Control Options for Emergent and Aquatic Weeds

Wise water management is necessary if aquatic vegetation control is to be more than temporary. Management must begin by evaluating all uses of a given body of water. Chemical, biological, and physical factors should be well understood and manipulated if possible to get the maximum use of the water for the benefit of the greatest number of people. Before undertaking management of aquatic weeds, consider the following:

  • Biological aspects
    • Identify the problem species
    • Identify other species present
    • Determine density, stand, or scope of problem and stage of weed growth
    • Determine fish species present
  • Water-use aspects
    • Irrigation, potable, recreation, fish production, livestock, and wildlife
    • Length of time water can be quarantined from each use
    • Amount and destination of outflow. Can outflow be regulated? If so, for how long?
  • Physical aspects:
    • Size of channel or pond to be treated
    • Water depth and movement or velocity
    • Water turbidity
    • Water temperature
    • Water quality

Our expanding population and water use have increased the need to prudently manage our limited water supply. Increased fertilization, for example, stimulates aquatic plant growth and may trigger harmful algae blooms (cyanobacteria). Prime sources of fertilization are: 1) discharging untreated or septic-tank-treated wastes from shoreline homes; 2) storm sewer drainage from fertilized lawns and gardens; 3) uncontrolled drainage from heavily fertilized farmland and livestock feeding lots; 4) discharging effluents from municipal sewage treatment plants; 5) drainage from garbage dumps near watercourses, and 6) discharging treated or untreated wastes from industrial plants. Eliminating faulty practices and using good land and water management practices are essential in controlling unwanted aquatic vegetation.

Other cultural practices may prevent weedy aquatic plants from establishing or may minimize their impact. Vegetative buffers, comprised of plants with substantial root systems, like many rushes or sedges, can minimize inflow of excess nutrients into ponds or other impoundments. At a minimum, avoid fertilization of turf grasses within 10 to 20 feet of open water. Constructing ponds, ditches and canals with steep banks will minimize the marginal areas available for emergent weeds to become established. Banks with a 1 to 1.5 percent slope that extends at least 3 feet below the designed water level are optimal. Remove fertile topsoil from the pond or reservoir basin. If a beach area for swimming and other recreational purposes is desired, remove the fertile topsoil and replace it with sand. If possible, prevent water heavily laden with silt and nutrients from entering an impoundment. For ditch banks, plant grasses and use a 2,4-D program to keep out most undesirable broadleaf plants. East of the Cascades, seed redtop or creeping red fescue at the waterline, and crested wheatgrass on the shoulders and top of the ditch bank. Provide roadways on both ditch banks for weed control and other operations. Provide means to control water levels and water flow.

Biological Control

A few approved biological control agents are available for treatment of aquatic weeds common in the Pacific Northwest. The efficacy of biocontrol measures is difficult to predict, and even successful biocontrol programs generally do not result in a weed being eradicated. While biocontrol may serve as a solitary control measure, it typically works better as a complement to other strategies. Many of our most common aquatic weeds do not have viable control options. This is a function of stringent rules established to protect natural resources, including agricultural crops and native plants. Despite these limitations, a few biocontrol options for aquatic plants are worth noting.

The milfoil weevil (Eurychiopsis lecontei) is a native weevil normally found on the native northern watermilfoil (Myriophyllum sibiricum). This native weevil prefers to lay eggs on the non-native Eurasian watermilfoil (Myriophyllum spicatum) but requirements for overwintering habitat and susceptibility to fish predation may reduce efficacy. No operational plan for the use of native insects has been published in a peer-reviewed journal.

Grass carp (Ctenopharyngodon idella) have been used in limited settings to control abundant populations of aquatic plants. Only sterile, triploid grass carp are permitted as biological control agents. Grass carp are generalist herbivores with some feeding preferences. They do not typically feed on emergent plants or water lilies, and strongly prefer some submersed species to others. The criteria and permit requirements for stocking grass carp vary from state to state. In Washington, a private fish stocking permit and possibly a Hydraulic Project Approval application (where inlets/outlets require screening) are required from the Washington Department of Fish and Wildlife. In Oregon, permits are issued by the Oregon Department of Fish and Wildlife only if: a) the waterbody is on private land (or land owned by an irrigation district or otherwise limited access to the public); b) is less than 10 acres or is a ditch or canal; c) and is outside the 100 year flood plain and is screened with ODFW approved screens.

These and other requirements limit the number of scenarios where grass carp are a viable option for weed control. Stocking rates vary according to the permitting agency and site specific conditions.

A number of classical biocontrol agents have been highly successful at controlling purple loosestrife (Lythrum salicaria). Two leaf-feeding beetles (Galerucella calmariensis and G. pusilla), one root-feeding weevil (Hylobius transversovittatus) and one seed-head weevil (Nanophyes marmoratus) have significantly reduced plant density and the number of flowering stems (thus reducing seed production) of purple loosestrife when populations have become established.

Manual or Physical Control

A number of physical control measures are available for control of emergent and aquatic weeds. These are generally those methods that are non-chemical and non-motorized. They include hand pulling, rakes, cutters, benthic barriers, drawdown, aeration, shading and weed rollers.

