Identification and Control of Weeds in Southern Ponds
George W. Lewis
Professor, Aquaculture and Fisheries
Warnell School of Forest Resources
James F. Miller
Extension Agronomist-Weed Science
Cooperative Extension Service
The University of Georgia
Excessive aquatic plant growth is one of the most frequent and frustrating
problems encountered in pond management. There are hundreds of aquatic
plant species that grow in southern ponds. Fortunately, most of these plants
are rarely a nuisance to pond management. Some people consider any aquatic
plant a weed which must be removed. However, aquatic plants need to be
controlled, but not eliminated, if good pond fishing is the management
goal. This publication is designed to aid the pond owner in identifying
the problem weeds and in selecting the best management approach for effective
and safe control. Only those aquatic plants which occur commonly in Southern
ponds and are a common nuisance to management are illustrated. To use this
publication successfully, observe the following: identify the nuisance
weed, evaluate the pond, and select and use the most practical control
method. The effectiveness of control measures can be quite variable from
one region to another. Herbicide use is subject to federal, state and local
regulations. Owners who are not certain of the identification of the pond
weed or the best control method should consult their county Extension agent
or state fisheries biologist. Another solution would be to hire a qualified
Identifying Common Aquatic Weeds
Aquatic plants are listed in four groups according to the habitats in which
they are usually found. The four groups are algae, floating weeds, emersed
(above water) weeds and submersed (underwater) weeds. Figure 1. To identify
the weed in question, first decide in which group it belongs, turn to that
section, and use the illustrations and descriptions to make your decision.
Remember, only the more common weeds are described.
Figure 1. Aquatic plant groups.
These plants occur in most all waters exposed to sunlight. Shape and size
vary from microscopic single-celled plants to branched-coarse plants resembling
submerged aquatic weeds. Algae, unlike other aquatic plants, do not produce
flowers or seeds. Based upon size and shape, algae can be divided into
three groups: microscopic (planktonic) algae, filamentous algae, and stoneworts.
Microscopic (planktonic) algae are single or multiple-celled plants that
cannot be identified without the aid of magnification. Figure
2. Specific identification is usually not essential for control. Rather,
it is important to recognize the plant group and not the individual species.
Most of the microscopic algae respond to the same control measures. They
occur in almost all Southern ponds.
Figure 2. Microscopic (planktonic) algae.
These algae are generally beneficial to ponds. They are the beginning
of the food chain in a pond, converting nutrients from the water into usable
food for insects and fish. Through photosynthesis, they provide dissolved
oxygen, essential to life in the pond. Pond fertilization promotes the
growth of microscopic algae as evidenced by the green color of the water.
Figure 3. There is rarely a need to control these plants in a pond; however,
excessive blooms can lead to oxygen depletion and fish kills. Excessive
blooms are usually the result of over-fertilization or organic pollution
(see section on Pond Fertilization). A few species, especially some of
the blue-green algae such as Microcystis, can cause fish kills and animal
and human health problems. Blooms of these algae occur occasionally and
are usually associated with organic pollution. Masses of these plants appear
rapidly and make the water seem like a "soupy" bright green mass on the
downwind side of the pond. These plants also give off a foul sewage-like
odor and can give an off-flavor to fish caught from the pond. Fish affected
by the toxins of this plant act drugged and may convulse. Fish usually
show these symptoms only during the daylight hours. A rapid and complete
fish kill, although rare, may occur.
Figure 3. Microscopic (planktonic) algae.
Unlike microscopic algae, filamentous algae are frequently a problem in
pond management and are usually visible to the naked eye as a floating
mat of thread-like filaments often called "pond moss." Figure 4. They usually
begin growth on the pond bottom in shallow water, later float to the surface
and may completely cover the pond surface.
Figure 4. Filamentous algae.
Except for a few species, all filamentous algae control methods are
similar. As with microscopic algae, it is more important to recognize the
plant group and not the individual species. Various species of filamentous
algae occur throughout the southern states.
One group of wide-spread algae which is difficult to control is Pithophora
spp. Pithophora resembles a mass of wet, green wool. Figure 5.
Figure 5. Pithophora.
