In a previous post, I described various preventive measures for aquatic vegetation control. While these measures will help prevent weed growth, they are not the answer for eliminating weed infestations that are already present. If this is the case, there are various mechanical, biological, and chemical methods available to control aquatic weeds.
Mechanical control involves physically removing the plants. This can be done by hand or with special hand-held devices designed for cutting and pulling pond vegetation, available from aquatic/fisheries suppliers. Most aquatic plants are perennial and will re-sprout if the plant is not entirely removed, including growth below the ground. For larger bodies of water, motor-driven underwater vegetation harvesters are available. This equipment is a major investment and is not a feasible option for most individuals on privately owned lakes and ponds.
In severe cases of emergent plant growth, such as cattails, complete removal with a backhoe or other equipment may be necessary.
Biological control refers to the use of organisms to control vegetation. Biological control of aquatic vegetation has received considerable publicity recently and research is currently being conducted to determine the ability of bacteria, fungi, and insects to control aquatic plant infestations. The research has shown some potential but it is too early to tell if this type of control will work on a widespread scale.
The most widely used biological control is grass carp. Grass carp consume some algae and most submersed plants but have the potential to completely remove all underwater vegetation in a body of water. Grass carp can provide good control of pond vegetation but they have a tendency to feed on beneficial native plants before the truly troublesome weeds, such as Eurasian watermilfoil or mat-forming algae. Also, their feeding rate slows after about five years, so more fish may need to be added. Grass carp are not effective controls for duckweed or watermeal.
Private landowners in Indiana do not need a permit to introduce grass carp into their ponds but there are restrictions. First, the fish must be purchased from a holder of an aquaculture permit and the permit holder must deliver and stock the fish. Next, the land surrounding the pond or lake must be totally in private ownership. Finally, precautions must be taken to ensure the fish do not escape the stocked area, such as erecting barriers at the spillways and outflows.
Chemical control is the most common method of aquatic plant control. A variety of chemicals are available to control different types of vegetation. When used properly, chemical controls are safe and effective. However, many are only temporary solutions and require repeated applications to prevent the vegetation from returning. When using chemical controls, read and follow all label directions. The type of chemical required depends on the type of vegetation you want to control. Many chemicals also have waiting periods on the use of water treated with the herbicides. These mostly have to do with fishing, domestic use, livestock watering, and irrigation.
The best time to apply aquatic herbicides is late spring. Do not wait until July or August. Late summer treatments can kill fish. By that time of year, vegetation is extensive and thick and the water is warm. Killing off vegetation under these conditions seriously depletes oxygen levels in the water and can result in fish kills. If you must treat in late summer, treat only a portion of the vegetation at a time.
Additional information on aquatic weed control, including specific herbicide recommendations, can be found in Purdue Extension Publication APM-3, Identifying and Managing Aquatic Vegetation.
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