Controlling aquatic plants and algae to support clean water

Hydrilla. Pondweed. Loosestrife.

These might sound like the newest Gen Alpha slang, but they are actually far more obnoxious, or you might say, "noxious." They are noxious weeds. And that isn't us name calling either. "Noxious weed" is the legal term for invasive, non-native plants that are so aggressive they harm local ecosystems or disrupt agricultural production.

Ecology has a role in controlling noxious aquatic weeds by providing a water quality permitting program for appropriate pesticide use. We are proposing updates to the Aquatic Plant and Algae Management General permit as part of our regular 5-year reissuance process.

Managing aquatic plants and algae

For decades, we've been required by state law to provide a permitting program that allows lake managers to use pesticides and other chemicals to control noxious aquatic weeds, which includes many invasive aquatic plants. Invasive aquatic plants can harm native fish habitat and clog recreation areas. The permit applies to fresh water bodies and shorelines as well as roadside ditches. Our permitting program only allows registered pesticides that are approved by the U.S. Environmental Protection Agency and the Washington State Department of Agriculture. To make sure these pesticides meet state laws, we further review these chemicals through an environmental impact statement.

An excerpt from RCW 90.48.447 helps explain why Washington has a general permit for Aquatic Plant and Algae Management:

The legislature finds that the environmental, recreational, and aesthetic values of many of the state's lakes are threatened by the invasion of nuisance and noxious aquatic weeds. Once established, these nuisance and noxious aquatic weeds can colonize the shallow shorelines and other areas of lakes with dense surface vegetation mats that degrade water quality, pose a threat to swimmers, and restrict use of lakes. Algae can generate health and safety conditions dangerous to fish, wildlife, and humans.

The general permit

Water quality permits are a legal tool to limit the discharge of pollution into the water. Each permit is different and sets specific methods for release, monitoring, and mitigating any potential environmental damage. A water quality general permit is a type of permit that regulates similar treatments or activities under one permit. If each applicant meets the requirements of the permit they are granted authorization to discharge. The permit holder must follow the conditions and requirements in the permit. Failure to do so can result in enforcement action by Ecology.

Each general permit is updated on a five year cycle, where we look at how the permit is working; incorporate feedback from permittees, Tribes, and interested parties; review any new, relevant science; and increase clarity.

We're currently working to update the Aquatic Plant and Algae Management general permit. Specifically, for this permit we also review monitoring data, modify timing windows or propose other actions that better protect sensitive species.

What's in the permit?

The Aquatic Plant and Algae Management general permit requires people to use management solutions that have the least environmental impact among reasonably available options. Chemicals are sometimes the least invasive solution to control plants and algae.

If a local authority - like an irrigation district, municipality, or HOA for a lake community - decides as part of an integrated pest management approach that registered pesticides or chemicals are needed to control a nuisance plant or algae, then they must apply for coverage under Ecology's permit. We do not provide permittees with consultation or advice on which approaches to use, we are only involved to make sure any chemical use is done safely and in accordance with the law under the permit. It is common to use a mix of approaches to get nuisance plants or algae under control. Non-chemical approaches can include mechanical weed harvesters, bottom barriers, and education and outreach programs such as the Clean, Drain and Dry program.

Our permit ensures the chemicals used, and the methods by which they're applied, minimize risks to animals and other plants. The permit limits which chemicals can used, where they can be used, and who may use them. It also requires public notification before a waterbody is treated.

Proposed changes

Ecology is proposing a few changes to the permit, based on permit implementation and our ongoing effort to incorporate recent science and data into our work.

Our proposed updates add flexibility around when some aquatic herbicides are used. This incorporates more detailed ecotoxicity data for products containing one of the seven active ingredients under the permit that have fish restrictions. Ecology is proposing this new process so that more effective plant management can happen while also protecting fish. Currently, the standard treatment window for some lakes is after the plants have grown and reproduced, which makes applying a pesticide far less effective. Ecology is working closely with toxicologists and staff at Washington Department of Fish and Wildlife to develop this approach.

Phosphorus control treatments

Phosphorus is a nutrient that can boost the growth of aquatic plants and lead to more frequent algae blooms. Under the permit, lake managers can use treatments to help control excess phosphorus, with the goal of reducing plant growth and algae blooms. Toxic algae blooms can be harmful to wildlife, humans and pets, requiring recreation areas to close to prevent exposure. When an algae bloom end and start to decompose it lowers the oxygen levels in water, which can be dangerous for aquatic life.

Alum treatments are a common and effective way to control phosphorus. There are other types of treatments using lanthanum and iron. All of these methods work by chemically binding to the phosphorus and then settling into the sediment. These treatments, when done properly, help improve water quality and prevent algae blooms. Unfortunately, across the country and in Washington, there have been incidents of alum treatments harming lake ecosystems. We are proposing additional monitoring requirements to better understand the effects of these treatments and how to prevent problems.

Also, to provide lake managers with additional phosphorus control options, we are proposing to add another type of product that is lanthanum-based and used to remove phosphorus from both lakes and streams or rivers that flow into them.

We invite your comments

We are accepting public comment on the draft permit from 9 a.m. on Sept. 10 through 11:59 p.m. on Oct. 24, 2025. The draft permit, fact sheet and ways to submit comment are on our permit webpage. There are multiple opportunities to learn more about the permit updates, each with question and answer sessions, and opportunity to provide testimony. Meeting details:

Oct. 14 Online workshop, 6 p.m.
Register on Zoom

Oct. 16 Online workshop, 2:30 p.m.
Register on Zoom

Next steps

After we review the feedback we receive on the draft permit and related materials, we will make any necessary revisions, prepare a response to comments, and make a decision on final permit reissuance. We expect to have a decision in early 2026.