Weeds overboard

Aquatic weed growth can become excessive due to several conditions. Your pond may have clear, shallow water that allows sunlight to reach the bottom. It might have an abundant nutrient supply, particularly of nitrogen and phosphorus. Whatever the reason, it is possible to manage the problem. Select a sight that reduces the chances of excess nutrients reaching the pond. Use high standards designing and installing the pond.

Also, you must prevent the introduction of potentially weedy plants to nearby bodies of water. If water gardens or ornamental pools are nearby, avoid discarding unwanted plants into the pond. Avoid planting highly invasive aquatic plants such as water hyacinths, parrotfeather, duckweeds and mosquito ferns in ornamental pools that are near waterways, lakes or ponds.

However, if you already have weedy growth, several management techniques can help you control it. The first step is to correctly identify the cause of the problem before choosing the most appropriate treatment.

Identifying the problem

To select the appropriate management procedures for your waters, first identify the weeds. To do this, you will need to bring a sample to a diagnostician. Diagnosticians prefer live samples for the identification of most aquatic weeds. Collect your weeds, including the roots and flowers, and wash off any mud or debris. Wrap them in damp (not wet), absorbent paper towels, and place them into a zipper-type plastic bag. Then, transport them immediately to a qualified person for identification. Do not leave your samples in a hot vehicle or allow them to dry out. Also, do not leave them in a container of water, or they will begin to rot within 24 to 48 hours.

Take notes on the size and average depth of the pond, use of the water, extent of infestation and the frequency and amount of water flow out of the pond. Also, know the current and intended uses of the pond (irrigation, fishing, aesthetics, etc.). They may limit your management options. For instance, water-use restrictions may apply if certain herbicides or dyes are used, or if you have endangered- or threatened-species restrictions.

Once you have identified your weeds and considered all the factors, you can use this information to make the appropriate management decision.

Selecting a control method

You can use manual, cultural, mechanical, biological and chemical methods alone or in combination to manage the weeds in your waters. After diagnosing the problem, choose which control method will work best for you.

• Manual techniques

Manual techniques include raking, seining and skimming plants. These are labor-intensive, inefficient and usually not effective for long-term control. These techniques can be used on small ponds, but their use in larger waters is not feasible.

• Cultural techniques

Cultural techniques include the use of pond dyes and water-level drawdown. Use pond dyes to manage filamentous algae and submersed (underwater) vegetation. Several pond colorants and one or two dyes are EPA-registered for aquatic-weed control. Pond dyes and colorants can be effective if there is little water outflow from the pond.

Dyes and colorants intercept sunlight needed by algae and other underwater plants for photosynthesis. Therefore, they are generally ineffective on floating plants like duckweed and water lilies and emergent (growing above the surface) plants like cattails and bulrushes. They are nontoxic and do not kill the plants, and they are safe for use in ponds for irrigation, fishing and livestock. However, they are not intended for use in large lakes with a lot of water flow or lakes used for public water supplies.

Water-level drawdown is a cheap, effective technique for submerged vegetation like Brazilian elodea, naiad and pondweed, and, to a lesser extent, filamentous algae. I normally do not recommend drawdown unless the pond is larger than 1 acre and has a control structure that allows you to adjust the water level easily. You should conduct the drawdown in the winter months when the combination of drying and cold temperatures will kill many aquatic weeds. Do not conduct drawdowns during the warm months. You will stress the fish population, and you may allow plants such as cattails, bulrushes and willows to spread.

• Mechanical removal

You can use almost any type of digging, dredging, cutting or mowing equipment to remove unwanted vegetation from ponds and lakes. The main advantage of mechanical removal is that you can quickly remove unwanted vegetation. You can use harvesters in rivers and on large lakes. Several companies market hand-held and boat-mounted weed cutters that may be useful for small ponds. These cutters leave the weeds in the pond. Therefore, you must remove the cut weeds or they may root again.

The drawback of mechanical removal is its expense — often more than $10,000 per acre if soil removal is involved. Weed fragments may also float away and cause problems elsewhere. Large-scale mechanical removal operations may require access to a landfill for weed disposal. For large scale projects, it might be beneficial to contract with someone who specializes in mechanical removal.

