This page was last updated March 11, 2008

What are Noxious Freshwater Weeds?

Noxious freshwater weeds are plants that are not native to Washington, are generally of limited distribution, and pose a serious threat to our state. Plants considered to be nonnative were not present in Washington prior to European settlement.  Because nonnative plants have few controls in their new habitat, they spread rapidly, destroying native plant and animal habitat, damaging recreational opportunities, lowering property values, and clogging waterways. Some noxious weeds can even harm humans and animals. The sap of giant hogweed, a plant that grows in wet areas, can cause severe burns.

Nonnative aquatic plants have been introduced to Washington as ornamental plants (purple loosestrife, giant hogweed), as water garden plants (parrot feather milfoil), and as aquarium plants (Eurasian watermilfoil, Brazilian elodea, fanwort).  They "escaped" into our waterbodies through floods, by people discarding aquarium plants, and by being deliberately planted.  Once introduced, these invasive plants rapidly outcompete our native plants, forming single-species stands, and reducing habitat for fish, waterfowl, and aquatic mammals and invertebrates. 

Many of these exotic weeds are on Washington's Weed List (see below) and some of these plants are prohibited for sale, transport, or transplantation by the Washington Department of Agriculture. However, not all introduced plants become problems. Many of our agricultural crops and landscaping plants are nonnative.  Most do not grow where they are not planted, and do not become invasive or aggressive.

Washington's Noxious Weed Control Board classifies noxious weeds based on the stage of invasion of each species.  The classification system is designed to prevent small infestations from becoming large infestations, and to contain already established infestations to regions of the state where they occur and prevent their movement to uninfested areas of Washington.  All weeds on Washington's Weed List are nonnative species.

Class A Weeds have a limited distribution in Washington.  The statewide goal for these species is eradication.

Class B Weeds are weeds that are established in some regions of Washington, but are of limited distribution or not present in other regions of the state. Because of the differences in distribution, treatment of Class B weeds varies between regions of the state.  In regions where a Class B weed is unrecorded or of limited distribution, prevention of seed production is required. In these areas, the weed is a Class B-designate, meaning it is designated for mandatory control.  In regions where a Class B species is already abundant or widespread, control is a local option.  In these areas, the weed is a Class B-non-designate, with containment, gradual reduction, and prevention of further spread being the chief goals.

Class C Weeds may be characterized as already widely established in Washington or of special interest to the State's agricultural industry. Placement on the list allows counties to enforce control if locally desired.  Other counties may chose to provide education or technical consultation.

A Monitor List of nonnative species is also maintained.  While there is no legal or regulatory aspect to the monitor list, information collected about the weed once it is placed on the monitor list may be used to justify its future classification as a Class A, B, or C weed.

Freshwater Nonnative Plants Considered Problematic in Washington State