What is a noxious weed?

Scotch broom in Skamania County's West End

"Noxious Weed" is the traditional, legal term for and invasive, non-native plant that threatens agricultural crops, local ecosystems or fish and wildlife habitat.  The term "noxious weeds" includes non-native grasses, flowering plants, shrubs and trees.  It also includes aquatic plants that invade wetlands, lakes, rivers and shorelines.  Noxious weeds cause damage that has considerable environmental and economic costs. 

It is important to note that about half of all invasive noxious weeds are escapees from gardens.  The rest are plants accidentally introduced to Washington through human travel and trade. 

  Not sure if your weeds are noxious?  Check out the Washington State Noxious Weed Control Board weed search tool for a comprehensive search by habitat, color or name.  If you have questions or need assistance, feel free to give us a call or email.

How are noxious weeds spread?

Noxious weeds are spread in a variety of ways.  While some of these are natural (wind, water, and wildlife), weeds are  often spread by human activity and pet traffic.  Seeds get caught in tires or the undercarriage of vehicles (including ATVs), and in the tread of shoes and boots,  get stuck on clothing or attached to recreational gear and accidentally transported .  Pets carry weed seeds when burrs and other seeds get caught in their coats and paws. 

Noxious weeds usually have more than one method of propagation. For example, they may send out rhizomes as well as generate seeds.  Knotweed is an example of a plant that spreads by seed and fragmentation.  A fragment as small as an inch  can break off and take root.  This is the primary method of invasion  flooding events occur or when knotweed debris is disposed of improperly.   

Does each state and county have different noxious weeds?

Noxious weeds vary from state to state and county to county. Please check with your local weed authority to determine the noxious weeds of concern in your area.

In Washington, an list is generated annually with three classifications. Class A weeds are non-native species with limited distribution in the state. Eradication of all Class A weeds is required by state law. Class B weeds are non-native weeds that are established in some regions of Washington, but have limited distribution or are not present in all regions of the state.  In areas where Class B weeds are present, control is optional;  however, when plants are found that have been previously undetected or have limited distribution,  seed prevention is required.  Class C weeds are widely established or of particular agricultural interest. A county weed board may designate weeds regardless of class for control within their county.

(For a complete list of Washington State's Noxious Weeds, visit http://www.nwcb.wa.gov/printable.htm).

How can I get rid of noxious weeds on my property?

The best strategy to control noxious weeds is to employ an integrated approach.  Integrated Management Plans or IMP's use multiple methods of control including manual or mechanical removal, the careful use of herbicides, and the introduction of biological controls such as seed or root boring weevils that impact a plants ability to reproduce.

(For a free home weed survey and technical assistance on weed management contact us at weeds@co.skamania.wa.us or call 509-427-3941.)

What kind of economic impact do noxious weeds have in Washington State?

Noxious weeds reduce crop yields and require farmers to spend more time and money on weed management. Estimates include some of these staggering costs:

  • Infestations of Knapweed and Yellow Starthistle result in an annual cost of $950,000 in lost forage production in Eastern Washingon.
  • As a whole, Washington suffers over $100 million in annual crop loss.
  • Scotch broom results in $47 million annual reduction of timber production


How do noxious weeds threaten local species?

Class A noxious weed, garlic mustard rosettes near Stevenson.

Noxious weeds threaten local species by reducing wildlife habitat and altering natural food sources. Often, noxious weeds form dense monocultures (the ubiquitous Himalayan blackberry is a great example of this), eliminating or drastically reducing biodiversity. Many noxious weeds are water-guzzlers, and drain our water resources rapidly. Some noxious weed species are allelopathic; which means they release toxins into the soil that inhibit growth of nearby plants. In addition, noxious weeds are sometimes the culprit of soil erosion, which in turn leads to reduced water quality.

Are noxious weeds harmful to my health?

Some noxious weeds can be harmful to your health. While some species contain toxins that can be harmful on contact, others must be ingested to cause a reaction. In Skamania County, Tansy Ragwort is a potential threat. It is poisonous to cattle, horses and sheep, and causes liver failure when eaten. Visit this link for a complete look at poisonous plants in this region.

Contact us

Office Hours:            6:30 - 4:30 Monday to Thursday

Office Location:      704 Rock Creek Drive Stevenson, WA 98648

Mailing Address:        PO Box 369   Stevenson, WA 98648

Phone: (509) 427-3941 Email: Weeds

Learn more

Noxious weeds: FAQ

Get help and participate

Noxious weed list and laws

Weed Prevention

Methods of control


Toxic Plants








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