California's Invasive Plants
California’s Mediterranean climate and broad range of habitats make it ideal for the invasion and spread of nonnative/invasive plants. Invasive nonnative plants threaten the health of our forest, riparian, wetland, and rangeland ecosystems, reducing biological diversity, altering water patterns, impacting rare species, degrading wildlife habitat, modifying vegetation structure and species composition, changing fire and nutrient cycles, and degrading soil structure.
On National Forest System lands, invasive plants are documented, treated, and surveyed whenever possible using the Early Detection/Rapid Response (EDRR) system, and an Integrated Pest Management (IPM) approach. Treatment of infested areas is often hampered by wildfire and a lack of funding and resources at the National Forest level. Therefore, the USFS has prioritized working with our State & Private partners, such as Weed Management Areas, and adjacent landowners to mitigate invasive plant impacts and stop their spread whenever possible.
Both California and the USDA maintain lists of plants that are considered threats to the well-being of the state or the country. The two lists differ significantly. Invasive plants in California are ranked according to two separate ranking systems, the Cal-IPC Invasive Plant Inventory, and CDFA’s Encycloweedia Weed Ratings. National Forests prioritize treatment utilizing federal and state rankings, as well as assessments of potential for spread, feasibility of eradication, and potential for control when proposing management options on National Forest System lands.
Priority Species Highlights
Yellow starthistle
Yellow starthistle (Centaurea solstitialis) is widespread throughout California, and there is an eastward leading edge in the Sierra Nevada. Within the last 20 years, yellow starthistle has increased from 1 to 12 million acres, about 12% of the State’s land base.
Photo: Yellow starthistle,
by Doug Greenburg
Scotch and French broom
Brooms are extremely invasive. Dense stands of older brooms present a high fire hazard. Scotch broom (Cytisus scoparius) and French broom (Genista monspessulana) were introduced in the mid-1800s as an erosion control planting
Photo: French broom,
by Joseph DiTomaso
Giant reed and tamarisk
Riparian habitat species like giant reed (Arundo donax) and tamarisk (Tamarix ramosissima) affect large areas of habitat in southern California and can affect water availability and fire regimes.
Photo: Giant reed,
by Joseph DiTomaso
Knapweed
Spotted knapweed (Centaurea stoebe ssp. micranthos) and diffuse knapweed (Centaurea diffusa) were likely introduced through movement of contaminated seed. New locations may be the result of vehicle or equipment movement, i.e. during wildfire suppression activities.
Photo: Diffuse knapweed,
by Joseph DiTomaso
Early Detection and Rapid Response (EDRR)
The USDA’s National Invasive Species Information Center defines EDRR as: “a coordinated set of actions to find and eradicate new and emerging invasive species in a specific location before they can spread and cause harm.” Our National Forests work closely with partners to detect and manage leading edge areas.
- In 2019, the CA Invasive Plant Council concluded a 5-year grant with the Forest Service. The grant supported work to develop and update the online database and tool “CalWeedMapper”, which shows landscape-level distribution of invasive plants and suggests management priorities by region.
- In 2020, the Forest Service awarded a special grant to the Agricultural Research Service for development of methods to mass rear, store, and release the rosette weevil, a new biological control agent of yellow starthistle.
- In 2022, the Forests Service partnered with the University of CA, Santa Barbara, and the Agricultural Research Service to support continued monitoring of Cape-ivy biocontrol and to begin a project piloting potential biocontrol options for invasive annual grasses, which threaten forest regeneration and increase uncharacteristic wildfires.
Integrated Pest Management (IPM)
IPM is a science-based, decision-making process. As defined in 7 U.S.C. § 136r, IPM is “a sustainable approach to managing pests by combining biological, cultural, physical, and chemical tools in a way that minimizes economic, health, and environmental risks.”
As of 2022, many site-specific NEPA documents approved to control invasive plants as part of an IPM program have been completed on 14 of the 19 National Forests in CA. Several forests have completed forest-wide NEPA for invasive plant management. We continue to work with partners on projects such as the Sierra National Forest’s Merced Canyon Invasive Weed Control Project and the Inyo National Forest’s Volunteer Program to detect and treat invasive plants.
For opportunities to partner with our National Forests on Invasive Plant management, please contact the Pacific Southwest Region Invasive Plant Program Manager.
Contact Us
Stacey Clark, Invasive Plants Program Manager & Pesticide-Use Coordinator
Cell: 707-980-9010
Email: stacey.clark@usda.gov
REPORT A PEST!
Please report a plant or pest that you suspect may be a new invasive species in your area.