This group of algae is quite often confused with underwater aquatic plants
because it is attached to the pond bottom. No part of the plant extends
above the water surface. Stoneworts do not produce flowers or seeds.
Chara (Chara spp.)
Chara has a distinctive musky odor when crushed. Figure 6. It is usually
grey-green and has a rough texture caused by calcium deposits on the plant.
This branched algae has whorled thread size "leaves" and the plant is anchored
to the bottom mud. It occurs throughout the south but is more commonly
found in hard water areas.
Figure 6. Clara.
Nitella (Nitella spp.)
Nitella (not illustrated) is similar in appearance to Chara. However, the
plant is dark green, does not have a musky odor, and does not hae the texture
of chara. It is more common in acid or softwater areas of the south.
Floating Aquatic Weeds
Plants in this group float in or on the water and obtain nutrients from
the water rather than the soil. They are rarely attached to the pond bottom.
Duckweeds are small floating plants which at a distance can be mistaken
for algae. Figures 7, 8. Depending upon the species, duckweeds may range
in size from microscopic to about 1 inch in diameter. Another plant group
similar to duckweed is waterfern (Azolla spp.) Leaves of these plants are
0.1 to 0.4 inches wide and overlap one another as scales on a fish.
Figure 7. Duckweed and waterfern.
Figure 8. Duckweeds.
Water Hyacinth (Eichhornia crassipes)
Water hyacinths are quite variable in size, and may range from 3 to 36
inches in height. Figure 9. Flowers may be blue, violet, or white and are
quite showy. They are occasionally found rooted to the bottom. The plants
are a serious problem along the coastal areas of the Southeast.
Figure 9. Water hycinth.
Coontail (Ceratophyllum Spp.)
The common name of this plant is very descriptive of its appearance. Figure
10. The leaves are 0.2 to 1.5 inches long in whorls on the stem, becoming
more and more crowded near the tip. Coontail is widespread throughout the
Figure 10. Coontail.
Bladderwort (Utricularia Spp.)
Bladderworts are identifiable by small bladders produced randomly on the
plant. Figure 11. Leaves have many fine thread-like segments. Although
bladderworts are rootless plants, stems are sometimes attached to the bottom.
Flowers of many species are yellow and grow above the water surface. Figure
Figure 11. Bladderwort
Figure 12. Bladderwort flower.
Emersed (above water) aquatic weeds
Plants in this group are rooted to the bottom but have several leaves or
parts which extend above the water surface. Some of the plants in this
group may grow in water 10 feet deep; most are shoreline or shallow water
Although not all the plants in this group are true waterlilies, they are
often confused with waterlilies. This group of plants may also be called
lily pad, cow lily, spatterdock and lotus. They are all rooted plants with
American Lotus (Nelumbo lutea)
American lotus leaves are large and circular, 8 to 25 inches in diameter.
Figure 13. The center of the leaf is depressed, forming a cup. Leaves produced
early in the season float on the water surface and as the stem grows, the
leaves become suspended above the water. The flower is pale-yellow and
about 8 to 10 inches in diameter. Figure 14. Seeds are produced in a large
distinctive fleshy receptacle which is yellow as the flower opens, then
turns green and later, dark brown. Unlike other waterlilies, American lotus
does not have a split or notched leaf.
Figure 13. American lotus.
Figure 14. American lotus flower.
Fragrant and White Waterlily (Nymphaea spp.)
Leaves of the true waterlilies are split or notched and are usually 6 to
8 inches in diameter. Fragrant waterlily, Figure 15, can be readily distinguished
from white waterlily by its sweet-scented white or pink flowers. Flowers
of white waterlily have little or no fragrance. Leaf veins of fragrant
and white waterlilies originate from the leaf center and extend to the
margin in a fan-like pattern. Figure 17.
Figure 15. Waterlily.
Figure 16. Spatterdock.
Figure 17. Waterlily leaf.
Spatterdock (Nuphar luteum)
Spatterdock is also known as pond lily, cow lily, and yellow waterlily.
