Aquatic plant life is desirable in aquatic habitats. One-celled aquatic plants (algae) are the basis of the food chain and supply oxygen to the aquatic system. Larger plants offer shelter and breeding habitat for many aquatic organisms. A balance between plants and other aquatic life is therefore beneficial.

When aquatic plants begin to flourish and affect human activities negatively, these plants are referred to as “weeds.” The “weed” determination may be based on the location in which the plants are growing such as boating lanes or around boat docks. Problems also arise when the aquatic plants interfere with the intended use of the body of water such as swimming, skiing or fishing.

Most problems are caused by introducing exotic species, which have no natural controls to keep growth in check. Without these natural control mechanisms, the plants quickly replace native vegetation. Exotic species that have become problems are water hyacinth, alligatorweed and hydrilla.

Factors Affecting Control

Environmentally sound and cost-effective management decisions should be the basis of any aquatic weed control program. Plant identification is critical because control methods are usually species-specific. All control measures will affect the environment, so it is important to consider the intended use of the water body. Physical constraints such as shallow water or obstacles can impair herbicide applications. Water quality variables such as total alkalinity or the possibility of dissolved oxygen depletions are important considerations. Potential impacts on fish and wildlife populations must also be considered.

Pond Construction

Prevention is the easiest and most economical method to control aquatic weeds. Proper site selection is the first step in preventing aquatic weed problems. Sites should be selected that minimize erosion, nutrient enrichment from runoff and high water flows through the pond. Avoid using a flowing stream as a water source, because the continuous flushing creates clear water and causes low contact times for herbicides and fertilizers. Maintain proper watershed-to-pond ratios. Limit livestock usage in the watershed to lessen erosion and levee destruction. Pond banks should be as steep as possible without causing excessive sloughing. Inside levee slopes should drop 1 foot for every 3 feet the slope extends into the water. Avoid areas shallower than 3 feet deep to minimize excessive weed growth. Encourage grass species along banks such as Bermuda and rye.

Preventive Fertilization

Fertilization provides nutrients for algal growth, which reduces light penetration below the level required for submerged plant growth. Once fertilization has begun, you must continue the program to prevent adverse effects on fish populations. Do not start a fertilization program until the current weed infestation is controlled. The plants you are trying to control will use the nutrients and increase their growth. Liquid fertilizers are preferred because they give faster results.

Drawdown

Drawdown is limited to lakes and ponds with adequate water control structures and a reliable source of water for refilling the pond. Drawdowns are usually conducted during winter to expose plants to drying and freezing. The advantages include low cost as well as oxidation and consolidation of sediment. Drawdowns also increase options for chemical control because some chemicals are more effective when applied to dry water bottoms. One disadvantage of drawdowns is that they may reduce desirable species and allow tolerant species to spread further. There may also be some loss of recreational benefits such as duck hunting and spring fishing.

Mechanical Control

Physical harvesting by hand or equipment can be effective at removing small populations of nuisance weeds such as duckweed, cattail and water hyacinth. This can be accomplished with various methods such as dip nets, sickles or blades, and by placing barriers across incoming streams to restrict floating plants. The main advantages to mechanical control are low cost and low environmental impact.

Biological Control

There have been many attempts to control aquatic plants with biological control methods. These include pathogens, insects and herbivorous fish. For the average pond owner, the use of herbivorous fish like triploid grass carp is the only biological control method available. Triploid grass carp are functionally sterile, meaning they cannot reproduce in ponds. The Louisiana Department of Wildlife and Fisheries has developed a permit to allow farm pond owners to use these fish to control vegetation. The number of grass carp that should be stocked depends on the type of weed, condition of the ponds and severity of the weed problem. Triploid grass carp prefer submerged plants, but some emergent species are also controlled. Some recommendations for stocking rates are given in the table.

Recommended Stocking Rates for Triploid Grass Carp
Weed Evaluation	Number of Fish to Stock per Acre
New pond or very slight weed problem	5
Moderate weed problem (10 to 20 percent coverage)	10 to 15
Severe weed problem	5 to 20 or more

Chemical Control

About 200 herbicides are registered in the United States, but only 10 are labeled for aquatic sites. These chemicals can control aquatic weeds effectively. However, correct weed identification and matching the proper herbicide to the particular weed problem are extremely important.

Aquatic plants which are usually considered problems are divided into four groups: algae, floating, submergent and emergent weeds. Algae are small, usually microscopic plants lacking leaves, roots and stems. These plants may be made up of only one cell, an aggregate of cells called a colony or a chain of cells called a filament. Algae may grow freely suspended in the water (plankton), floating at the water surface (pond scum) or attached or unattached on the bottom. Free-floating weeds such as duckweed and water hyacinth have leaves and stems above the surface and roots that suspend in the water below.

Submergent weeds are usually rooted at the bottom and often extend to the water surface. Common weeds in this group are pondweed, coontail and watermilfoil. Emergent vegetation is also a problem in ponds and lakes. Growth usually occurs on or near shore and in shallow water areas. Plants are rooted in the bottom, and their leaves and/or stems extend above the water surface. Common emergent weeds are smartweed, alligatorweed and cattail.

When considering herbicides for control of aquatic weed problems, remember two important points. First, the label on the herbicide container provides specific information on the proper use of the chemical. Protect yourself and others by reading and abiding by directions and warnings on the product label.

A second consideration is that as dead aquatic plants decay, oxygen in the surrounding water is used in the process. If large quantities of plants are killed with one treatment, dissolved oxygen in the water may be reduced to the point that fish and other aquatic organisms die. Therefore, it is usually desirable to treat only a portion of a weed problem at a time. This allows the body of water to recover lost oxygen before subsequent treatments. The possibility of low oxygen becomes more serious as water temperatures rise in late summer. If possible, weed problems should be dealt with when water temperatures are between 70 and 80 degrees F.

There are several methods for applying herbicides. Some herbicides can be applied directly from the container. Handheld or backpack sprayers are used for spraying emergent vegetation around the shoreline. Boat-mounted tank sprayers can be used for either surface spray or subsurface injection. Hand-operated or mechanical spreaders are used for dispensing granular material. Granular material can also be dissolved by towing it in a bag behind a boat.

When considering chemical control, check with the parish Cooperative Extension Service office or a fisheries biologist for plant identification and current herbicide recommendations. Additional information on aquatic weed control and related topics is available from your local Extension office. Request these publications: Handbook for Common Calculations in Finfish Aquaculture, Pub. 8903; Aquatic Weed Management: Herbicides, Pub. 2398; Aquatic Weed Management: Control Methods, Pub. 2410.