Roesiger Invasive Aquatic Plant Control

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


Milfoil Plant Close-Up

Eurasian Watermilfoil 

5 year goal: Eradicate

Eurasian watermilfoil is currently 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: 

Roesiger Draft Invasive Aquatic Plant Control Plan #1

Fragrant Waterlily Control

5 year goal: North and South Basins - Eradicate 

Middle Basin - 40 to 50% Reduction

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


2023 Actions

  • The County has obtained all necessary permits and contracted with a licensed aquatic herbicide applicator. 
  • Invasive lilies will be treated with imazamox (ClearCast) with using the adjuvant, AgriDex, both of which were approved in the plan. A notification letter was sent to all Lake Roesiger residents.
  • The treatment area will include all patches of invasive lilies in the in the north and south basin and approximately 25 acres in the middle basin as shown in the treatment map.  
  • The first treatment occured the week of Aug 10.
  • The second treatment will occur the week of September 11. The exact dates within that time frame depend on wind and weather conditions.
  • Shoreline properties within 400 feet of the treatment will also be posted by the contractor no more than 48 hours before the treatment.
  • Landowners may choose to do additional control such as hand-pulling cutting etc. per WDFW's guidelines (note additional permits may be required for control methods beyond hand removal). 
Boat draining on boat ramp to remove invasive species

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 invasive plant and animals to be introduced to the lake. Users leaving the lake can also spread Roesiger invasive species to other lakes. Outreach to boat users on how to Clean, Drain and Dry their boat is the best method to prevent the spread of invasive species. aquatic plants and animals into the lake


2023 Actions

  • Social media posts and other outreach to the general public and Roesiger residents on how to prevent introductions (Clean, Drain and Dry your boats) 
  • Coordination with advisory group and community on potential to have volunteers provide prevention education at boat launch on busy weekends.
  • Continued research into feasibility of boat cleaning station

Purple Loosestrife Patch and Flower

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 has reached out to all known properties with knotweed offering free treatment which will occur in August or September. 
  • Yellow flag iris & purple loosestrife - Landowners can take individual actions to reduce yellow flag iris and purple loosestrife. 
  • Additional educational workshops and outreach support on best control methods are planned for 2024.
Grass-leaved Arrowhead Patch and Flower

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

  • The County is in the process of contracting for Diver Assisted Suction Harvesting (DASH) to remove slender arrowhead. 
  • The contractor will conduct 8 days of DASH removal of slender arrowhead. 
  • DASH removal is scheduled for August 2 - 4 and August 7 - 11
  • The focus will be on removing plants in high traffic areas that are also shallow including the boat launch and near the entrance to and from the middle lake. The goal is to reduce the possibility of boats catching plant fragments which can spread to other lakes.  
Milfoil Removal Boat

Project Success Monitoring 

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


2023 Actions

  • The County will conduct surveys in June of 2023 (pre-treatment) and 2024 (post-treatment) of areas identified for lily or arrowhead control plus some control areas. The goal is to determine the efficacy the management strategies as well as to determine if native vegetation is re-established following successful control.  
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

  • Establish and Coordinate Advisory Committee 
    • Recruitment completed via via mail and email
    • Interviews conducted to select members
    • Two of three annual committee meetings held to guide this and future year's work. 
  • Community outreach on project including: 
    • Project webpage created and regularly updated
    • Project kickoff mailer sent 
    • Permit-required notifications for herbicide treatments sent
    • Updates sent via email and NextDoor
      • Notification of upcoming plant control activities
      • Increase awareness of "Clean, Drain, Dry Boats" to prevent spread of invasive species 
      • Post-season summary of actions