Figure 16. Leaves vary from nearly round to lance shaped and have a deep
notch. The flower is yellow and shaped like a ball. Spatterdock can be
distinguished from fragrant and white waterlily by the pattern of its leaf
veins. Figures 17, 18. Spatterdock leaf veins originate from a main lateral
vein and are not fan like. Figure 18.
Figure 18. Spatterdock leaf.
Watershield (Brasenia schreberi)
Watershield may also be called dollar bonnet. Figure 19. The plants have
floating, oval or elliptical leaves 1 to 4 inches in size. Stems and the
undersides of leaves are coated with a thick layer of gelatin-like material.
This gelatinous material interferes with the uptake of some chemical treatments.
Figure 19. Watershield.
Water Pennywort (Hydrocotyle umbellata)
Water pennywort is usually found growing in water less than 2 inches deep.
Figure 20. Dense stands may occasionally break loose and float in deeper
water. Leaves are nearly round and are 0.5 to 1.2 inches in diameter.
Figure 20. Water pennywort.
Frogbit (Limnobium spongia)
Frogbit has two leaf forms. The floating or underwater leaves are heart
shaped and have a deep notch. Figure
21. The underside of the leaf is thick in the center and spongy. Figure
22. The out-of-water leaves are also heart-shaped but not as thick
in the center or deeply notched and have a leathery feel. Frogbit may be
found either growing rooted to the bottom mud in shallow water or floating
on the water surface. It usually grows three inches or less in height.
Figure 21. Frogbit.
Figure 22. Frogbit--underside of floating leaf.
Pickerelweed (Pontederia codata)
Pickerelweed usually grows in shallow water areas and is more common in
the coastal areas of the Southeast. Figure 23. Leaves grow in clusters
2 to 6 inches wide and 4 to 12 inches long. The flowers are violet-blue
Figure 23. Pickerelweed.
Alligatorweed (Alternanthera philoxeroides)
Alligator weed grows in a wide range of soil and water conditions. Figure
24. Growing plants usually form an interwoven mat which may be free floating,
rooted and above water, or in a dry field. It is more common in the coastal
areas. Leaves are 2 to 5 inches long, lance shaped, and have a distinct
mid-rib. Flowers are white.
Figure 24. Alligatorweed.
Smartweed (Polygonum spp.)
Smartweed is a shallow water plant. Figure 25. Each joint or node is covered
by a thin white to brown sheath. Leaves are linear or elliptic and alternate.
Flowers are white, pink, pinkish-white or green.
Figure 25. Smartweed.
Arrowhead (Sagittaria Spp.)
Arrowhead is a shallow water plant. Figure 26. Leaf shapes are highly variable;
however, they are usually in the shape of an "arrowhead". Leaves are usually
above the water, but they may be under the water or floating.
Figure 26. Arrowhead.
Water Primrose (Jussiaea Spp.--Ludwigia Spp.)
Water primrose grows along the shoreline and in shallow water areas. Figure
27. Leaves are light green, about 3 inches long, and lance-or-oval shaped.
Stems may appear reddish-green. Flowers are bright yellow, about one inch
in diameter and have 5 petals.
Figure 27. Water primrose.
Cattails (Typha Spp.)
Cattails usually grow along the shoreline and are sometimes found in water
3 to 4 feet deep. Figure 28. Plants may attain a height of 6 to 8 feet.
Leaves are about 1 inch wide, and ribbon shaped Flowers are produced on
the end of stalks in cylindrical clusters; whence the name "cattail."
Figure 28. Cattail.
Sedges and Rushes
There are many species of rushes and sedges in the Southeast. Few of them
are ever a problem in pond management. Examples of some members of this
family are illustrated. Figure 29.
Figure 29. Rushes.
One species, slender spikerush (Eleocharis acicularis), is a major problem
in some Southeastern ponds. Figure 30. Slender spikerush plants are small
and hair-like varying from 2 to 6 inches long. Leaves arise from the base
of the plant in tufts. Plants grow along the bottom, but may break away
and form living floating mats.
Figure 30. Slender spikerush.
There are a large number of woody plants that may grow along the edges
of a pond which are potential problems to pond management. There are 3
species in the Southeast that are a common problem; buttonbush (Cephalanthus
occidentalis), willows (Salix spp.) and alders (Alnus spp.)
