Wash. Rev. Code § 17.04.010
The boards of county commissioners of the respective counties may create a weed district or districts within their counties and enlarge any district, or reduce any district or create or combine or consolidate the districts, or divide or create new districts, from time to time, in the manner hereinafter provided, for the purpose of destroying, preventing and exterminating, or to prevent the introduction, propagation, cultivation or increase of, any particular weed, weeds or plants, or all weeds or plants, including Scotch broom, which are now or may hereafter be classed by the agricultural experiment station of Washington State University as noxious weeds, or plants detrimental to or destructive of crops, fruit, trees, shrubs, valuable plants, forage, or other agricultural plants or produce. Any such district shall include not less than one section of land, and the boundaries thereof shall be along an established road, railroad, scab, uncleared or grazing land, or property line, or established lines, or some natural boundary, and shall include only cultivated or farming lands and shall not include any scab, uncleared or grazing land, except such as shall lie wholly within cultivated or farming lands within the districts, or which lie adjacent to such cultivated or farming lands and which are infested, or which may reasonably be expected to become infested, with the particular weed or weeds to be destroyed, prevented and exterminated by such district: PROVIDED, That any quarter section of land, or lesser legal subdivision in single ownership, fifty percent of which is cultivated or farming land, shall be considered cultivated and farming land within the meaning of this chapter.
RCW 17.04.010