Washington State law requires counties to have Noxious Weed Control Boards. The State enacted RCW 17.10 Noxious Weeds - Control Boards, in 1969 to limit economic loss due to the presence and spread of noxious weeds. Revisions to the law were made in 1987. This weed law mandates the control of many weed species. Noxious weeds are defined in the law as "..a plant which when established is highly destructive, competitive or difficult to control by cultural or chemical practices."

Each year, the Washington State Noxious Weed Control Board updates the state noxious weed list. It is published in WAC 16-750: State Noxious Weed List and Schedule of Monetary Penalties. The list is divided into three different categories of weeds:

• Class A weeds are the highest priority species on the list. They are non-native species with a limited distribution in Washington. Preventing new infestations and eradicating existing infestations is required for all Class A species.
• Class B weeds are the second highest priority. They are non-native species that infest some regions of the state but not others. Class B species are designated for control in regions where they are not yet widespread. Preventing infestations in these areas is a high priority. In regions where a Class B species is already abundant, control is decided at the local level, with containment as the primary goal.
• Class C weeds are established throughout much of the state. Long-term programs of suppression and control are a local option, depending upon local threats and the feasibility of control in local areas.

The local noxious weed control boards and weed districts carry out the state's noxious weed law at the local level.