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 and areas to set priorities beyond what the law requires but must request a change in a weed’s designation from the state board to reduce control requirements.

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). 

2026 Weed List Updates

This year’s King County Noxious Weed List additions include 4 species that were all added from the Washington State Noxious Weed List. Click on a species name below to learn more about the weed.

1. delta arrowhead (Sagittaria pltyphylla) – Class A (regulated)
2. waterhemp (Amaranthus tuberculatus) – Class A (regulated)
3. black locust (Robinia pseudoacacia) – Class C (non-regulated)
4. Norway maple (Acer platanoides) – Class C (non-regulated)
5. Poison hemlock (Conium maculatum) – Class B (selectively regulated)

Find the weed list on our website here.

Black Locust and Norway Maple – 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 black locust and Norway maple on your property, especially if you live near any natural areas or forests.

Black locust (Robinia pseudoacacia) – Non-regulated Class C Noxious Weed. It is a Class C due to how widespread it is in Washington state, and thus not required for control. Black locust can form dense islands of vegetation, shading out other native species. It can impact a variety of habitats from prairie, meadow, forest edges, and riparian zones. It can spread through seed and extensive clonal suckering.

Norway maple (Acer platanoides) – Non-regulated Class C Noxious Weed. It is also a Class C due to how widespread it is in Washington state, and thus not required for control. Norway maple was planted for the shade it provides, hardiness, and adaptability. Unfortunately, these traits combined with its ability to spread beyond where it is planted causes issues in our parks and forests where it displaces native plants and causes dense shade that prevents native seedlings from growing and reduces ecosystem resiliency. It is also a host for Sooty Bark Disease that can transfer to our native maple trees.

Delta Arrowhead (Sagittaria platyphylla) – 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 delta arrowhead 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.

Delta arrowhead – Regulated Class A Noxious Weed due to its limited populations in Washington state and potential for harm. It is a highly invasive aquatic weed in Africa, Australia, Asia, and western North America. It outcompetes native vegetation, reduces wildlife habitat, and clogs waterways, hindering recreation.

Waterhemp (Amaranthus tuberculatus) – 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 waterhemp 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.

Waterhemp – Regulated Class A Noxious Weed due to its limited populations in Washington state. Waterhemp is fast growing, has high seed production, and is drought tolerant. It is highly invasive in crop fields. Due to herbicide resistance, waterhemp is very difficult to control once established. A single plant can produce over 1 million seeds under non-competitive conditions. Seeds are viable for many years and are readily dispersed by water, farm machinery, and wildlife.

Poison hemlock – an updated strategy

Regulation of poison hemlock has shifted based on our experiences from the past few years to best control the spread and prevent the greatest impacts. It is now regulated on high priority private land, public land and rights-of-way including railroad rights-of-way. The high priority sites are determined by our board-approved management plan. As part of that we now have a best management practices (BMP) document for poison hemlock control.

King County Best Management Practices for Poison Hemlock (Conium maculatum)

Looking ahead to 2027

The state weed board has already opened the application period to make changes to the 2027 weed list. If you are interested in learning more about or participating in this process, please read our post: Noxious Weeds & Prohibited Plants: A Listing Guide (kingcountyweeds.com) 

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