Wash. Rev. Code § 20.01.120
The licensee shall prominently display license plates issued by the director on the front and back of any vehicle used by the licensee to transport upon public highways unprocessed agricultural products which he or she has not produced as a producer of such agricultural products. If the licensee operates more than one vehicle to transport unprocessed agricultural products on public highways he or she shall apply to the director for license plates for each such additional vehicle. Such additional license plates shall be issued to the licensee at the actual cost to the department for such license plates and necessary handling charges. Such license plates are not transferable to any other person and may be used only on the licensee's vehicle or vehicles. The display of such license plates on the vehicle or vehicles of a person whose license has been revoked, or the failure to surrender such license plates forthwith to the department after such revocation, shall be deemed a violation of this chapter.
RCW 20.01.120