Buttonbush is a small to medium sized shrub. Figure 31. The flowering heads
of this shrub resemble buttons at a distance. Leaves are whorled, 2 to
6 inches long, elliptic or oval shaped, and coarse textured.
Figure 31. Buttonbush.
Willows are large shrubs or trees with alternate leaves which are several
times longer than wide. Figure 32. The leaves are usually toothed on the
margin. The wood is soft and light, and the bark is aromatic but has a
Figure 32. Willow.
Alders are large shrubs or small trees whose alternate leaves are coarse
textured, heavy veined, and toothed. Figure 33. Flowers occur in clusters
and resemble small pine cones when mature.
Figure 33. Alder.
There are many grass species that can be a management problem in ponds.
Maidencane (Panicum hemitomon) and southern watergrass (Hydrochloa caroliniensis)
are probably the most common and difficult to control.
The stalks are long and narrow. Figure 34. Stems may be 3 to 8 feet tall
growing in up to 2 feet of water. Leaf blades are 4 to 10 inches long and
0.2 to 0.6 inches wide. Leaves are rough on the upper surface and smooth
on the under surface.
Figure 34. Maidencane.
Stems are branched and the underwater portion is usually leafless. Figure
35. Floating leaf blades are 2 to 4 inches long and 1/8 to ¼ inch
Figure 35. Southern Watergrass.
Submersed (underwater) Aquatic Weeds
Plants in this group grow under and up to the water surface. Some submersed
plants have seed heads which extend above the surface and may be confused
with emersed plants. However, unlike emersed plants, most submersed weeds
are dependent upon water for support. When submersed plants are removed
from water, they may be limp and unable to support themselves in an upright
position. Submersed weeds can be the most difficult group to identify and
control, yet are some of the most common weeds interfering with pond uses.
Naiads (Najas Spp.)
This group of plants, also known as bushy pondweed, is very common in the
Southeast. The margins of the leaves have a "sawtooth" appearance, but
in some species these teeth are barely visible without magnification. Leaf
size and appearance can vary with different species from a small, thread-like
(0.4-1.5 inches long and 0.1 inches wide) shape to a broader, saw-like
shape (0.4-1.5 inches long and 0.6-2.0 inches wide). The leaves occur opposite
and some species have three leaves in each whorl. One of the most wide
spread species is southern naiad. This plant is dark green to purple-brown
in color. Leaves are 0.3 to 1.8 inches long and 0.2 to 0.3 inches wide.
Marginal teeth on the leaves require magnification to be seen. Leaves are
usually opposite or in whorls. Figure 36.
Figure 36. Naiad.
Watermilfoil (Myriophyllum Spp.)
There are several species of Myriophyllum common to the Southeast.
Generally, this group of plants can be confused with emergent (above water)
plants. In some species, the tip may extend 3 to 12 inches above the water.
Also, above water leaves may appear to be different from underwater leaves.
In all species, underwater leaves are "comb-like" or "feather-like" in
appearance and are whorled. Three species are a common management problem
in Southeastern ponds.
Parrotfeather (M. brasiliense)
Parrotfeather is rooted to the bottom but may extend 3 to 12 inches above
the water surface. Figure 37. Underwater the stem is quite stout with leaves
arranged in whorls with relatively long internodes. Each leaf is 0.8 to
2 inches long and has 10 to 18 narrow comb-like teeth on each side of the
mid-rib. Above water, leaves are compacted on shorter internodes and are
more delicate in appearance. Above water, leaves are also whorled and vary
from yellow to green in color. Parrotfeather is found in most areas of
Figure 37. Parrotfeather.
Eurasian Watermilfoil (M. spicatum)
This species is primarily a problem in the coastal areas of the southeast.
38. This plant is apparently spreading throughout the Southeast. The underwater
leaves look like weather-beaten feathers and are whorled. The above water
tip has no leaves and may be 2 to 4 inches long.
Figure 38. Eurasian watermilfoil.
