Although aquatic vegetation can provide a variety of benefits to both a pond and pond owner, it can also be a nuisance when it becomes too abundant. Overabundant vegetation can prevent good fishing, inhibit domestic or agricultural water uses, and ruin the appearance of a pond. Excessive algae can lead to summer fish kills and dense submerged vegetation can contribute to winter fish kills. Decomposition of plants can cause water to smell. Dense vegetation can attract insects and unwanted animals. Fish production can be reduced when thick vegetation prevents effective predation of small fish by larger fish. Swimming, boating, and fishing also become restricted if plants become too thick.
Methods for controlling aquatic vegetation:
Mechanical Control. Mechanical control is simply the physical removal of plants. It consists of either pulling the plants out at the roots, or cutting the actively growing part of the plant. Pulling can be done by hand or with elaborate equipment. Since most plants grow in water less than five feet deep, this can often be accomplished by wading into the water and scooping them up by hand. A garden rake may expand your reach. Extensive mechanical control involves using heavy equipment to remove plants and is usually expensive. These extreme measures are only necessary for heavy concentrations of plants with strong root systems, such as cattails or spatterdock. Cutting is a mechanical control method that has limited application in most situations. Emergent vegetation can be cut with tools such as garden shears, weed whips (rakes), and gasoline powered trimmers (weed whackers) equipped with brush-cutting blades. Gasoline trimmers are a very good choice for this type of work. Remember, never use electric-powered equipment around the water. Use of electric-powered equipment in wet areas can result in electrocution.
Biological Control. Biological vegetation control reduces the amount of aquatic vegetation by stocking plant-eating fish. The fish most often stocked to control vegetation is the grass carp, sometimes called the white amur. Certified triploid grass carp were legalized for stocking in 1988 specifically for this purpose. These fish are incapable of reproducing in your pond.
Grass carp stocking rates vary depending on the amount and kind of vegetation in the pond. For ponds with a surface covered by more than 60 percent vegetation, stock them at 10 fish per acre, but reduce the rate to 5 fish per acre for ponds with 40 to 60 percent vegetative cover, and to 2–3 fish per acre for ponds with 20 to 40 percent vegetative cover. Grass carp are not recommended for less severe problems. More fish can be added if these stocking rates do not provide adequate control. Wait at least three years after your initial stocking before deciding if you need more fish. Grass carp may not be effective for controlling milfoil, water meal, filamentous algae, or pond lilies.
There are a couple of things to remember about grass carp. First, pond owners should not expect them to solve a vegetation problem overnight. The fish will eat more as they grow older and bigger, and it may take them a year or two to reduce plant growth. Second, it’s much easier to put grass carp into a pond than it is to remove them. Grass carp should not be stocked in a new pond until there is a vegetation problem, and applications should start at low initial stocking rates and be increased later as necessary.
Chemical Control. The most commonly used method of controlling vegetation is the application of chemical herbicides. This approach has both good and bad points. On the good side, herbicides often provide quick and effective vegetation control, are easily obtained and applied, and safe when properly used. They must also undergo rigorous testing to be labeled for aquatic use. On the negative side, herbicide use is becoming more and more controversial due to growing concerns about accumulation of chemicals in the environment.
Herbicides can be dangerous to both the applicator and the environment when improperly used. To ensure that the herbicide chosen will be effective, proper plant identification, herbicide selection, and herbicide application are essential. Failure to do this can waste money on a product that simply cannot do the job. The most important aspects of using herbicides are reading the product label, carefully following application instructions, or contracting a licensed applicator to select and apply the appropriate chemicals. Herbicides can be categorized by the way they kill plants. The most commonly used products are contact herbicides, systemic herbicides, and shading products. The type of product needed depends upon each situation.
Contact Herbicides. A contact herbicide only kills the plant parts that it touches. This is why effective cover of the plants is imperative for successful treatment. Contact herbicides usually work within a few days. Plants that grow from seeds each year (annuals) can be readily controlled with one application of a contact herbicide. However, plants that grow from the same rootstock each year (perennials) will require a number of treatments because contact herbicides will not kill the root system. Copper sulfate has long been the contact herbicide of choice for the control of filamentous algae. Many people know it by the name “bluestone,” which comes from its color.
Copper sulfate is available in a variety of sizes, from relatively large, gravel-sized chunks to finely crushed granules that resemble refined sugar. The smallest size, commonly called “snow,” is the easiest to work with. This is usually applied by dissolving the required amount in water and spraying or pouring the solution over and around the treatment area. Another commonly used method for applying copper sulfate is to place the needed amount of bluestone in a burlap bag and drag it around the pond, either behind a boat or by wading. This can be very difficult if the filamentous algae is thick. Copper sulfate by-products can be toxic to fish eggs and newly hatched fry, so avoid using it during spring spawning periods.
