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
Extension Service Office can also help in plant identification. Once the plants are identified, physical, mechanical,
or a combination of these control methods can be used.
Physical Techniques for Weed Control
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
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
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
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.