What does it mean to be a noxious weed? 

“Noxious weed” is a legal designation. To become a noxious weed a plant must first meet 4 criteria. 

1. Introduced to Washington state (non-native)
2. Spread beyond where they are planted
3. Create harm where introduced*
4. Difficult to control

*This harm can be agricultural, such as reducing crop output, ecological, such as out competing our native plants, or physical, such as toxic to humans or livestock. They can impact wildlife, human health, land value, recreation, and natural resources.

Anyone can nominate a species, but only the Washington State Noxious Weed Control Board defines which plants are noxious. They also determine where weeds are required for control based on how widespread they are and their impact. County Boards have the power to select species to have more selective/additional control requirements but must get approval from the state board to reduce control requirements set at the state level.

To better understand the weed law and how listing works, check out our 2023 blog post: Noxious Weeds & Prohibited Plants: A Listing Guide (kingcountyweeds.com). 

2025 Weed List Updates

This year’s King County Noxious Weed List additions include 3 species that were all added from the Washington State Noxious Weed List. Click on a species name below to jump to information about its listing.

1. Common holly (Ilex aquifolium) – Class C (non-regulated)
2. Roundleaf bittersweet (Celastrus orbiculatus) – Class A (regulated)
3. Marsh thistle (Cirsium palustre) – Class A (regulated)

Preview the weed list below or find web and PDF versions on our website here.

Common holly (Ilex aquifolium) – Class C

Class C Non-regulated Noxious Weeds are impactful species that are often so widespread that control is not required, but strongly encouraged. Due to the problems caused when it escapes, we recommend controlling common holly on your property, especially if you live near any natural areas or forests.

Common holly (Ilex aquifolium) – Non-regulated Class C Noxious Weed, with the exception of commercially grown holly. It is a Class C due to how widespread it is in Washington state, and thus not required for control. Common holly is a small tree or tree-like shrub that acts more like an understory plant in Washington. It does not grow as tall as our trees, but it grows tall enough to shade out native understory vegetation. It also stops or slows down the growth of surrounding vegetation, including trees, although the way it does that is still being studied. Common holly is also highly flammable and may increase the risk of a forest fire. 

Roundleaf Bittersweet (Celastrus orbiculatus) – Class A

All Class A Noxious Weeds are priority species that are legally required to be controlled in Washington state due to their limited distribution (we still have a chance to prevent it from spreading). If you think you’ve found roundleaf bittersweet on a property you manage in Washington state, please report it to your local county weed board so they can control it before it can spread.

Roundleaf Bittersweet (Celastrus orbiculatus) – Regulated Class A Noxious Weed due to its limited populations in Washington state. Only 4 infestations of this species have been found in the state, one here in King County and the three others in Clark, Okanogan, and Snohomish County. Roundleaf bittersweet is a woody vining species that can grow 100 ft+ up and around vegetation. Its woody vine twines around other species to get higher into the tree canopy, adding loads of weight and sometimes constricting around the species its growing over. This species is documented as invasive or naturalized in the Eastern United States, New Zealand, and parts of Europe.

Marsh Thistle (Cirsium palustre) – Class A

All Class A Noxious Weeds are priority species that are legally required to be controlled in Washington state due to their limited distribution (we still have a chance to prevent it from spreading). If you think you’ve found marsh thistle on a property you manage in Washington state, please report it to your local county weed board so they can control it before it can spread.

Marsh Thistle (Cirsium palustre) – Regulated Class A Noxious Weed due to its limited populations in Washington state. Only 1 infestation of this species has been found in the state in Spokane County. There are no known occurrences in King County. Marsh thistle is a herbaceous (non-woody) species that has been documented to hybridize with a more common noxious weed, creeping thistle (Cirsium arvense). Its most concerning impact is its ability to thrive in moist niches (i.e. wetlands, marshes, swamps) where it can outcompete similarly growing native species. Like most invasive thistles, this plant has potential to impact forage for livestock if allowed to spread in agricultural areas. It is documented as invasive or naturalized in the Northeastern and Midwestern United States, British Columbia, Ontario and Nova Scotia in Canada and portions of New Zealand.

Looking ahead to 2026

The state weed board has already opened the application period to make changes to the 2026 weed list. If you are interested in learning more about or participating in this process, please read our post from last fall: Noxious Weeds & Prohibited Plants: A Listing Guide (kingcountyweeds.com) 

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