One of the problems with copper sulfate is that it can bind to organic materials and suspendedclay particles in the pond. This reduces its effectiveness in controlling algae. A number of companies have developed more effective variations of the same chemical compound thatallow the copper to remain in solution and stay active longer. These herbicides are more effective than plain copper sulfate, but they are also more expensive. Be sure to use stainless steel or plastic equipment for application of copper products because they are extremely corrosive to most metals. As with copper sulfate, timing the applications of modified copper products is important because they are also toxic to fish eggs and newly hatched fry.
Systemic Herbicides. A systemic herbicide travels through a plant and eventually reaches all parts of the visible plant and its roots. Systemics are usually slower acting than contact herbicides, but provide good control of perennials with only one application. Systemic herbicides tend to be more expensive than contact herbicides, which makes them a somewhat less desirable option for controlling annuals.A pond owner’s choice between contact or systemic herbicides may depend upon the plant. At this point, it becomes necessary to balance the high initial cost of the systemic herbicide against the need for repeated applications of a contact herbicide. The total cost of vegetation control
from each type of herbicide often balances out.
Shading Products. Shading products work by reducing the amount of light available to aquatic plants. If shading is done properly, it can totally prevent weed growth in some areas of the pond by slowing down photosynthesis and retarding plant growth. Several commercial products are available that reduce plant growth by shading. These contain safe dyes that color the water blue. The dye pigments absorb sunlight and prevent light penetration. These products are not licensed for water that will be used for human consumption. In addition to shading chemicals, other shading products are available that are constructed from fabric screens and are placed on the pond bottom. They prevent sunlight from reaching the bottom and prevent plants from rooting.
Before Applying Control Measures
In order to select and use aquatic herbicides or other chemicals effectively and safely, a pond owner needs to know the: 1) volume of water in the pond, 2) use of the water, 3) temperature of the water, and 4) type of vegetation that needs to be controlled.
Determining the Volume of Your Pond. The easiest way to determine the volume of a pond is to check with the people who built or designed it and ask to see their records. Without this information the volume will have to be calculated by the pond owner with the formula: volume=surface area (acres) x average depth (feet).
The first step is for the pond owner to determine the surface area of the pond. If the pond is a circle, rectangle, triangle, or some other standard geometric shape, estimating surface area is pretty straightforward. If the pond is irregular in shape, the best thing to do is divide it into workable shapes and then add the areas of the smaller units to get the area of the whole. The length and width, or diameter of the pond, or the divisions of the pond should be measured to the nearest foot. Formulas in Table 4.1 can be used to determine the pond’s surface area in square feet. Surface area in acres is simply obtained by dividing the surface area by the number of square feet in an acre (43,560).
A pond owner can estimate average pond depth by measuring the depth of the water in a number of places throughout the pond, adding these measurements, and then dividing the total by the number of measurements. Measurements can be acquired with an electronic depth finder, sometimes called a “fish finder,” or simply with a weight that is attached to a string marked in feet. Use a ruler to measure how many inches there are above the last foot marker and record the depth at this location. A reasonably accurate estimate can be made with 10 to 15 measurements for every acre of water. Measurements should be randomly scattered over the entire surface of the pond to provide the best estimate. Pond volume is then determined by plugging the estimated average depth into the formula.
Water Use. Before using herbicides, a pond owner must consider the use or anticipated uses of the pond’s water on his property and the uses of the same water by downstream neighbors. Most herbicides carry use restrictions for treated water that may range from hours to months. Many problems can be avoided by selecting herbicides that are compatible with water uses and considerate of downstream neighbors. A product that is unsafe for animals would be inappropriate if a downstream neighbor uses the water for livestock. Pond owners should carefully read product labels before they buy or use a chemical.
Temperature. The water temperature of a pond should always be checked before using herbicides. This will protect fish and ensure chemical efficiency. Some chemicals will not work below certain temperatures, whereas others may kill fish eggs or newly hatched fry. To reduce the chance of killing fish eggs or fry, simply avoid herbicide applications during spring spawning periods. In addition, higher water temperatures mean lower dissolved oxygen concentrations, therefore it is a good idea to treat vegetation before the hottest part of summer. Sometimes conditions will force a pond owner to treat their pond well after the water has warmed in the summer. If this is the case, the chances of a fish kill due to oxygen depletion can be greatly reduced by treating no more than one fourth to one third of the pond at any one time and then waiting about two weeks before further treatment. Only smaller areas of thick vegetation should be treated at any one time.