How to Get Rid of Cattails and Duckweed in a Wastewater Lagoon

Almost everything written about cattail and duckweed removal is written for private ponds. A treatment lagoon is a different animal: it discharges under a permit, its berms and liner are infrastructure, and every pound of vegetation you kill and leave in the water becomes oxygen demand your cells have to treat. This guide covers control methods that work in the wastewater context, and the permit and infrastructure traps the pond articles never mention.

Why vegetation control is different in a treatment lagoon

Four reasons the pond playbook does not transfer cleanly:

1. You discharge under a permit. Pesticide applications to waters, including treatment lagoons in many circumstances, can require coverage under your state’s Pesticide General Permit, a licensed applicator, or both, and product labels restrict use around discharging systems. The label is law, and your state rules sit on top of it. Two phone calls, to your state environmental agency and your county extension or ag department, settle your local requirements before you spray anything. The federal program your discharge permit sits under is NPDES.
2. Dead vegetation becomes load. In a private pond, decaying cattails are an aesthetic problem. In a lagoon, a berm-line of killed cattails or a treated duckweed mat decomposes into BOD and TSS inside your treatment train. Kill it, then remove it, in sections.
3. Your berms and liner are the asset. Cattail rhizomes work into embankments, and cattail stands are prime muskrat habitat. Muskrat burrows are a leading cause of berm damage and liner penetration in earthen lagoons. Vegetation control is partly infrastructure protection.
4. Sun and wind are part of your process. Facultative treatment depends on sunlight for algal oxygen and wind for mixing and surface reaeration, which is why guidance for lagoon systems consistently calls for keeping banks mowed and clear. A wall of cattails is a windbreak and a shade panel exactly where you do not want one.

Getting rid of cattails

Mechanical control. Cutting cattails below the waterline deprives the rhizomes of oxygen and can drown the stand, especially with a repeat cut when regrowth appears. Tools range from a weed razor or gas-powered aquatic cutter on smaller cells to amphibious mowers on larger systems. Mechanical control is permit-simple (no pesticide involved), immediate, and repeatable, and it pairs naturally with routine berm mowing. Its limit is labor: dense, established stands come back until the rhizome energy reserve is exhausted.

Herbicide control. Systemic aquatic-labeled herbicides (aquatic glyphosate and imazapyr formulations are the common tools for emergent cattails) translocate to the rhizome and kill the plant at the root. Applicators commonly time treatment for late summer into fall, when the plant is actively moving energy down to the rhizome and carries the herbicide with it. In a lagoon context, three rules on top of the label: confirm your state’s permit and applicator requirements first, avoid treating immediately ahead of a discharge event, and treat large stands in sections so the decay load hits your cells in stages instead of all at once. Wait until treated stands are fully brown, then cut and remove the material rather than letting it fall into the cell.

Cattail control methods compared

Drawdown and excavation. If a cell is coming down anyway for sludge removal or liner work, mechanical removal of cattail fringe and rhizome mat during the outage is the most thorough reset available.

Prevention. Cattails establish on shallow, quiet margins. Steeper interior side slopes maintained per your design, routine mowing above the waterline, early removal of pioneer stems, and keeping muskrats from establishing all cost far less than fighting an acre of mature stand.

Getting rid of duckweed

Duckweed is a signal before it is a problem: it thrives on nutrient-rich, still water. A cell wearing a complete duckweed blanket has told you it is nutrient-heavy and under-mixed, and the blanket then makes things worse by blocking the sunlight and surface reaeration a facultative cell depends on, driving dissolved oxygen down.

Agitation and aeration. Duckweed cannot establish on moving water. Surface aerators and the surface turbulence from a well-designed diffused system are the most reliable long-term control, and they treat the underlying oxygen problem at the same time. If duckweed keeps returning, read it as an aeration and loading diagnostic, not just a housekeeping chore. Our aeration guide covers what adequate mixing actually looks like.

Physical removal. Skimming, booming, and netting work, and wind will often raft the mat to one bank for you. Some operators put the biology to work deliberately: duckweed (genus Lemna) takes up nutrients, and moving skimmed or corralled duckweed to a final polishing cell, where its shading suppresses algae-driven TSS, is a legitimate old-school technique. The rule is control: a managed mat on a polishing cell is a tool, a total blanket on an aerated cell is a problem.

Herbicide. Aquatic-labeled contact and systemic products exist for duckweed, with the same lagoon rules: state permit requirements, label compliance, sectional treatment, and removal of the dead mat before it sinks and rots.

The permit-safe checklist

Before any vegetation treatment in a discharging lagoon:

• Confirm whether your state requires Pesticide General Permit coverage or a licensed applicator for applications to the water.
• Use only products labeled for aquatic use.
• Read the label for discharge and irrigation restrictions.
• Schedule around discharge events.
• Treat in sections sized to your cells’ oxygen budget.
• Remove killed biomass.
• Log everything, since your inspector will ask.

When vegetation is really an oxygen problem

Fighting vegetation and low dissolved oxygen at the same time is usually one problem wearing two hats. A duckweed blanket suppresses reaeration, low oxygen slows treatment, slow treatment banks sludge, and the sludge feeds nutrients back that grow more duckweed. Breaking that loop almost always starts with mixing. If you are also seeing odor episodes, start with odor control, which reads the same symptom from the other end.