Wash. Rev. Code § 17.06.040
At the time and place fixed for such hearing, with the chair of the principal board acting as chair, the respective boards shall determine by a majority vote of each of the boards of county commissioners of the counties whether such intercounty weed district shall be created, and if they determine that such district shall be created, the respective boards shall fix the boundaries of the portion of the proposed district within their respective counties, but they shall not modify the purposes of the petition with respect to the weed or weeds to be destroyed, prevented, and exterminated as set forth in the petition, and they shall not enlarge the boundary of the proposed district, or enlarge or change the boundary or boundaries of any district or districts already formed without first giving notice, as provided in RCW 17.06.030, to all landowners interested. If the respective bodies shall determine that the weed district petitioned for shall be created each such board shall thereupon enter an order establishing and defining the boundary lines of the proposed district within its respective county. A number shall be assigned to such weed district which shall be the lowest number not already taken or adopted by an intercounty weed district in the state, and thereafter such district shall be known as "weed district No. . . . .", inserting in the blank the number of the district.
If any county represented does not by a majority vote of its board of commissioners support the petition for an intercounty district, the petition shall be dismissed.
RCW 17.06.040