Aquatic Weed Control


Category 5

Adapted from North Central Regional Extension Publication No. 241, Carole A. Lembi, Aquatic Weed Specialist, Purdue University and Aquatic Pest Control, University of California Leaflet 2961


Specific requirements for commercial applicators in the Aquatic Pest Control category relate generally to a practical knowledge in four areas-

Effective management of aquatic pests requires an integrated approach that incorporates cultural, mechanical, biological, and chemical methods as appropriate.

Why is aquatic weed control necessary?

Plants are natural and important components of the aquatic environment. Microscopic plants (algae) form the base of the aquatic food chain. Larger algae and plants provide habitat for fish and food organisms, and all plants produce oxygen as they photosynthesize during the daylight hours. However, excessive growths of these plants can have a detrimental effect on a body of water and its inhabitants. Many shallow, nutrient-rich ponds, lakes, and drainage ditches provide ideal conditions for abundant aquatic weed growth.

Some of the problems caused by aquatic weeds are as follows:

Water Use Situations

The demand for water resources for recreation, agriculture, and industry is increasing. Many kinds of plant and animal aquatic pests can interfere with water uses. Control of aquatic pests must be done without harm to people and the environment.

Habitats for aquatic weeds involve various proportions of water and soil, including intermittently wet ditches, ditches which always hold standing water, streams, stock ponds, farm ponds, lakes, ornamental ponds, and intermediate habitats. This manual considers three types of water situations - static, limited flow impoundments, and moving water.

Static Water

Static water is confined for considerable periods of the year, or totally confined within a known area, with no downstream movement. However, even totally enclosed bodies of water often have appreciable water movement because of wind and changes in water temperature. Weeds commonly grow in static water up to 12 feet deep. Weeds may grow in very clear water that is more than 20 feet deep. If a herbicide is applied for weed control, there is no reason to expect that any appreciable downstream effect may occur, unless there is overflow resulting from unusual storm conditions.

Limited -flow Water Impoundments

Ditches may be intermittently wet or dry, depending upon climatic conditions. However, herbicides applied to these habitats may move downstream following an influx of water from surrounding areas. The purpose of the ditch is to drain the surrounding land area so considerable amounts of water must pass through it.

Many farm ponds may be characterized as having limited flow because there nearly always is an overflow pipe and an emergency overflow channel (spillway). The overflow pipe is designed to permit passage of a continuous and relatively well-defined amount of water at all times. The emergency spillway is provided to release from the pond when storms dump in excess amounts of water in a short time. In these situations, small amounts of pesticides may be carried downstream from the application site. Larger amounts may be found downstream after sudden rain storms, which interrupt or come immediately after pesticide application.

Moving Water

Moving water is found in small streams, creeks, streams, and rivers where there is always some detectable downstream current. Applied pesticides may be found in downstream locations in varying amounts away from the area of original application. Such situations present the greatest potential for concern as an environmental hazard.

Aquatic Weed Identification

Identification is the first step in managing aquatic weeds. Most control methods are aimed at specific weeds or groups of weeds with similar growth habits. Aquatic weeds can be divided into two botanical groups; algae and flowering plants. Algae are usually very simple in structure with no apparent leaves or stems. However, some (for example, Chara) can resemble flowering plants. For effective chemical control, it is essential that you distinguish between algae and flowering plants.


Microscopic algae form scums and/or color the water green or yellow-green. Sometimes they cause red, black, or oily streaks in the water called "blooms." Blooms usually occur where abundant nutrients are reaching the water. They should be treated with chemicals before they cause a noticeable color but a sudden die-off of these algae can cause fish kills.

Filamentous algae (also known as moss) form floating, mat-like growths which usually begin around the edges and bottom of ponds in the early spring. Moss is probably the most common in lakes and ponds in the Midwest. Often, repeated chemical treatments during the summer season are necessary for effective control.

