Invasive Plants/Noxious Weeds

Existing Conditions and Assessment Conclusions

California’s varied topography, geology, and climate enables many exotic plants to colonize and become established. These invasive plants and noxious weeds are a serious threat to the State’s biodiversity and native habitats. Invasive plants are problematic because they adversely impact native vegetative communities by altering patterns of nutrient cycling, hydrological processes, and the intensity of fire.

The Pit RCD and Upper Pit River Watershed are infested with invasive plants and noxious weeds. The Lassen and Modoc County Agricultural Commissioners maintain a list of California Department of Food and Agriculture (CDFA) rated invasive plants known to occur in Modoc and Lassen Counties. View the weed list table here. Many of the weeds in this table occur in the Pit RCD and are negatively affecting the watershed condition of the area.

There are presently weed control projects underway in Modoc and Lassen Counties, some of which occur in the Pit RCD. Most notably, amongst these weed control projects, is the elimination of Scotch thistle in the Big Valley Area. Scotch thistle, like many invasive plants, is a native of Europe and eastern Asia and is probably an escaped ornamental. Scotch thistle stands are dense and practically impenetrable due to the weed's spiny nature and large size. Dense stands of Scotch thistle prohibit the colonization and establishment of native vegetation, thus negatively affecting the overall condition of the watershed.

Another noxious weed of major concern within the Pit RCD is the recent emergence of small plots of Perennial Pepperweed, also known as Tall Whitetop, in the Big Valley and Ash Valley areas. Tall whitetop is an extremely invasive weed that quickly spreads by root fragments and seed. Population’s form large, monoculture stands that crowd out desirable vegetation, compete with desirable crops, and alter ecosystem function including soil properties and the hydrologic cycle. Recent statewide surveys identify tall whitetop as one of the top five weed problems in California, and both the California Department of Food and Agriculture (CDFA) and California Invasive Plant Council (Cal-IPC) list it as a noxious weed of greatest ecological concern.

• Impacts on Agriculture: Tall whitetop is threatening the viability of Lassen County’s agriculture by decreasing grazing land productivity, hay quality, hay marketability, and forage yields. Management of tall whitetop also increases agricultural producers’ annual weed control costs decreasing profit margins and land-value.
• Impacts on Wildlife and Natural Resources: Tall whitetop threatens habitat for several waterfowl, mammal, and insect species by dramatically altering plant community make-up. The dense, monoculture populations also interfere with waterfowl nesting and wildlife movement.

There are currently three agencies that bear responsibility for the treatment and control of noxious weeds within the Pit RCD. They are the Lassen and Modoc County Agricultural Commissioner Offices and the Big Valley Pest Abatement District.

Management Strategy

A. The Pit RCD will work with the responsible weed control agencies to educate landowners on the need to take action to control noxious weeds.

B. The Pit RCD will encourage producers and landowners seeking cost share assistance to include pest management practices in their contracts.

C. The Pit RCD will require and include noxious weed monitoring and follow up treatment in all projects that involve ground disturbing activities in which they participate.

D. The Pit RCD will work with partner agencies and with partnerships to obtain needed funding for noxious weed treatment and control.

Scotch thistle stands are dense and practically impenetrable due to the weed's spiny nature and large size.

Perennial Pepperweed, also known as Tall Whitetop is a major concern within the Pit RCD.

Diffuse Knapweed is one of the noxious weeds known to occur in Modoc and Lassen Counties.