Source: California Invasive Plant Council

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Invasive Plant Definitions

What Makes a Plant "Invasive"?

Bull thistle (Cirsium vulgare) invades Tioga Pass in Yosemite National Park
Bull thistle (Cirsium vulgare) invades Tioga Pass in Yosemite National Park
Photo courtesy Bob Case

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. Cal-IPC focuses on plant species that impact natural areas, sometimes called "wildland weeds." (See our California Invasive Plant 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.

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