What does it mean to be a noxious weed? 

First things first, what is a noxious weed? And why do we care? “Noxious weed” is a legal designation. To become a noxious weed a plant must meet 4 criteria. 

1. Introduced to Washington state 
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. Chances are several of those are relevant to your life! 

Anyone can nominate a species, but only the state Noxious Weed Control Board decides which plants are noxious. They also decide where they should be controlled based on how widespread they are. County Noxious Weed Boards can select additional species to educate about on a more local level, but do not have the power to select weeds for designated control without state approval (more on this below). 

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). 

2024 List Updates 

Overview 

Select the bullet point link below to jump to the respective update.

• Required additions from state list: Palmer Amaranth & Variable leaf watermilfoil hybrids 
• County-selected addition: beachgrasses (marram) & their hybrid 
• County-selected subtraction: buffalobur de-selected for regulation 
• County requested designation change: egeria selection (Green Lake now non-regulated) and poison hemlock selection (now regulated along railroads rights-of-way)
• Common name changes for 9 different listed species on the County list

Preview and download the updated 2024 King County Noxious Weed List below:

2024 King County Noxious Weed List by Common Name

2024 King County Noxious Weed List by Scientific Name

Additions FROM the State Weed List 

Palmer Amaranth (Amaranthus palmeri) is a new Class A species. It is an annual weed, native to the Sonoran Desert. It is often found in disturbed sites and is of concern as a noxious weed primarily due to its agricultural impacts. Palmer Amaranth can have allelopathic effects on crops. Studies of Palmer Amaranth have shown it to reduce agricultural yields. Current climate models indicate that most of Washington is suitable for its establishment, including King County.

Thus far, Palmer Amaranth has only been found in two locations in Washington. Once in Spokane County and once in Walla Walla county. There are no known occurrences in King County. Because it is so rare, we still have a chance to stop it from spreading. This is why it is listed as a high priority plant and required to be controlled statewide.

More information on the state noxious weed list update can be seen on the Washington State Noxious Weed Control Board’s website. 

Variable leaf milfoil hybrids (Myriophyllum heterophyllum hybrids) is a new Class A species, very recently identified in 2011 and was not found in Washington until 2022. The hybrids of Myriophyllum heterophyllum are very difficult to distinguish without genetic testing.

The reason for adding variable leaf milfoil hybrids to the state list are the aquatic impacts of these species. Similar to spiked watermilfoil Myriophyllum spicatum, the variable leaf milfoil hybrids can form dense mats that shade out other aquatic plants, degrades water quality, inhibits water flow, and impacts recreational activities. There are currently no known sites of variable milfoil in King County. 

County-Selected Addition

Beachgrasses (marram) and their hybrid (Ammophila arenaria, A. breviligulata, and A. arenaria x breviligulata) is a new Class C weed. It is a nearshore species that has spread along the coastlines of California, Oregon and Washington. While it is widespread along the coast, it is relatively less common in the Puget Sound region with only two known sites in King County (one at Discovery Park and one at Richmond Beach). 

Beachgrasses may be inadvertently introduced to restoration sites, potentially outcompeting native dune grass (Leymus mollis). While these two species of nearshore species may seem similar the introduced beachgrasses do not support the same level of biodiversity in our region and have been shown to sequester less carbon than the native dune grass.

Selection changes 

While King County Noxious Weed Control Board does not have authority to list a species as a noxious weed, they may select additional Class C and Class B Non-Regulated species to require for control within the county.

Buffalobur (Solanum rostratum) is no longer required for control in King County (change from Class C regulated to Class C non-regulated). Buffalobur remains a Class C weed at both the state and county level. This decision was passed by the County Weed Board.

The story of buffalobur is one of successful monitoring and of responsiveness of the law to changing awareness and conditions. Buffalobur was introduced as a noxious weed out of concern for its impact on agriculture and requested to be selected for control in King County. Other solanum species have had negative impacts in this region. King County Noxious Weed Control Program tracked 17 sites for 25 years. Over these years the program observed that infestation areas did not exceed 20 square feet and that control efforts were often successful after one year of treatment. Of the 17 total sites the Program has tracked throughout its history, there are only currently 2 active sites, neither of which has had any buffalobur since 2017.  

Designation changes 

Egeria (Egeria densa) no longer selected for control in Seattle’s Green Lake. 

Since our 2021 Green Lake egeria blog post we’ve learned that there is much more egeria in the lake than anticipated. Weeds are required for control often to prevent spread, and requiring total control of egeria in this lake would require immense time, resources, and potentially have a bigger impact than the species itself. Therefore, our program petitioned the state to de-select Green Lake for control.

However, the de-selection does not mean that Green Lake will be left to egeria! The City of Seattle is actively working with partners (including the King County Noxious Weed Control Program) to address the impacts of noxious weeds in the lake and develop an integrated pest management plan. The coming year will bring an engaging planning process to determine options for how to care for this valuable community resource.

Poison hemlock (Conium maculatum) now required for control on railroad rights-of-way. 

Poison hemlock was listed as a Class B noxious weed in 1988, but was not required for control on public lands and public rights-of-way until 2019. This regulation requirement was put into place to protect public health. This year, railroad rights-of-way were added for required control. Railroad rights-of-way are often public facing, and this control effort will support the protection of the public from this toxic plant. 

Common name changes 

We changed the common name of nine plants this year. These changes are part of an ongoing effort to remove location based and nationalist names for plants in favor of descriptive ones. How we refer to plants is important and can reinforce nationalistic and xenophobic rhetoric. Using descriptive language can help people learn and identify plants. Our team began this process in 2021 and with the approval of the King County Noxious Weed Control Board have now selected 25 common names we believe better describe the species we wish to talk about.  

To select new common names our team looked for common names that are already currently in use by peer organizations, names that help identify or describe the species, or names that connect to the scientific name. Perhaps you will already be familiar with some of these names!

Check out our 2022 weed list update blog post for more details on the 9 common name changes that were made for the 2022 list and the 2021 weed list update blog post for the 6 common name changes made on that list. 

For more information about any of the mentioned weeds, find their detail pages on our King County Noxious Weeds website or on the Washington State Noxious Weed Board website. 

 Looking ahead to 2025 

The state weed board has already opened the application period to make changes to the 2025 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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