Roesiger Invasive Aquatic Plant Control

The table below shows each of the plan elements and the actions being taken in 2023. All work is managed by Snohomish County under the guidance of the Lake Roesiger Invasive Plant Advisory Committee. To stay up-to-date on the project, sign up to receive email updates.


Milfoil Plant Close-Up

Priority 1 - Eurasian Watermilfoil 

5 year goal: Eradicate

Eurasian watermilfoil is currently found in scattered locations in each of the three basins. The low densities are thanks to over two decades of diver surveys and hand-pulling. While these efforts have helped, they have not been consistent enough to eradicate this plant. 


2023 Actions: 

  • The County contracted Lake Defense Force, a professional diving company.
  • Divers conducted 5 days of diver surveying and hand-pulling.
  • Only 14 plants (see maps) were found and removed in the entire lake!

Next Steps: 

Roesiger Draft Invasive Aquatic Plant Control Plan #1

Priority 2 - Fragrant Waterlily 

5 year goal: 

North and South Basins - Eradicate 

Middle Basin - 40 to 50% Reduction


Fragrant waterlily covers around 0.7 acres in the north basin, 0.6 in the south basin and 28.9 in the middle basin. 


2023 Actions

  • The County acquired necessary permits and contracted with AquaTechnex, a licensed aquatic herbicide applicator.
  • Treatment Notification letters were sent to all Lake Roesiger residents and shorelines were posted per the permit.
  • Three herbicide treatments were conducted on a total of 26.9 acres of lilies with some areas being treated twice (see contractor treatment maps).
  • Three treatments were required instead of two, as lake levels were low and the contractor had ongoing mechanical issues with their airboat.
  • Landowners with dense lilies should expect some areas to have small lines of lilies in boat lanes that were not effectively treated. This is caused by the airboat turning and will become less prevalent once boating lanes are established for treatment in dense patches.

Next steps:

  • Conduct survey in spring 2024 to assess efficacy.
  • Work with contractor and advisory committee to improve treatment in 2024 including:
    • Begin treatment earlier in the season, July instead of August (lilies in middle basin brown earlier than is typical for this region).
    • Ensure contractor's airboats are in good working order.
    • Improve communication to landowners that it can take 3-5 weeks for plants to die back following treatments.
Boat draining on boat ramp to remove invasive species

Priority 3 - Invasive Species Prevention

5 year goal: Prevent new introductions of invasive aquatic species


Lake Roesiger is a popular destination for boaters putting it at high risk for new introductions of invasive plants and animals. Visitors can also carry invasive plants from Roesiger to other area lakes. 


Everyone can help if they Clean, Drain and Dry their boat to prevent the spread of invasive species. Some work began in 2023, but outreach will be a major focus for 2024.


2023 Actions

  • Sent Clean, Drain, Dry posts on social media to Snohomish County residents. 
  • Included Clean, Drain, Dry messaging on project update emails. 

Next Steps

  • Coordinate with advisory group and community on 2024 outreach plan which may include:
    • Social media campaign
    • Boat launch education program
    • Research into feasibility of boat cleaning station

Purple Loosestrife Patch and Flower

Priority 4 - Shoreline Invasive Species 

5 year goal: Prevent further spread and reduce coverage

Invasive shoreline plants at Lake Roesiger include invasive knotweed, purple loosestrife and yellow flag iris. 


2023 Actions

  • Knotweed control - the Snohomish County Noxious Weed Control Board treated 3 properties this year; 8 properties have eradicated or nearly eradicated knotweed.
  • Yellow flag iris & purple loosestrife - Landowners can take individual actions to reduce yellow flag iris and purple loosestrife
  • The Roesiger Community and Boat Club held a workshop that included how to manage shoreline invasive species.

Next Steps

  • Continue knotweed eradication efforts.
  • Work with advisory committee to develop additional educational workshops, videos or other outreach support - this is a focus of 2024. 
Grass-leaved Arrowhead Patch and Flower

Priority 5 - Slender Arrowhead 

5 year goal: Prevent further spread and reduce coverage (initial target of 20% per year that would be scaled based on funding)


2023 Actions

  • Contracted with Lake Defense Force for Diver Assisted Suction Harvesting (DASH) services. 
  • Conducted 6 days of DASH removal near boat launch.
  • In total, 107 bags of plants were removed across 0.5 acres (see map).
Next Steps
  • Assess efficacy in spring 2024.
  • Work with advisory committee to decide next steps including exploring more cost effective control options. 
Milfoil Removal Boat

Project Success Monitoring 

5 year goal: Track the effectiveness of the project in meeting the project goals


2023 Actions

  • Conducted pre-treatment survey in June of 2023. 

Next Steps

  • Conduct a post-treatment survey to assess efficacy of lily and slender arrowhead control efforts.  
Roesiger Draft Invasive Aquatic Plant Control Plan #2

Project Communications

5 year goal: Ensure community members are kept up-to-date on project actions and outcomes


2023 Actions

  • Recruited, established and held three meetings with new Advisory Committee.
  • Created and updated project webpage throughout season.
  • Sent 2 postcards via mail. 
  • Sent 10 project updates via email including a post-season summary of actions.

Next Steps

  • Continue regular project communications to the lake community via advisory group meetings, mail, email and NextDoor.