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.  Click here for Milfoil FAQs.

2023 Milfoil Treatment Plan:

Based on milfoil surveys and information received from residents throughout the year, Cascade treated for milfoil in two phases in 2023. Phase one was completed the week of June 26, 2023, and phase two was completed the week of August 7, 2023. Residents in planned treatment areas were notified regarding time and dates of treatment prior to actual application.

This year, Cascade used the State Department of Ecology approved aquatic herbicide ProcellaCOR®. Cascade used this herbicide to treat the reservoir for the past four years with good results.  It is environmentally friendlier than herbicides used in the past, and results have been significantly better. This treatment may affect water that is used for irrigation, but, as a reminder, pumping or drawing Lake Tapps Reservoir water for irrigation is never permitted.

Milfoil is found in most Northwest lakes. It has been in Lake Tapps on and off for years. Cascade has no legal obligation to address milfoil, however, Cascade is as interested as residents and recreation enthusiasts in keeping the reservoir clean and safe. This is why over the past decade, Cascade has treated and addressed milfoil management on a regular basis at its own cost.  Cascade does not manage native vegetation.

Cascade’s Treatment Sites for Eurasian Milfoil:

Phase 1 – Week of June 26, 2023:

Phase 2 – Week of August 7, 2023:

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

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

Pilot Studies:  Cascade conducted two pilot studies in 2018 and 2019 on controlling milfoil with bottom barriers and dry-land herbicide treatments. Neither pilot project was successful in controlling milfoil.

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®.

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.