Wash. Rev. Code § 15.09.060
Each owner of land containing any plant or plants shall perform or cause to be performed such acts as may be necessary to control and to prevent the spread of horticultural pests and diseases, as such pests and diseases are defined under RCW 15.08.010, as now or hereafter amended, or as such pests and diseases are defined by the director of the department of agriculture in accordance with the purpose of this chapter and with the provisions of the Administrative Procedure Act, chapter 34.05 RCW. The word "owner" as used in this section shall mean the possessor or possessors of any form of legal or equitable title to land and entitlement to possession. For purposes of liability under this chapter, the owners of land shall be jointly and severally liable.
RCW 15.09.060