• Biological control

Chinese grass carp (white Amur) are the primary organisms used for biological control in the United States. They may live up to 10 years or longer and reach sizes greater than 50 pounds. They feed entirely on vegetation and, thus, do not compete with other fish for food resources.

Grass carp are the most effective and least costly method for control of submersed plants like naiads, pondweeds and hydrilla in small ponds. They occasionally will feed on free-floating, floating-leaved and emergent plants, but they are rarely an effective control of these plants. Also, they usually do not provide effective control of submersed plants in the milfoil family such as parrotfeather, variable-leaf milfoil, Eurasian watermilfoil and others.

Stocking recommendations vary by state, but usually fall in the range of 10 to 15 fish per acre for small ponds. You should stock fish that are at least 10 inches in length — smaller fish are prey for bass and wading birds. It's best not to stock grass carp in large lake systems. They may migrate to other waters where they may feed on desirable vegetation. Most states require a permit for carp purchase and require that the fish be certified as triploid (sterile and non-reproductive). Check with your state fisheries or natural resource agencies for specific regulations. For additional information, see Using Grass Carp for Aquatic Weed Management, which is available online at: http://www.ces.ncsu.edu/nreos/wild/aquatics/weed/grasscarp/index.html.

Fish are not the only organisms used in weed management. Insects are used to control alligatorweed in the southern United States and to control purple loosestrife in the northern United States and southern Canada. Use the alligatorweed flea beetle to control alligatorweed in the southern coastal area of South Carolina to Florida and west across the Gulf Coast. For additional information or to obtain flea beetles, contact the Aquatic Plant Control Operations Support Center at (904) 232-2215. For additional information on purple loosestrife insects, consult the Cornell University biocontrol website at: http://www.nysaes. cornell.edu/ent/biocontrol/weedfeeders/wdfdrtoc.html.

• Chemical control

You can also use several formulations of algicides and herbicides to control weeds in ponds and lakes. Most of these products have water-use restrictions, including fishing, swimming, irrigating, preparing other pesticide sprays, watering livestock and drinking. However, proper use of algicides and aquatic herbicides does not pose any significant threat to humans, animals or general environmental health. Fish kills from herbicide toxicity are rare. When they occur, they are usually the result of errors such as excessive treatment rate or use of an inappropriate product.

If you are going to apply an algicide or herbicide, be sure that you have identified your target weeds and that your choice of product(s) is appropriate for the weeds present and the intended uses of the pond. Check the label to find out how to use the product and to be sure that it will control your weed problem. It may be necessary to use more than one product when several different weeds are present. Always read the label and wear appropriate clothing and safety gear when mixing and applying algicides or herbicides.

• Types of chemicals

Chemicals that can be used include copper, glyphosate, endothall, 2,4-D, diquat dibromide and fluridone. Copper formulations are used largely as algicides, although several also are labeled for aquatic flowering plants. Glyphosate formulations are used for emergent vegetation and occasionally for control of certain floating-leaved plants, such as water lilies. Endothall formulations are used almost exclusively for submersed vegetation, although the amine salt of endothall can be used as an algicide. Formulations of 2,4-D are used for broadleaf weeds and are particularly effective on the watermilfoil group, water lilies and waterhyacinths. Most formulations of 2,4-D are not labeled for aquatic use, so check the label to verify that the 2,4-D formulation you plan to use is labeled for use in ponds. Diquat dibromide is used for submersed weeds, duckweeds (except watermeal, which is resistant) and, occasionally for waterhyacinths and other free-floating weeds.

Most of these chemicals kill plants quickly. This may be what you need, but be aware that it may result in loss of oxygen due to decomposing vegetation in the water. This can be a problem if you have stocked fish. Fluridone is a slow-acting herbicide, and takes several weeks to several months to kill weeds, so it does not cause problems with oxygen depletion. It is used especially for the control of submersed weeds, such as hydrilla and Eurasian watermilfoil. It also is the only product currently available that will effectively control watermeal.

Water-use restrictions, which are found on product labels, vary with active ingredient, formulation used, site of application and application rate.

