Cal. Lab. Code § 1156.4
Recognizing that agriculture is a seasonal occupation for a majority of agricultural employees, and wishing to provide the fullest scope for employees' enjoyment of the rights included in this part, the board shall not consider a representation petition or a petition to decertify as timely filed unless the employer's payroll reflects 50 percent of the peak agricultural employment for such employer for the current calendar year for the payroll period immediately preceding the filing of the petition.
In this connection, the peak agricultural employment for the prior season shall alone not be a basis for such determination, but rather the board shall estimate peak employment on the basis of acreage and crop statistics which shall be applied uniformly throughout the State of California and upon all other relevant data.
Ca. Lab. Code § 1156.4