Broadleaf Watermilfoil (M. heterophyllum)
Broadleaf watermilfoil (not illustrated) is more coarse in appearance than
parrot feather. Underwater leaves are also whorled and sparse. Above water,
the spike may be 2 to 6 inches long. Leaves are also whorled above water
but do not have a feather-like appearance. They may be 0.2 to 1.2 inches
long and are serrated along the edges.
Included in this group are hydrilla (Hydrilla verticillata), egeria
or Brazilian elodea (Egeria densa) and elodea (Elodea canadesis).
Hydrilla is a significant problem in Florida and is becoming established
in many of the other Southern states. Figure 39. Egeria is also more common
in Florida but is also established in other areas. Figure 40. Elodea is
more common to Northern and Midwestern states and is occasionally found
in the Southeast.
Figure 39. Hydrilla.
Figure 40. Egeria.
Hydrilla is a serious threat to fresh water habitats and for this reason
it should be distinguished from egeria. The midrib of hydrilla leaves may
be red. Leaf margins have strong serrations and large soft spines on the
back of the midrib. Usually hydrilla has a harsh scratchy texture while
egeria lacks these characteristics. Hydrilla is very difficult to identify
because of the variable characters it has in different habitats. Any aquatic
plant identified as egeria, elodea or hydrilla should be sent to a specialist
for positive identification since hydrilla is such a serious threat to
fresh water habitats. It is only through early identification and concentrated
control methods that there is any hope of eliminating hydrilla. Fishermen
or boaters in waters known to have hydrilla should made every effort not
to accidently transport hydrilla from one lake or pond to another.
Fanwort (Cabomba Spp.)
The submersed leaves of fanwort are "fan-like" in shape and made up of
many thread-like elements. Figure 41. The stem may be covered with a gelatinous
material. The floating leaves are few and of different shapes. Floating
leaves may be long and narrow or oval in shape. Fanwort is found in most
areas of the Southeast.
Figure 41. Fanwort.
Pondweeds (Potamogeton Spp.)
Pondweeds are the largest group of aquatic plants. Figures 42, 43. Different
species are quite varied in their appearance. Some members of this group
are very difficult to control. In many species, the leaves are usually
alternate and the underwater leaves are often ribbon-like and less firm
in texture than the floating leaves. The flowers and fruits are on spikes
extending above the water surface. Leaves may vary from thread-like to
large oval-or lance-shaped.
Figure 42. Pondweed.
Figure 43. Pondweed.
Evaluating a pond is a simple, critical, and often overlooked step in the
successful control of aquatic weeds. An evaluation of the pond will help
the owner select and apply the most efficient, effective, and economical
control measure. A pond evaluation should include the following: an inventory
of pond water and watershed uses; an appraisal of the physical conditions
of the pond and watershed; knowledge of some of the water chemistry; and
the pond water volume.
Pond and Watershed Uses
Most pond owners already know the various uses of their pond and watershed.
However, pond and watershed uses need to be considered when attempting
to control aquatic weeds. This is especially true if herbicides are used.
For example, aquatic herbicides applied to a pond used for irrigation may
have a disastrous effect upon the irrigated crops. In most instances, a
conflicting water use (such as irrigation) can be temporarily stopped until
the herbicide has dissipated from the water. In other situations, this
may not be practical and an alternative control measure should be used.
If herbicides are used, water uses should be compared to restrictions listed
on the label. Some water uses which may interfere with aquatic weed control
are: irrigation, boating, swimming, watering livestock, sportfishing and
commercial fish production.
Various physical conditions may contribute to an aquatic weed problem or
interfere with attempts to control it. Ponds which have extensive areas
of shallow water or receive organic runoff usually have aquatic weed problems.
Attempts to control weeds in these ponds are usually unsuccessful unless
the pond is deepened or the source of organic runoff is removed or diverted
around the pond. Deepening a pond or eliminating organic runoff can be
considered control methods and are discussed in the Methods of Aquatic
Weed Control" section. Excessive amounts of water flowing through a pond
can interfere with aquatic weed control. Ponds which are constructed on
constantly flowing streams, or have an excessively large watershed, or
have a large number of springs can be difficult to manage for aquatic weeds.
Excessive water flow may interfere with the following weed control measures:
fertilization, herbicides, and fall-winter drawdowns.