Equipment costs for manual or physical control are typically very low, but such measures are labor intensive and are therefore better suited to small, less established weed populations. Hand pulling and raking may result in turbid or murky water and may create plant fragments that can subsequently spread to new sites.

Mechanical Control

Many mechanical control technologies are available and may prove a reasonable choice depending on site specifics.

On ditch banks, mowers, scythes or string trimmers will temporarily reduce stands of emergent plants; searing or burning of plants with a propane torch may further retard growth, but needs to be done repeatedly for plants with established root systems as well as new weed seedlings.

In open water, cutting or shredding boats can slice through swaths of thick emergent or floating weeds to allow navigation channels between open water and moorage sites or docks. Rotovators, essentially large-scale underwater rototillers, work by tilling up lake, pond or river sediments to chop and loosen plant roots, which may then be skimmed from the water surface. These work well in loose cobble sediments where shallow rooted plants are easily dislodged.

Growth of emergent plants like cattails can be reduced by dredging, where fertile sediments are removed and water depth is increased to 6 to 10 feet. However, dredging is costly and will not generally prevent the growth of submersed plants. Additionally, dredging may require a Section 404 permit from the US Army Corps of Engineers, and/or a removal-fill permit from Oregon Department of State Lands or a Hydraulic Project Approval (HPA) from the Washington Department of Fish and Wildlife. Mechanical harvesting machines cut aquatic plants approximately 5 feet below the water surface and then convey the material to an on-board storage bin which may then be off-loaded for disposal. Mechanical harvesters are maneuverable around fixed structures like docks, but indiscriminately harvest everything in their path, potentially resulting in undesirable by-catch of fish, other wildlife or desirable native plants.

Chemical Control

When used in, over or near surface waters, herbicides must be registered for aquatic use.1 Considerations in choosing a herbicide should include safety, effectiveness and selectivity, residues, and cost. Use restrictions for fishing, swimming, domestic use, livestock watering or irrigation are explicitly stated on labels and serve to protect people, domestic animals and wildlife. Check with local state game department or other regulatory personnel before applying herbicides to lakes, ponds, irrigation canals and other impoundments or conveyances.

In response to Sixth Circuit court decision (National Cotton Council, et al. vs. EPA) that required the EPA to develop permits for certain pesticide applications in, over and near waters of the State, NPDES-authorized states including Oregon and Washington have developed permit programs regulating applications of aquatic herbicides. In Oregon, the Department of Environmental Quality (OR DEQ) NPDES Program has developed two permits: the Pesticide General Permit (2300A) and the Irrigation District General Permit (2000J). These were required as of October 31, 2011.

In Washington, the Washington Department of Ecology (WADOE) manages the Aquatic Noxious Weed Control NPDES General Permit, Aquatic Plant and Algae Management General Permit, and The Irrigation System Aquatic Weed Control General Permit. These permits regulate the use of pesticides applied to manage aquatic plants in the surface waters of the State of Washington. Refer to the WADOE website ( to obtain the latest permit updates and legal requirements for the use of aquatic herbicides in Washington. In addition, products applied to aquatic sites in Washington are regulated as state restricted-use pesticides; their purchase and application must be made by properly licensed applicators who have an aquatic endorsement.

1 All herbicides discussed in this chapter were approved for use in aquatic habitats as of June 2017, according to the Pesticide Information Center Online (PICOL) Database.

Calculations for Treating Flowing Water

Irrigation canals, drainage ditches, etc.


ppm = (1 milligram or milliliter/L)

ppb = parts per billion (1 microgram or micro liter/L)

cfs = flow in cubic ft/ second

ppm = (Number of gallons of solvent x 1,000,000) ÷ (Number of cfs x 450 x time in minutes)

Gallons of solvent per cfs = (450 x time in minutes x ppm) ÷ 1,000,000

Total gallons of solvent = 450 x time in minutes x ppm x cfs ÷ 1,000,000

Calculations for Treating Ponds and Lakes

Concentration based on parts per million by volume (ppm)

Q = A x D x C x 0.33

Q = Number of gallons of herbicide required

A = Area of water surface in acres

D = Average depth of the pond or lake in feet

C = Desired final concentration of herbicide in ppm

Concentration based on parts per million by weight (ppm)

P = A x D x C x 2.72

P = Number of pounds (active ingredient) of herbicide required

A = Area of water surface in acres

D = Average depth of the pond or lake in feet

C = Desired final concentration of herbicide in ppm

Concentration based on parts per billion by weight (ppb)

P = A x D x C x 0.0027

P = Number of pounds (active ingredient) of herbicide required

A = Area of water surface in acres

D = Average depth of the pond or lake in feet

C = Desired final concentration of herbicide in ppb


The information provided in this handbook is not intended to be a complete guide to herbicide use. Before using any chemical, read the recommendations on the label. Before a chemical can be recommended for a specific use, it must be thoroughly tested. Recommendations on the manufacturer's label, when followed, can prevent many problems arising from the improper use of a chemical.