2021 King County Noxious Weed List is official

The 2021 King County Noxious Weed List, adopted by the Noxious Weed Board at its January meeting, gained two new species and a number of plant name changes. The first addition to the list is a new Class A noxious weed Turkish thistle (Carduus cinereus), an introduced invasive species discovered recently in Oregon and Idaho and not yet known to occur in Washington. Turkish thistle is most likely to be found in rangeland, meadows, pasture, and fields where it competes with native plants and reduces available forage for livestock. Class A noxious weeds have the highest level of priority under the state noxious weed law because of their potential impacts and limited distribution in the state. Early detection and rapid response is the goal for Class A weeds, so please tell your county weed board if you see Turkish thistle in Washington.

Turkish thistle flowers
Turkish thistle (Carduus cinereus) was added to the Washington State Noxious Weed List in 2021. Photo by Joel Price, ODA.

The second addition is a new King County Weed of Concern, horse-chestnut (Aesculus hippocastanum), an ornamental tree that has spread beyond gardens and parks into forests where it competes with native plants for space and water. Horse-chestnut fruits are toxic to people but they are attractive to squirrels and other animals who carry them into natural areas. Weeds of Concern aren’t noxious weeds according to the state noxious weed law. They are plants that King County educates residents about due to the plants’ local impacts. People are not required to control Weeds of Concern but they are encouraged to do so where it would be beneficial to ecosystems or agriculture. For more information on the weed list categories, see Weed Lists and Laws.

horse-chestnut tree
Horse-chestnut (Aesculus hippocastanum) is an introduced tree species found in King County that has spread outside of cultivation.
horse chestnut flowers and leaves
Horse-chestnut is an ornamental tree that was introduced to Washington because of its attractive foliage and flowers. It was added to the King County Weeds of Concern list due to its spread into natural areas and its impacts in forest habitats.

The other changes to the King County weed list are plant name updates. Because language matters as a way we communicate our values, the King County Weed Board is searching for more inclusive ways to talk about noxious weeds. As an effort in that direction, the Board adopted several name changes that avoid labeling plants as belonging to other countries. The purpose of this effort is to emphasize that the negative impacts of the plants in their new location are the problem, not what country they come from. In fact, many other plants native to the same countries are well-behaved food plants or ornamentals perfectly suited to landscapes and gardens. This process will be done a few plants at a time to reduce confusion with the name changes, and attempts will be made to choose non-national names already in common usage where available.

King County will begin calling this plant itadori knotweed instead of Japanese knotweed to emphasize that the impacts of invasive plants are the problem, not the countries they come from. Photo by Michael Gasperl, License CC by SA 3.0.

For 2021, the Board adopted the following common name changes:

Bohemian Knotweed to Hybrid Knotweed (or simply knotweed)
Hybrid knotweed is a hybrid between two other species of knotweed that was first identified in Bohemia, Germany, which is where it got the name Bohemian knotweed (Fallopia X bohemica). It is the most common species of knotweed in Washington.

Japanese Knotweed to Itadori Knotweed (or simply knotweed)
Although this knotweed, Fallopia japonica, isn’t as common in Washington as the hybrid, the name Japanese knotweed is the most familiar to most people. There isn’t another commonly used English name besides Japanese knotweed, so it was decided to use the Japanese name for the plant, itadori, or itadori knotweed.

Brazilian Elodea to Egeria
Brazilian elodea is a confusing name because the plant closely resembles a different water plant native to Washington called Elodea (Elodea canadensis). But Brazilian elodea isn’t an Elodea, but rather an Egeria (Egeria densa).

Canada Thistle to Creeping Thistle
Creeping thistle is the common name of this very common thistle species (Cirsium arvense) in much of the English-speaking world. Also, in spite of the name, it isn’t native to Canada, but rather parts of Europe and Asia. Creeping thistle also accurately describes how this thistle spreads by creeping underground rhizomes.

English Laurel to Cherry Laurel
Cherry laurel is a popular common name for this plant and reflects that it is in the cherry genus Prunus (although cherry laurel is not really a laurel so that is still confusing!). Also cherry laurel is not native to England, but instead to regions bordering the Black Sea in southeastern Europe and northeastern Asia. It is in fact invasive in the British Isles as well as here.

English Hawthorn to Common Hawthorn
Common hawthorn is often used for this species of hawthorn and it is less confusing than English hawthorn which is also used for other hawthorns.