Pond Water Chemistry
When controlling aquatic weeds with herbicides or fertilizer, it is important
to know something about the chemistry of the water in the pond. Water hardness
can affect herbicide and fertilizer applications. The water hardness also
should be considered when using herbicides containing copper. Water hardness--Generally,
water hardness is a measure of the calcium and magnesium in the water.
In hard waters (above 50 parts per million hardness) it may be necessary
to apply greater amounts of herbicide in order to achieve control. In soft
waters (below 50 parts per million hardness) some herbicides are more toxic
to fish and plants. Herbicides which may be affected by water hardness
have precautions listed on their labels. Water hardness is also an indicator
of the lime requirement for the pond. See section on Pond Fertilization.
Many areas of the south have softwater. Pond owners should be familiar
with the water hardness of their pond. Pond water hardness can be measured
by most state Extension services or state game and fish departments. Copper--In
softwater some heavy metals, especially copper, can be toxic to fish. Some
herbicides contain copper and should be used with caution in softwater
ponds (less than 50 parts per million water hardness).
Pond Water Volume
Pond owners should know the water volume of their ponds. Most chemical
application rates are based upon water volume. This is true for chemicals
used to treat aquatic weeds, fish diseases, and oxygen depletions. The
volume of water in a pond is usually expressed in acre-feet. An acre-foot
of water is one surface acre that is one foot deep. A pond having three
surface acres and an average depth of 6 feet has a total water volume of
18 acre-feet. Table 1 can be used to convert acre-feet to other expressions
of volume and weight. Most county Soil Conservation Service offices can
assist pond owners in determining the water volume of their ponds. Assuming
the surface area of the pond is known, the following method can be used
to determine the average depth of a pond. Average depth can be determined
by use of a sounding line at regular intervals along several transects
of the pond. Both deep and shallow areas of the pond should be included
in the transects. Average depth is computed by adding all of the depth
measurements and dividing by the number of measurements. The average depth
multiplied by the surface area should give an accurate estimate of the
pond water volume.
Table 1. Equivalents of I acre-foot and amount of chemical added
per unit volume to give one part per million (ppm) (Conversion Factor).
|1 acre foot = 43,560 cubic feet
||1 ppm = 2.72 pounds per acre foot
|1 acre foot = 325,830 gallons of water
|1 acre foot = 2,718,144 pounds of water
Methods of Aquatic Weed Control
Methods of aquatic weed control include preventive measures, mechanical,
biological, and herbicidal controls. Each method has advantages and disadvantages.
The best approach is to consider preventive measures first. If they are
not practical or do not produce the desired results, then other control
methods should be considered. It is always easier and more economical to
prevent a weed problem than to cure one. Even when preventive measures
are only partially successful, they quite often facilitate the effectiveness
of other control measures.
Preventive measures include proper pond location and construction, fertilization
and fall-winter drawdowns. If one or all of these practices were followed,
the need for herbicide use in many southern ponds would be reduced.
Careful selection of a pond site can help prevent weed problems. A flowing
stream is not a good location for the construction of a pond. Excessive
water flow will prevent successful fertilization and complicate herbicide
applications. This is also true of ponds with excessive watershed-to-pond
surface area ratios. The recommended watershed-to-pond ratio depends on
land use, vegetative cover, soil type, and slope of the land. Generally,
a watershed of 10 to 20 acres per surface acre of pond is recommended for
woodland, while 5 to 10 acres is recommended for pastureland. Construction
of ponds in watersheds that have highly fertilized fields, barnlots, septic
tank fields, or other sources of organic runoff should be avoided. Enrichment
of a pond with organic material will promote weed growth. Existing ponds
with excessively large watersheds or sources of organic pollution in the
watershed may benefit from the construction of a diversion ditch to direct
some of the runoff around the pond.
Most algae and submersed and emersed rooted aquatic weeds usually start
growing in shallow water 2 feet deep or less. Once established, they will
often extend into deeper water areas. Ponds should be constructed so there
is little if any water less than two feet deep. In existing ponds it may
be practical to deepen shallow water areas.
