[ Search PSU ]   

[Visible Title of Page]

Bryan Swistock, Extension Associate, School of Forest Resources

Most ponds in Pennsylvania have some amount of aquatic plant and algae growth. Depending on the intended use for the pond, this plant and algae growth may be undesirable. There are numerous techniques available to help manage this growth but they can be complicated and pond owners need to carefully weight each option and its consequences before proceeding. This short fact sheet introduces the basic steps for controlling aquatic plants and algae and provides references to a more detailed Cooperative Extension publication. Readers are strongly encouraged to consult these more detailed references before proceeding with aquatic plant management.

Identifying the Problem

The offensive plant or algae must first be accurately identified before you can determine how to control it. There are a variety of resources that may be available to help identify a plant specimen including local offices of Penn State Cooperative Extension, Pennsylvania Fish and Boat Commission or a local school or college biology department. In addition, professional pond consultants experienced with aquatic plant identification may also be found in your local yellow pages. There are also numerous web pages that provide pictures, drawings, and descriptions of aquatic plants and algae. Guides to identifying aquatic plants can be purchased from the North American Lake Management Society (phone 608-233-2836, on-line at http://www.nalms.org/). Finally, you can send a close-up color picture or fresh dried sample in newspaper to Bryan Swistock, 132 Land and Water Building, Penn State University, University Park, PA 16802 for identification. 

Control Methods

Prevention: Like most problems, prevention of excessive aquatic plant and algae growth is usually more desirable than attempting to control it after it has become a problem. Growth of rooted aquatic plants may be discouraged by deepening shallow edges of the pond or by lowering the water level over winter (drawdown). State permits may be necessary for physical changes to the pond structure including water drawdown. Consult with your local Pennsylvania Department of Environmental Protection (DEP) office to determine if a permit is necessary in your case.

Growth of both aquatic plants and algae can sometimes be controlled by reducing or eliminating nutrient inputs to the pond. This might be accomplished by reducing fertilizer and manure applications near the pond, diverting nutrient-rich runoff away from the pond, or creating vegetative buffer strips to trap nutrients. Nutrient reduction will not immediately reduce plant growth but it will have a long-term benefit. 

There is some evidence that applications of barley straw to a pond may help prevent growth of algae. More detail on barley straw applications can be found in a fact sheet entitled “Using Barley Straw to Control Pond Algae” available from your county Cooperative Extension office.

Mechanical Removal: Mechanical removal methods consist of cutting, mowing, raking, digging, or pulling plants and algae. As you might expect, these methods are physically demanding but inexpensive if you do them yourself. They are most efficient for small quantities of plants and algae near shorelines.

Grass Carp: Grass carp can be very effective in controlling many species of submerged plants like pondweeds, naiads, elodea, coontail and muskgrass. They are generally not effective against algae or floating plants like water lily. Their sale and use is regulated by the Pennsylvania Fish and Boat Commission. Pond owners must obtain a permit before stocking grass carp. A permit application to buy grass carp and a list of commercial hatcheries that sell them can be obtained from your local Pennsylvania Fish and Boat Commission office.

Chemical Control: Aquatic herbicides offer a solution to many otherwise difficult aquatic weed problems. But chemical control of aquatic weeds must be undertaken with adequate planning and considerable care. Once you have identified the problem plant or algae, follow these steps.

1. Carefully calculate the pond area and volume. Knowledge of the approximate pond surface area and average depth are critical before attempting to use an aquatic herbicide. Labels on herbicides will usually give the amount of chemical that should be applied for a certain area or volume of pond water. These calculations must be done carefully to ensure that enough chemical is applied to control the plant without causing fish kills or other adverse effects. These measurements are also required to receive a state permit to apply an aquatic herbicide (see #3 below). Pond area can be estimated by taking some rough pond measurements. For roughly square or rectangular ponds, take several measurements to determine the average length and width in feet. Try to include both the longest and shortest distances in calculating average length and the widest and narrowest distances for determining average width. In addition to the extreme locations, take measurements in between to get a good representative sample of the pond dimensions. The more measurements you take the more accurate your final calculation will be. After taking the measurements, find the average length and width and multiply those together to get the approximate pond area in square feet. The area in square feet can be converted to acres by dividing by 43,560.
   
   If the pond is circular in shape, take multiple measurements of the diameter and average them. The diameter is simply the distance from one bank of the pond to the opposite bank through the center of the pond. Divide the average diameter in half to get the average radius. Square the pond radius and multiply by 3.1416 to get the square feet of pond area for the circular pond. 
   
   The average depth of the pond can be determined by taking a number of depth measurements at numerous points in a transect along the widest and longest parts of the pond. Average all of the depth measurements to get the average pond depth in feet. 
   
   Many aquatic herbicide labels give dosage rates per acre-foot of water. An acre-foot is simply one acre of surface area with one foot of water depth. You can obtain the number of acre-feet of water in your pond by multiplying the surface area (in acres) by the average pond depth (in feet).
   
   Make sure you check and recheck all of your calculations to ensure that your final surface area and volume estimates are as accurate as possible! 
   
   
2. Choose the correct chemical to treat the problem. A variety of chemicals are registered in Pennsylvania as aquatic herbicides for controlling plants and algae. Once the problem plant or algae is identified, consult the Cooperative Extension publication entitled “Pond Management and Aquatic Plant Control” (see below for more information) to determine the best chemicals for your particular problem. 
   
   
3. Obtain a state permit application. A state permit is required by law to apply herbicides to all waters in Pennsylvania including private ponds. The two-page permit application and instructions can be obtained from your local Fish and Boat Commission office, your local DEP Office, and some county Cooperative Extension offices. The permit and instructions can also be downloaded from the Fish and Boat Commission web page at www.state.pa.us/PA_Exec/Fish_Boat/pfbchom2.html. Allow at least four weeks for the permit to be processed. Click here to download the permit and instructions to apply an aquatic herbicide.
   
   
4. Purchase the proper aquatic herbicide. After you have received your permit, purchase the specified aquatic herbicide from a local farm supply or chemical supply store. 
   
   
5. Apply the herbicide according to the label instructions. Once you’ve obtained the permit and chemical, carefully follow the label instructions on the herbicide. Care must be taken when using chemical herbicides. Many can be toxic to fish and other animals when over-applied or unevenly distributed. Newly hatched fish appear to have less tolerance to herbicides than do older fish. In some cases, minor fish kills may occur when safe dosages are exceeded because of an uneven distribution of the chemical. A fish kill also may occur as an indirect result of chemical weed control because oxygen is consumed by the rapid decay of plants. To minimize this danger, treat only one-third of the pond at any one time, even when plants are present in the entire pond. Make applications at least one week apart, or as specified on the label, so that rapid plant decay will not reduce oxygen content of the water to a dangerously low level. Make sure you also note and follow any water use restrictions after application of the chemical.
   
   Herbicide treatments are generally most effective when applied in late spring or early summer when plants are young and actively growing. Treatment at this time of year usually gives the best control with the least amount of chemical. Applications in late summer or early fall require more chemical and usually give slower, more erratic control.

For more detailed information on control of aquatic plants and algae

Pond Management and Aquatic Plant Control (AGRS-76)

Can be ordered or purchased directly from your local Cooperative Extension Office