Milfoil & Invasive Species

Cascade operates Lake Tapps for eventual municipal water supply for its members, but works diligently to keep it clean and healthy today. That includes addressing milfoil and other invasive species. Milfoil is a non-regulated weed in Pierce County and Cascade has no regulatory obligations to control it in Lake Tapps.  However, Cascade will continue to manage milfoil. Cascade does not remove native, aquatic plant species.

What is milfoil?

Eurasian milfoil (Myriophyllum spicatum) is a non-native, invasive aquatic plant. Milfoil is native to Eurasia and Africa, but is now found on all continents but Australia and Antarctica. The leaves of milfoil are submerged, and grow in whorls of four, with many thread-like leaflets. This plant can grow from broken-off stems and can spread quickly, creating dense mats that may crowd out native plants and inhibit recreational activity in the water.

Click here for Milfoil FAQs.

What is Cascade doing about milfoil?

Lake Tapps reservoir is assessed annually to determine areas in greatest need of treatment. Since 2010, Cascade has managed milfoil growth in the Lake Tapps Reservoir, and has spent over $2 million to date on milfoil treatments.  Click here for Milfoil FAQs.

Chemical Treatment:  Since 2010, Cascade has primarily managed milfoil growth with chemical treatments. Cascade has used the herbicide ProcellaCOR® for treatment since 2019.

2024 Milfoil Treatment Plan:  Areas designated for herbicide ProcellaCOR® treatment in 2024 are designated on the map below. Treatment is anticipated to occur the week of June 24-28.  Residents in planned treatment areas will be notified regarding exact dates of treatment prior to actual application.

2024 Milfoil Treatment Map Phase 1


TappsWise Program:  Since nutrients from lawns and septic tanks increase aquatic vegetation, Cascade has partnered with the Tacoma-Pierce County Department of Health with the TappsWise program,  to promote natural yard care and septic maintenance to keep Lake Tapps clean and healthy.

Research:  There are multiple strains of milfoil, some resistant to herbicides. Cascade hired a research team to sample and identify the various genotypes in the reservoir and eleven strains were found. Cascade also supported Washington State’s approval of use of a new herbicide, ProcellaCOR®. Additionally, Cascade conducted two pilot studies in 2018 and 2019 on controlling milfoil with bottom barriers and dry-land herbicide treatments; neither pilot project was cost-effective in controlling milfoil.

2024 Milfoil Treatment Pilot Project For Homeowners And HOAs

Every year, Cascade chemically treats the most heavily concentrated areas in the Lake Tapps Reservoir to control the spread of milfoil.

As the owner of the reservoir and the lake bed up to 545 feet elevation, Cascade is the only entity that is permitted by the Washington Department of Ecology to apply herbicides in the water. Cascade’s applicator (Aquatechnex) applies an herbicide called ProcellaCOR® twice each summer. ProcellaCOR® has a better environmental profile than many other herbicides.

Although Cascade is able to treat much of the milfoil in the reservoir, it doesn’t have the funds to treat all impacted areas, such as small patches that affect a limited number of homeowners. To address these needs, and per the request of some Lake Tapps homeowners, Cascade conduct a pilot project in 2023 and is continuing the program in 2024.

The 2024 pilot project allows homeowner associations (HOAs) and individual homeowners, at their own expense, to use Cascade’s permit and contract directly with Aquatechnex to chemically treat milfoil in front of their residences. Treatment will be limited to milfoil and will not include other non-native or native plants. Aquatechnex will likely perform two rounds of treatment – in June and again in August.

If you would like more information about the pilot program, contact Paula Anderson, Cascade’s Land Use Administrator, at or (425) 283-4294.

Other Homeowner Options to Manage Milfoil

If Lake Tapps waterfront property owners plan to remove aquatic plants from in front of their property, they must follow rules outlined in Washington Department of Fish and Wildlife’s Aquatic Plants and Fish: Rules for Aquatic Plant Removal and Control. A summary is below; click here for more detailed information.

Hand Removal of Aquatic Plants:  Hand removal of aquatic plants can help manage growth and can be effective for small, confined areas. Remove the entire plant when possible – milfoil can re-grow from pieces floating in the water – and dispose it at an upland site where it cannot re-enter the water.

Bottom Barriers:  Bottom barriers are synthetic or natural fiber material used to cover and kill plants growing on the lake bottom. The material is anchored with sandbags or rocks. Bottom barriers are best used in small, confined areas where control of all plants is needed.  For more information on bottom barriers click here and also see the Rules for Aquatic Plant Removal and Control.

Reduction or Elimination of Excess Nutrients: Nutrients from lawns and septic tanks can significantly increase vegetation in the reservoir, so reducing fertilizer applications and keeping septic tanks maintained and working properly will reduce aquatic plant growth. Residents are encouraged to participate in the TappsWise program to help keep Lake Tapps clean and healthy.