Chara, or stonewort usually grows in very hard water and is often calcified and brittle. The plant is rooted, and leaves are arranged along the stem in whorls. It grows completely underwater and has a musky smell. Chara can be difficult to control once it has become established and has a heavy coating of calcium carbonate. Use contact herbicides when the plants are still young and not heavily calcified. Although this plant resembles some flowering plants, it is an alga.

Flowering Plants

Flowering plants can be grouped into broad categories according to where they are found in a body of water.

Submersed plants are rooted in the bottom sediments and grow up through the water. Flowers or flowering spikes sometimes emerge above the water surface. The main criteria for identification are leaf arrangement and leaf shape.

Common underwater weeds are

Floating weeds

Free-floating plants, such as duckweed and watermeal are seed-bearing plants which float free on the water's surface. They never become rooted in the soil, and are propagated by sexual and asexual means. They can completely cover the surface of a pond. Both are extremely small. Duckweed is no more than 1/4 inch in diameter, and watermeal is even smaller. Both plants are found in nutrient-rich waters. Input of waste water from sources such as livestock feedlots and septic tank fields should be eliminated.

Rooted floating plants include waterlily, spatterdock, and water lotus. Spatterdock is usually the weedier of the three and completely fill in shallow areas less than 3' or 4' deep. Spatterdock is a massive, difficult to kill underground rhizome from which new plants sprout. It differs from waterlily in having heart-shaped leaves that come above the surface of the water and a yellow flower. Waterlily has round leaves.

Emergent (shore or marginal) plants commonly include cattails, bulrushes, spike rushes, reed canarygrass, and other grass-like perennial plants. Broadleaves include willow trees and creeping water primrose.

Managing Aquatic Weeds

The basic control approaches are
  1. Preventive Measures

    Proper design and construction of ponds is an important factor in preventive control of weeds. Shallow water at the margins provides an ideal habitat for immersed weeds, such as cattails. These weeds can spread then to deeper water. Banks should be sloped steeply so that very little water is less than 2' to 3' deep.

    Proper design and construction of ditches and channels makes weed control easier in the future. If the banks are leveled and smoothed, hard-to-reach places will be eliminated. Lining canals will help to alleviate water weed problems, too.

  2. Mechanical Control

    Even with preventive measures, many ponds still have severe waterweed infestations. In some cases, use of a herbicide may not be possible if the water is used for livestock, drinking, or fish. Hand-pulling the weeds or dredging the pond are possible methods of control. But often the infestation is so severe that these methods are impractical or uneconomical.

    Motor-driven underwater weed cutters are available and can be used for the control of such plants as waterlilies and watermilfoil. Some mowers simply cut the weeds loose beneath the water surface. Aquatic weed harvesters collect weeds for removal. Disposal of harvested weeds can be a problem. Most mechanical control methods fragment weeds. Many weed species can spread and reproduce from these pieces.

    Mechanical control is usually slower and more expensive than use of herbicides. Underwater weed cutting must be done continuously during the summer and usually represents a long term financial investment.

  3. Cultural Control and Habitat Alteration

    Certain methods of manipulating or altering the aquatic environment can be effective in controlling aquatic weeds. One of the more successful methods is the drawdown technique in which water levels are lowered over the winter. Exposure of the sediments in the shallow areas of a lake or pond to alternate freezing and thawing action will kill the underground rhizomes of many aquatic weeds (the majority of aquatic weeds are perennial and come from rhizomes).

    This method has been quite successful for the control of Eurasian watermilfoil and waterlilies, although the degree of control depends somewhat upon the severity of the winter. There are several advantages to a winter drawdown in addition to weed control. As the sediment dries, it is compacted, thereby increasing the depth of shallow areas. Drawdown also concentrates the fish which increases the predation of the smaller fish by the larger ones. Fishing quality can often be improved following a drawdown.