• Application of herbicides

Herbicide application rates for ponds and lakes are expressed in three ways: pounds or gallons per surface acre, parts per million (ppm) or pounds or gallons per acre-foot (the equivalent of 1 surface acre of water 1 foot deep). When the application rate is given by the second or third method, you need to calculate the volume of water to be treated. This may be only part of a pond or lake or, in some cases, the whole pond.

To do this, lower a long, calibrated pole (such as a cane fishing pole marked at 1-foot intervals) to the bottom of the pond at least four times at equally spaced intervals across the pond or treatment area. Record the depth of each measurement and divide the sum of these measurements by the total number of measurements. Repeat this process at least three times, going in different directions across the area to be treated. Multiply the average of your measurements by the surface area (acres) that you will treat. This is the volume of water (in acres-feet) in the treatment area.

Herbicide applications in small ponds do not require sophisticated technology. In some cases, you can simply open the container and pour the contents into the water. However, dilute the herbicide and apply it in several places around the pond to get better distribution. When spraying, you can use a simple backpack sprayer and either walk around the pond or, if necessary, apply the herbicide from a boat.

Timing your control

Fish kills following herbicide applications are usually the result of oxygen depletion from weed decay. They usually occur during the summer when treated ponds are nearly covered with weeds. Cooler water holds more oxygen than warm water so, if possible, wait until spring before treatment. However, if possible, wait until early in the growing season when the water temperature is at least 65°F. The plants will be actively growing, and they will absorb herbicides more readily

If you must treat your weeds during the summer, treat only a small portion of the pond (not more than one-fourth of the total pond). Wait until the treated weeds decay before treating another patch of weeds. Place an aerator in your pond or use irrigation equipment to spray pond water into the air. When it splashes back into the pond, it will help aerate the water and reduce oxygen depletion.

There are some exceptions to the early application rule. Treat perennial, rhizomatous plants such as cattails after they flower so that the herbicide will move down the stem and kill the root system. Early treatment on perennials usually results in temporary top burn, followed by immediate regrowth.

You have many factors to consider when choosing an aquatic-weed-control strategy. The best control option depends on the type and size of the waters, time of year, intended uses, type of weeds present and budget and time constraints. Consider all of these before you make your decision. Monitor the presence of weed species and keep notes of when problem species develop and what conditions prompted outbreaks. With this information, you will be better prepared to efficiently manage your waters.

Dr. Stratford Kay is an associate professor at North Carolina State University (Raleigh, N.C.).


For more details on the management of aquatic weeds, visit our Aquatic Weed Management web site at: http://www.cropsci.ncsu.edu/aquaticweeds. The site also contains numerous links to other web sites related to aquatic-weed management. For information on pond management and fish stocking, see our Pond Management Guide at: http://www.ces.ncsu.edu/nreos/wild/aquatics/pond/. Information on algicide and herbicide formulations, application rates and techniques, water-use restrictions and tables of effectiveness for control of specific weeds may be found in the following online publications:

  • Weed Management in Small Ponds (http://www.cropsci.ncsu.edu/aquaticweeds/ag-437.pdf);

  • Weed Control in Irrigation Water Supplies (http://www.cropsci.ncsu.edu/aquaticweeds/ag-438.pdf);

  • The Aquatic Weed Control Section (updated annually) of the North Carolina Agricultural Chemicals Manual (http://ipmwww.ncsu.edu/agchem/chptr8/823.PDF).

For more information on identification, consider several books on aquatic plants. The Aquatic and Wetland Plants of South Carolina is a color-picture guide published by the South Carolina Department of Natural Resources. It costs about $20 and is widely applicable to common aquatic plants in the Southeast and mid-Atlantic regions (ordering information is available at http://water.dnr.state.sc.us/water/envaff/aquatic/aquabook.html). You also will find excellent online photographs of many aquatic plants and other information at the University of Florida's Center for Aquatic and Invasive Plants website: http://aquat1.ifas.ufl.edu.


Aquatic plants include algae, mosses, ferns and flowering plants. We also group aquatic plants on the basis of their growth habits: submersed (completely underwater), free-floating (not rooted in the mud), floating-leaved (roots in the bottom with leaves floating on the water's surface) and emergent (roots in the bottom, with most of the leaves and shoots extending above the water's surface). All four of these groups overlap somewhat. Management of plants within a group generally is similar.

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