The 2021 King County Weed List also incorporates updates to Latin names made by the Washington State Weed Board to reflect current accepted taxonomy. One of the big updates for plants of interest in King County is the switch from Polygonum to Fallopia for the genus of the three main invasive knotweed species.

As of 2021, the Washington State Noxious Weed Board will be using the Latin name Fallopia x bohemica instead of Polygonum x bohemicum for Bohemian knotweed, also called hybrid knotweed.

Here are all of the scientific name changes for 2021:

  • Anchusa arvensis, annual bugloss to Lycopsis arvensis
  • Centaurea x moncktonii, meadow knapweed to Centaurea × gerstlaueri
  • Polygonum cuspidatum, Japanese (itadori) knotweed to Fallopia japonica
  • Polygonum sachalinense, giant knotweed to Fallopia sachalinensis
  • Polygonum x bohemicum, Bohemian (hybrid) knotweed to Fallopia × bohemica
  • Matricaria perforata, scentless mayweed to Tripleurospermum inodorum
  • Saccharum ravennae, Ravenna grass to Tripidium ravennae
  • Silene latifolia ssp. alba, white cockle to Silene latifolia
  • Zostera japonica, Japanese eelgrass to Nanozostera japonica

Each year, the King County Noxious Weed Board considers changes to the King County weed list as allowed under the State Noxious Weed Law, RCW 17.10. The Board meets regularly throughout the year and welcomes public input on the weed list and the operations of the Noxious Weed Control Program. The Washington State Noxious Weed Board also meets regularly and considers proposals for weed list changes and additions each year. To learn more about participating in the weed listing process, visit the Washington State Weed Board’s web page or email the King County Noxious Weed Program.

Learn more:

King County Noxious Weed Lists and Laws

2021 King County Noxious Weed List

King County Noxious Weed Control Board

Washington State Noxious Weed Board What’s New


  1. Do there also have to be changes in the entire lexicon in restoration ecology with respect to “native” and “non-native” plants? Am I inadvertently sending the wrong message to my grandchildren when I laud the benefits of planting native plants in their backyard forest, especially if I use the word “invasive plant” in the context of “non-native?”

    • That is an excellent question and the very one we are grappling with. I agree with using invasive plant instead of non-native to distinguish between plants that have negative impacts and plants that just happen to be from somewhere else. They way I might put it is that there are many benefits to protecting and restoring native plants and ecosystems everywhere in the world including here to maintain our global biodiversity as best we can. While not all non-native plants create problems, there are some introduced plants that overwhelm the systems they are introduced to, or they escape into, making it difficult for the ecosystems to thrive. It’s that impact that we should look for, and those plants would be considered invasive plants in those habitats. When I teach about invasive plants and noxious weeds, I like to stress that they are plants first and so they do have benefits that all plants have, but when they create harm to an ecosystem they’ve been introduced to, we need to act to reverse that, and help build back the native plant community so it can better withstand those impacts. It’s not always black and white, but we can try to keep our focus on helping to reverse impacts brought about by the introduction of plants that become invasive, instead of focusing on the origin of the plants.

  2. I find this name change issue a bit ludicrous to put it mildly. Can’t use a name of country as has been used “forever” because it is not PC. But we add a new Class A weed – Turkish Thistle. Anyone see any irony in this? Come-on, let’s not lose sight of the problem. The problem is the weed not the name. This is PC gone too far. On top of the basic loss of the real objective this will accomplish nothing other than add confusion as to what is being discussed because most folks will have no idea what weed is being talked about. You guys are better than this – or so I thought.

    • Thank you for sharing your perspective on this. It is true that using different names will add a challenge to teaching people about these plants. This is why we are only trying this with a small group of plants at first. We will continue to use both names where needed so people aren’t confused about what plant we are talking about, but hopefully with time we can teach people the new names. And yes we see the irony of the new Class A weed being called Turkish thistle, but that is a choice made at the state level. At the county level, we are weighing the benefits of using more inclusive language to communicate better with audiences from diverse backgrounds and nationalities with the disadvantages of the confusion that might come from using different plant names. We are starting with just a few plants that are common in King County and for the most part have commonly used, or easily understood, alternative names that are in some cases even more accurate. This is a work in progress and an experiment, and if it doesn’t achieve our goal of focusing on the impacts of the weeds instead of their country of origin, then we will need to reconsider. I appreciate hearing your opinion, and it is useful to hear about the disadvantages of this effort. Thank you for voicing your concern.

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