Early Weed Detection Protects Wetlands

Over half of Washington’s Class A weeds are not currently known to occur in King County. Our team of noxious weed specialists keep an eye out for these species to prevent them from spreading and becoming as impactful as ivy or blackberry.

This preventative measure of being familiar with specific plants and prepared to take action upon finding them is known as Early Detection and Rapid Response, or EDRR. In other words, the sooner a plant is identified, and we are able to take appropriate action, the less the plant is able to spread and cause damage to the local ecology and economy. This method applies to more common weeds showing up in any space that they have not been seen before but is especially impactful when it is a rarer weedy species that has not yet established locally.

Here in King County, WA we are dealing with just that. Floating primrose-willow, Ludwigia peploides, is an aquatic Class A noxious weed that until fall of 2022 had only been found at one site in the entire state of Washington.

stems, green leaves and yellow flowers above water below.
The beautiful floating primrose-willow is native to Oceania and parts of South and North America. Unfortunately, in Washington, it has the potential to dramatically alter our streams and wetland and upend our efforts to restore Salmon habitat. Photo credit: Katja Schulz 

In fall of 2022, the Washington Department of Transportation (WSDOT) staff identified floating primrose-willow at a mitigation site along Evans Creek in Redmond and pulled the few plants they found. In January 2023 WSDOT notified the King County Noxious Weed Control Program (KCNWCP), and thus begun the EDRR process for the second ever known floating primrose-willow site in the state of Washington.

First, we mapped and identified the extent of the current spread. Then control work began with hand pulling smaller plants. Our team, led by Aquatic Noxious Weed Specialist Ben Peterson, is set to return monthly until October 2023 to manually remove plants and control larger patches with herbicide if necessary. This process will repeat in 2024 from May through October when the plants are growing and visible. Continued collaboration with WSDOT and monitoring for many years will ensure that this patch of floating primrose-willow does not spread throughout the watershed.

Floating primrose-willow // impact

Floating primrose-willow, Latin name Ludwigia peploides, is in the evening primrose/willowherb family, Onagraceae. Floating primrose-willow is native to native to South America, Central America, West Indies, Cuba, and in North America, Mexico, and portions of the United States, particularly in (but neither limited to nor inclusive of all) the southern regions.

Preventing its spread is crucial for many reasons:

  • It is an incredibly fast-growing plant.
    • In France, a 400 square feet patch grew to cover 320 acres in 5 years.
  • Plants form dense root mats that can impede boat navigation, clog waterways and irrigation canals, increase sedimentation, and reduce water flow.  
  • Dense infests change water chemistry by reducing pH (acidification) and dissolved oxygen. Both of these changes can reduce fish hatching success and harm aquatic fish and insects, lowering their survival rates.  
  • Reduction of food for wildlife: unpalatable to livestock and is typically not eaten by other animals.
  • Reduces wildlife habitat: it thrives in disturbed areas and easily outcompetes native vegetation used by waterfowl and other wildlife
  • May indirectly cause an increase in local mosquito populations by making mosquito larvae inaccessible to mosquito eating fish and waterfowl.

PNW presence

Our knowledge of how flowering primrose-willow has impacted similar regions/ecosystems informs our decisions in making this weed an even higher priority.

In Oregon, this plant is a classifed noxious weed but is still an EDRR priority because it is regionally abundant but has limited distribution in some counties. Read more about the impacts in Oregon here: ‘The game changer’: Invasive ludwigia | oregonmetro.gov 

Taken from article (linked above). “Ludwigia has completely infested a side channel at Black Dog Landing near Albany. Photo by Travis Williams”.

There first and only other known occurrence of floating primrose-willow in Washington is in a ponded area along the Taylor Creek tributary to the Cedar River in King County. That site was first identified in 2004, and after about two decades of removal and monitoring our team has not found any floating primrose-willow on the Taylor Creek site since 2021. Our team is hoping to have the same success at Evans Creek.  

behind leaning trees a small pond is covered with green plants. 
Floating primrose-willow at the first King County site along Taylor Creek, a tributary of the Cedar River.

The Evans Creek site in Redmond is part of a broader effort to preserve wetland habitat and mitigate the impacts of construction along nearby State Road 520. WSDOT crews have been working to restore this wetland for the last decade and have recently been joined by a resident beaver who has constructed multiple dams along Evans Creek and created a large pond. Controlling noxious weeds alongside a beaver means anticipating where the beaver might be gathering and moving materials and checking those spots to ensure that the floating primrose-willow doesn’t follow along. Disruption to the ecology of Evans Creek adds complexity (which is great) but may give weeds an opportunity to spread, so our team checked each beaver dam for signs of new floating primrose-willow growth.  

a person stands, looking at the difference in water levels on either side of a mound of vegetation. Large grasses in background of the image.  
Aquatic Noxious Weed Specialist Ben Peterson inspects a beaver dam looking for evidence of Floating primrose-willow spread.

Identification & reporting

We encourage everyone to keep an eye out for Floating primrose-willow if you’re out enjoying the streams, lakes and rivers this summer. Please report any sightings to our office by emailing noxoius.weeds@kingcounty.gov or calling 206-477-9333.

Check out its species webpage here or see the identification tips in the photo captions below.