Start Now To Control Next Year's
Aquatic Weeds

This article was published in the August 2001 issue of the West Virginia Farm Bureau News.

It happens every year, and each year you resolve to solve the aquatic weed problem in your pond. Before long, it is the middle of summer and aquatic vegetation covers the pond surface. Timing is critical, and once again it is too late to solve the problem.

But it's not too late for a midyear resolution to solve next year's problem.

Now is a good time to identify the problem. What kind of weed is it? How much of the pond surface is covered? If you can determine the kinds of weeds growing in your pond, you can to easily determine the appropriate treatment. For example, filamentous algae is commonly treated with a chelated copper product like Cutrine +. However, this treatment is ineffective on a rooted plant like eurasian milfoil. It is nearly impossible to correctly identify a plant over the telephone. Seeing a sample of the plant itself is the best bet. Photographs, drawings, photocopies, and scanned images are also helpful. For photographs of some common aquatic weeds, you may wish to visit the image gallery on our aquaculture Web page ( http://www.wvu.edu/~agexten/aquaculture/gallery.htm   ).

Estimating surface area will require some measure of pond dimensions. A pond 200 feet long and 75 feet wide has a surface area of 15,000 square feet (200 x 75). Since there are 43,560 square feet in an acre, the surface area of a pond with these dimensions will be 15,000/43,560 or about a third of an acre.

Estimating the amount of water in the pond will be important if you are treating the entire pond with a herbicide at a specific concentration. Volume is usually measured in acre-feet. (One acre-foot is the volume of water 1 foot deep over an area of 1 acre. One part per million of 1 acre foot is 2.7 pounds.) If the pond above had an average depth of 5 feet, it would have 1.65 acre feet of water (.33 acres x 5 feet deep).

It is handy to measure depth in the warm summer months and mark the water level at that time. When it comes time to treat, you can check that reference for the depth. For example, if the water level in the above pond was half a foot higher than when the pond was 5 feet deep, there would be 1.8 acre-feet of water in the pond (.33 acres x 5.5 feet).

There are a variety of ways to manage an aquatic weed problem.

1. Use herbicides to kill vegetation directly. (Remember to read the label and follow instructions.) There is great appeal in immediate results. Plants controlled with herbicides frequently grow back. If applied properly, herbicides will not kill the fish. However, treating the entire pond with herbicide at this time of year can lead to oxygen depletion, which could cause a fish kill.
2. Do nothing. Maybe the problem is more in your mind than in the pond. Biologists in the W.Va. Division of Natural Resources (DNR) point out that the best bass populations are found in ponds with weed coverage of up to 40%. If bass fishing is important to you, it may be wise to do nothing.
3. Limit addition of nutrients. Weeds grow in response to fertilization. There may be ways to reduce flushing of manure and other nutrients into the pond. Fewer nutrients may lead to less weed growth.
4. Limit light penetration. Some people suggest fertilization as a way to create a "bloom," which promotes growth of the microscopic algae and makes the water green. This strategy works well in the southern United States where warm water temperatures commonly favor growth of this algae. Unfortunately, fertilization may also promote growth of the weeds you are trying to control. There are products on the market that make the water blue. This effectively shades the bottom, controlling plant growth in water deeper than 3 feet. Such products are most commonly used in ponds where appearance is critical.
5. Remove weeds physically. Mechanical removal of the plants is effective but can be a lot of work Floating plants like duckweed will be skimmed from the surface if the overflow drain pipe draws from the surface of the pond. Expect the weeds to grow back.
6. Stock fish that eat weeds. Grass carp tend to eat submerged rooted aquatic vegetation that is tender. They may provide long-term and cost-effective control. Like cattle, they will eat preferred plant, first and then graze less-desirable plants. Grass carp may not control some plants like filamentous algae especially if the water is cold. Israeli carp and koi carp may be effective against duckweed and filamentous algae. The DNR permits sterile grass carp in West Virginia. Israeli carp and koi carp require no permit. For details, check our Web site
   ( http://www.wvu.edu/~agexten/aquaculture ).
7. Draw down water in winter. Lowering water level will subject shallow areas to freezing and may kill weeds in these areas.
8. Design ponds properly. If you plan to build a pond consult the Natural Resources Conservation Service. You can avert many weed problems if the pond is designed properly.

There are many ways to manage an aquatic weed problem. Integration of two or more strategies give you even greater flexibility. For example, treating with herbicides gives immediate control, and stocking with grass carp after treatment may provide long-term control.

When the water begins to warm up in the spring a good time to take action, but you can begin to solve next year's weed problem now.