Encycloweedia: Weed Ratings

A noxious weed is a plant that has been defined as a pest by law or regulation. Both California and the U.S government maintain lists of plants that are considered threats to the well being of the state or the country. The two lists differ significantly.

In California, biologists of the California Department of Food and Agriculture recommend plants for listing, after consultation with outside experts and the Agricultural Commissioners of California's counties (CACs). If a plant is found to probably be "troublesome, aggressive, intrusive, detrimental, or destructive to agriculture, silviculture, or important native species, and difficult to control or eradicate", the Department will designate the plant as a noxious weed.

At the time that CDFA lists a species, it also receives a rating of A, B, C, D, or Q. These ratings reflect CDFA's view of the statewide importance of the pest, the likelihood that eradication or control efforts would be successful, and the present distribution of the pest within the state. The ratings are not laws, but are policy guidelines that indicate the most appropriate action to take against a pest under general circumstances. Local conditions may dictate more stringent actions at the discretion of the CAC, and the rating may change as circumstances change.

The precise policy language is given below. This system has been in place since at least 1977, and revised in May 2009. The term "commissioners" refers to the County Agricultural Commissioners.

Action Oriented Rating System Purpose

To advise commissioners as to the Department's policy regarding any pest action.


  • A pest of known economic or environmental detriment and is either not known to be established in California or it is present in a limited distribution that allows for the possibility of eradication or successful containment. A-rated pests are prohibited from entering the state because, by virtue of their rating, they have been placed on the of Plant Health and Pest Prevention Services Director’s list of organisms “detrimental to agriculture” in accordance with the FAC Sections 5261 and 6461. The only exception is for organisms accompanied by an approved CDFA or USDA live organism permit for contained exhibit or research purposes. If found entering or established in the state, A-rated pests are subject to state (or commissioner when acting as a state agent) enforced action involving eradication, quarantine regulation, containment, rejection, or other holding action.