Wash. Rev. Code § 17.06.060
The board of directors of an intercounty weed district shall have the same powers and duties as the board of directors of a weed district located entirely within one county, and all the provisions of chapter 17.04 RCW are hereby made applicable to intercounty weed districts: PROVIDED, That in the case of evaluation, assessment, collection, apportionment, and any other allied power or duty relating to taxes in connection with the district, the action shall be performed by the officer or board of the county for that area of the district which is located within his or her respective county, and all materials, information, and other data and all moneys collected shall be submitted to the proper officer of the county of that part of the district in which the greatest amount of acreage is located. Any power which may be or duty which shall be performed in connection therewith shall be performed by the officer or board receiving such as though only a district in a single county were concerned. All moneys collected from such area constituting a part of such district that should be paid to such district shall be delivered to the principal county treasurer who shall be ex officio treasurer of such district. All other materials, information, or data relating to the district shall be submitted to the district board of directors.
Any costs or expenses incurred under this section shall be borne proportionately by each county involved.
RCW 17.06.060