Community News
Aquatic Weeds Have Many Forms
By Mitchell May Decatur County Extension Agent
Mar 21, 2006, 17:19

There are many different types of aquatic plants that can be found in and around fish ponds.  These plants range from microscopic plankton algae which helps tint pond water green to larger plants such as the  water lily which is rooted on the pond bottom.  Some of these plants are essential for a healthy pond environment.  But many times these plants become nuisances by interfering with fishing in recreational ponds or commercial fish production.

Aquatic weeds that cause problems are generally placed into four groups: algae, floating weeds, emersed weeds (foliage above water) and submersed weeds (majority of  foliage below water).  Certain types of algae produce compounds which cause a musty flavor or odor in fish flesh.  These compounds are absorbed by the fish and can cause a highly offensive taste known as off-flavor.   This condition can be corrected if fish are moved to water that does not contain these off-flavor compounds.  There is no evidence at this time that thinning the plankton bloom with chemicals will reduce off-flavor problems.

Floating weeds float in or  on the surface of the water and obtain their nutrients from the water  rather than the soil.  Examples of common floating weeds include Duckweed (Lemna minor) and water meal (Wolffia spp.)

Emersed weeds are rooted to the bottom, have stems, leaves and flowers which extend above the water.  They primarily occur on the shoreline and in the shallow water up to 10 feet deep. Common types of emersed weeds are water lilly, cattail, watershield and alligator weed.

Submersed aquatic weeds grow under and up to the surface.  Most submersed weeds have flowers and seed-heads that extend above the surface of the water.  Examples of common submersed weeds include hydrilla, coontail, purple fanwort and watermilfoil.

Aquatic weed control incorporates a variety of preventive methods such as proper pond construction and maintenance, biological methods such as the grass carp and the use of labeled aquatic herbicides.  When developing an aquatic weed control plan its success is dependent upon correctly  identifying the problem weeds and selecting control methods that are compatible with how the owner wants the pond managed.

It is easier and less costly to prevent weed problems than it is to control them once they develop.  Careful pond site selection and proper pond construction practices are the first steps in preventing aquatic weed problems.  Rooted aquatic weeds and algae usually begin growing in shallow water.  Edges of the new and existing ponds should be deepened to reduce the shallow water areas.

Biological control such as the grass carp offer a practical and economical way to control certain types of pond weeds.  Grass carp effectively control weeds with tender succulent vegetation such as filamentous algae and duckweed, but are less effective in controlling weeds that have tough, woody vegetation such as waterlily and cattail.  Many states regulate the use of grass carp.

Herbicides may be used to control weeds in recreational and commercial ponds.  The first step in successfully using chemicals for aquatic weed control is accurately identification of the problem weed.  After the weed has been identified, a herbicide that is labeled for aquatic use may be selected.  The herbicide label must be read and fully understood by the user prior to application.

The most successful approach in controlling aquatic weeds is through integrated weed management.  Herbicides should be considered as a temporary control method.  Depending upon the herbicide selection and the weed species, duration of control can range from a few weeks to several months.  Long-term weed control can be achieved by using a combination of recommended aquatic weed methods.  For example, use of the proper herbicides followed by grass stocking can be an effective in controlling many aquatic weed problems.

Source: Aquatic Weed Management- SRAC Publication No. 360

Mitchell May

Mitchell May is a Decatur County Extension Agent for the University of Georgia Extension Service.  Call the Decatur County Extension Office at 248-3033 for more information.