Weeds

Weeds are simply plants in the “wrong” place. Many types of aquatic plants are beneficial for wildlife, and plants form part of a healthy pond. Farm pond owners may consider leaving vegetation in and around ponds to provide cover and food for wildlife. However, excessive growth of plants can interfere with other uses of the pond, such as watering cattle, fishing and swimming, making the plants “weeds.” Floating weeds, such as duckweed, can become so abundant that the pond surface becomes covered, cutting off light and oxygen to the fish below. On occasion, weeds can literally take over a pond and cause serious problems for the farm pond owner.

The best way to avoid weed problems is prevention. Building the pond correctly (see pond construction), limiting excessive nutrient loading, and keeping invasive weeds out of a pond are crucial. However, when weeds become a problem, there are several ways to manage the infestation. First, you need to identify the species of weeds that are causing the problem(s). The Center for Aquatic Invasive Plants at the University of Florida or Aquaplant from Texas A&M University are good places to start. Your County Cooperative Extension Service Office can also help in plant identification. Once the plants are identified, physical, mechanical, biological, chemical or a combination of these control methods can be used.

Physical Techniques for Weed Control

Shallow areas where light reaches the pond bottom are ideal for the growth of rooted aquatic weeds, and plants can be expected to grow in these areas. In most cases, measures to control weeds in such shallow water are futile. Deepening pond edges so that the water depth quickly reaches 2 1/2 to 3 feet helps reduce weeds. This may not be an appropriate option for ponds close to homes where the safety of children is a concern. An alternative is to use pond dye, which provides a shading effect and prevents light penetration to the bottom where rooted plants attach and grow.

Drawing down the water level 3 to 4 feet during the late fall and winter can help control rooted weeds and is also good to reduce overpopulation of prey fish. Shallow weeds are exposed during drawdown and subjected to drying an freezing. An expensive alternative is to use a pond liner which prevents plant roots from penetrating the soil. This option must be installed before filling the pond, or the pond must be drained.

Mechanical Techniques for Weed Control

Physical control by cutting or pulling plants is possible for small ponds or isolated patches of weeds. Weeds that are cut often grow back quickly and they have to be cut again. Floating weeds often are blown into a corner of the pond, where they can be scooped out with a fine mesh net. For filamentous weeds, dragging a chain through the pond is sometimes an effective method to harvest weeds.

Chemical Techniques for Weed Control

Chemical control is risky, expensive, and should generally be considered as a last resort. Cooperative Extension Service publication MP44 provides recommendations for selection and use of aquatic herbicides. When using chemicals, proper identification of the weed is important, as many herbicides are selective, that is, they only work on certain types of weeds. Be sure to follow label instructions, and note that the use of a chemical may restrict uses of the pond water for other purposes, such as irrigation or watering cattle.

Spot treatments of weedy areas usually can be accomplished without problems, but when whole pond treatments are required, actually measuring the pond area is important. To visually estimate the area of a pond is amazingly difficult, and even “experts” can be off by several-fold. Decomposition of weeds killed by herbicides removes oxygen from the water and can even result in a fish kill, especially in the summer months. When using a fast acting herbicide, treating only a section (up to a quarter of the pond area) at a time will reduce the chances of oxygen problems. Unless the herbicide is intended for whole pond application (i.e., fluridone), treating only a portion of the weeds at a time allows affected weeds to decompose before the next application. Typically, the heavier the growth of weeds, the smaller the area that should be treated in a single application. The best time to treat aquatic weeds is during the spring when the plants are growing rapidly and water temperatures are cooler (70ºF to 80ºF).

If you have fish in your pond and are thinking of using copper sulfate (sometimes called “bluestone”) for algae control, be sure to have the alkalinity of your pond water tested. Copper sulfate is toxic to fish in low alkalinity waters (below 50 mg/L), and the correct dose is based upon the alkalinity.

Biological Techniques for Weed Control

Biological control is feasible for some types of aquatic weeds. Grass carp prefer tender, succulent vegetation submerged in the water but will not control tough, fibrous plants that grow up out of the water, such as alligatorweed and cattails. Other types of weeds may or may not be eaten by grass carp, depending on how hungry the fish become, so that results are not predictable. For more information on grass carp and the types of weeds they control, see SRAC Fact Sheet #3600, “Using Grass Carp in Aquaculture and Private Impoundments.”

Grass carp are readily available in Arkansas, and they provide cost-effective and long-term control. Either normal (diploid) or sterile (triploid) fish can be used in Arkansas, and no permit is required. New ponds can be stocked with 2" to 6" grass carp at 4 to 5 fish per acre. In ponds with existing bass populations, grass carp at least 8" to 10" long must be stocked to avoid having them eaten by the bass. If you have a problem with a weed that grass carp are known to consume, stocking rates of at least 15 to 30 fish/acre are required to provide control within a year or two. Grass carp are capable of fast growth and can reach 20 to 25 pounds in weight. As these fish become older and mature, their rate of weed consumption declines, so restocking with additional fish after 4 to 5 years becomes necessary. Grass carp will also escape when heavy rains cause ponds to over-flow. A parallel-bar spillway barrier can be built to reduce fish. When more immediate results are required, applying an herbicide followed by stocking of grass carp (once the treated weeds have decomposed) may be the best option.