In properly constructed ponds, establishment and growth of aquatic weeds
is best prevented by fertilization. Fertilization stimulates the growth
of desirable algae so sunlight cannot penetrate to the bottom and rooted
aquatic weeds cannot become established. Figures 44, 45. In ponds with
established aquatic weeds, these plants generally die down during the winter.
If fertilization is begun before weeds begin spring growth, in most instances
they will not become re-established. If fertilization is begun after weeds
have become established, fertilizer will be taken up by the weeds. This
will produce heavier weed growth. Fertilization should begin as early as
possible in spring before weed growth starts. Herbicidal treatment of a
weed problem in the spring followed by a fertilization program is often
a good weed control measure. In some areas of the southeast, pond fertilization
may not be recommended or recommendations described below may be modified
because of area differences in watershed uses and fertility. For local
fertilizer recommendations, consult your county Extension agent or state
game and fish personnel.
Begin fertilizing with 40 pounds of 20-20-5 per surface acre in late
February or early March when water temperature stabilizes above 60°F.
If after two weeks you can see a bright object more than 18 inches beneath
the surface, proper water color resulting from plankton algae growth has
not been developed. Fertilize again at the same rate. If proper color does
not develop following the third application, test the pond for lime requirements.
Proper color can be maintained by fertilizing with 40 pounds of 20-20-5
per surface acre when a bright object can be seen more than 18 inches into
the water. Fertilization will probably be necessary about once a month.
Stop fertilization when water temperature stabilizes below 65°F. This
is usually in October or November.
Method of Application
The most economical method of application is to pour the fertilizer on
a platform constructed 12 to 18 inches below the water surface. Figure
46. There should be at least three square feet of platform for each surface
acre in the pond. Fertilizer placed on platforms will dissolve slowly in
the upper layer of water where desirable algae production occurs and be
distributed by wind. Locate the platform 10 to 15 feet from the shore so
the fertilizer will receive maximum wind distribution. Other acceptable
methods of application include: broadcasting fertilizer by hand in shallow
water less than four feet deep, or pouring it in a line 15 feet from shore
in shallow areas, or slitting fertilizer bags and placing them in shallow
Figure 46. Fertilizer platform.
Lime is necessary for plant production in ponds as it is on fields. Softwater
ponds should be limed before fertilization can be effective in producing
a bloom of desirable algae. Many Southern ponds should be limed every two
to four years. Liming requirements can be determined by your county Extension
agent or state game and fish personnel. Agricultural lime is used for liming
ponds. Although agricultural lime may be added at any time, it takes about
three months for it to go into solution. Lime should be distributed throughout
the pond during fall so it will be in solution the following spring when
Decreasing the water level of a pond is called a drawdown. Drawdowns are
beneficial in controlling weeds and correcting mildly crowded bream populations.
If a drawdown is used for two or four consecutive years, as much as 90
percent of the submerged vegetation in a pond can be eliminated. Drawdowns
expose the shallow areas of a pond to winter weather and drying, thus eliminating
some of the weeds. Figure 47. A pond should be drawn down in November and
this lowered water level maintained until spring (February or March). Drawdowns
should never be used in warm weather months or in ponds smaller than one
surface acre. The pond water surface area should be reduced at least one-third
and not more than one-half.
Figure 47. Fall-winter drawdown.
Some rather sophisticated machines have been developed to control aquatic
weeds. These machines either cut or drag weeds from a pond. Unfortunately
their operation is expensive and not practical for most privately owned
ponds. Seining or raking weeds out of the water can be an effective control
method in small ponds.
Because of the labor involved, it is not practical in larger bodies
of water. Seining or raking weeds can be used to temporarily clear small
shallow bays or beach areas in large ponds. Sun screening or shading materials
have been used to successfully control weeds in some ponds. These materials
either restrict or shade-out plant growth. Generally, their use is not
practical in large ponds. Black plastic sheeting, for example, can be spread
out and weighted down on the bottom in a swimming beach area to control
submersed weeds. This plastic should be left in place for about two weeks
in order to achieve temporary control.
