due to the wide-ranging impacts of invasive plants, many organizations and individual citizens now work to protect and restore native habitats by removing invasive plants. These “weed workers” range from professional land managers employed by government agencies, to local landowners, to volunteers spending their weekends working in local parks or creeks. The resources in these web pages support this growing effort to address invasive plants.

For More Background