    Many aquatic weeds or their seeds are carried into a pond by wind birds, fish introduction, fishermen, etc. These weeds infest a pond only if the water conditions are just right. This usually means that nutrients are entering the pond from runoff or stream inflow. To help prevent serious weed infestations you can do the following things:

    All of these measures will help prevent weed growth, particularly in a new pond. In older ponds these measures will probably aid in reducing infestations of floating plants such as algae and duckweed.

    Other types of habitat manipulation include riprapping shorelines and anchoring screens (e.g., Aquascreen) or black plastic sheets on the bottom sediments to prevent rooted plant establishment. Dyes such as Aquashade are used to inhibit light penetration through the water. This blue dye can be applied right out of the bottle along the shoreline. It mixes throughout the body of water within 24 hours. The dye intercepts light normally used for photosynthesis by underwater plants. The dye can only be effective if its concentration is maintained.

    Some general rules for using Aquashade are as follows:

    1. Do not apply where water outflow will reduce Aquashade concentration.
    2. Apply in March or April before weeds reach the water surface. Midsummer reapplication is usually necessary. It is effective only on rooted underwater plants growing at depths greater than 2 to 3 feet. Supplemental treatments of copper sulfate might be needed for algae control.
    3. Do not use in muddy water.

    Aeration has been publicized as another method of weed control. Although aeration is definitely beneficial for fish life and can help prevent fish kills, there is no evidence that aeration inhibits weed growth.

  4. Biological Control

    Biological controls for aquatic vegetation have received considerable publicity. Several species of fish are herbivorous in that their principal diet is aquatic vegetation. One such species, the grass carp (also known as the white amur or Chinese carp), is being tested in various parts of the United States and is legal in several states. However, it is illegal to introduce these fish to the ponds, lakes, and streams of many states. Check with your local or state fisheries department for regulations regarding the grass carp.

  5. Chemical Control Methods

Control with Herbicides

Chemicals used in aquatic weed control are classified as herbicides. Herbicides used primarily to control algae may be called algicides, even though they also kill other aquatic plants. For most aquatic weed problems, properly-used herbicides control vegetation without harming the fish. Aquatic herbicides are effective and commonly used means of controlling aquatic vegetation.

Four zones of a body of water may be treated

Surface Generally, only 1/4 to 1/3 of the surface area of the water should be treated at a time. This helps to protect fish from a possible shortage of oxygen. Surface area (in acres) of a rectangular body of water equals length in feet times width divided by 43,560 (the number of square feet in an acre).
Total water volume The whole body of water from the surface to the bottom is treated OR you can treat 1/4 to 1/3 of the water volume (based on surface area) at a time. Calculate the volume of the body of water and add chemical to obtain the required dilution.

The concentration of chemical needed to control aquatic plants is often very small and is stated in parts per million (ppm). For example, if the toxic concentration for a particular plant is 2 ppm, then the chemical should be applied at the ration of 2 parts of active ingredient to one million parts of water (2:1,000,000) in the area to be treated.

1) Calculate the acre-feet of the body of water to be treated. Multiply the surface acres by the average depth in feet. An acre-foot of water weighs 2.7 million pounds (2,700,000). 2) 2.7 * ppm concentration wanted * acre-feet = pounds of active ingredient needed.

The following calculation shows how to calculate the number of pounds of active ingredient needed to treat a body of water containing 10 acre feet at the rate of 0.5 ppm.

2.7 * 0.5 * 10 = 13.5 pounds of active ingredient

Bottom 1 to 3 foot layer of water Treating the bottom 1 to 3 feet of water is especially useful in deep lakes where it is impractical to treat the entire volume of water. Treatments are generally made by attaching several flexible hoses at 3 to 5 foot intervals along a rigid, weighted boom. Each hose has a nozzle at the end. The herbicide is applied as a blanket in the lower 1 to 3 feet of water.
Bottom soil surface Herbicide applications may be made to the bottom soil of a drained pond, lake, or channel.

Aquatic herbicides generally are available in sprayable or granular formulations.

Sprayable formulations- Most herbicide formulations must be mixed with water and applied so that they disperse evenly. These include-