Biological controls of aquatic weeds are a new and promising development
in pond management. A biological control is some living agent, whether
it be fish, insect, or bacteria, which is harmful to and attacks the nuisance
aquatic plant. Most biological agents are organisms which are not native
to this country and thus must be studied to determine whether they may
have any negative effects upon the environment prior to release. In a few
Southern states, the grass carp or white amur (Ctenopharyngodon idella)
is being used to aid in control of aquatic weeds. Figure 48. The grass
carp is not a native fish to this country. The use of grass carp for aquatic
weed control is presently highly controversial. It is the general opinion
of many biologists that the grass carp is a potential hazard to our native
fish and waters and not enough information is available to recommend its
use. In most states it is illegal to stock this fish. Do not stock grass
carp in ponds in states where their use is prohibited. And, never stock
the fish without first consulting with state game and fish department personnel
or a qualified fisheries biologist.
Figure 48. Grass carp.
Aquatic herbicides are safe and effective if they are selected and applied
properly. Choosing the right herbicide does not guarantee success. It must
also be applied properly and all warnings and precautions concerning use
should be understood and observed. Fortunately, all of this information
is on the herbicide label. Anyone who uses an aquatic herbicide should
always thoroughly read and understand the herbicide label before purchasing
and applying the chemical.
County Extension agents and state fisheries biologists can advise pond
owners who have questions concerning aquatic herbicide selection, methods
and rates of application, precautions, and state and Federal regulations.
The information in this section is not intended to replace the information
on the herbicide label. Rather, it should be used to supplement label information.
Selection of an Aquatic Herbicide
Identification of the weed, knowledge of pond water volume or surface area,
and water uses will help the pond owner select the most effective and economic
No single aquatic herbicide is capable of controlling all kinds of weeds
that are potential problems in the management of a pond. For this reason,
it is important to identify the weed before attempting to control it with
a herbicide. Herbicide labels list the aquatic weeds which will respond
to treatment with that particular herbicide. Table 2 lists the aquatic
weeds described in this publication and the herbicides which are usually
effective in their control.
Pond Water Volume and Surface Area
Herbicide application rates are determined either by the pond's water volume
or by surface area. Most herbicide labels list application rates according
to one or the other of these measures. Some herbicide application rates
will vary depending upon the weed species and the extent of infestation.
Prior knowledge of the pond's water volume and surface area will aid in
the selection of the most economical herbicide and insure the proper amount
is purchased and applied.
Pond Water Uses
Restrictions and cautions of herbicide use will vary depending upon the
herbicide. This information is on the herbicide label. If pond water uses
are not compatible with the restrictions and cautions listed for a specific
herbicide, an alternate herbicide should be selected, or another aquatic
weed control measure used.
It is not enough to select the right herbicide. The herbicide must also
be applied properly. The herbicide applicator must be knowledgeable of
the herbicide formulation, methods and rates of application, precautions
and warnings, and other information specific to the herbicide.
Herbicide formulations vary in the amount of active ingredients present.
The active ingredients actually are the chemicals which kill the pest or
weed. Inert ingredients are added to improve the convenience, safety, and
handling of the herbicide. Herbicide application rates are based upon the
active ingredient in the herbicide formulation. This is one reason why
it is always important to determine herbicide application rates from information
printed on the label.
Methods of Application
Some herbicides can be applied directly from the container (ready for use)
and others need to be diluted with water or some other diluent before application.
Always follow label directions. Dispersion of the herbicide is also important.
In small ponds, for shoreline areas or "spot treatments," many of the herbicides
can be applied by simply broadcasting the chemical over the area. Most
of the granular formulations can be scattered directly. In small areas,
some liquid formulations can be poured over the area but others need to
be diluted and may be applied with a sprinkling can. Treatment of large
areas usually require the use of mechanical sprayers or spreaders and a
boat with an outboard motor to insure that the chemical is adequately dispersed.
Most aquatic herbicides can be applied with a hand operated seeder, pressurized
sprayer, or boat bailer. Injecting the chemical near the outboard motor
propwash will help in dispersion. Some herbicides are "contact killers,"
that is, they must come into direct contact with the plant. For this reason,
contact herbicides must be evenly dispersed in or on the water to have
maximum effectiveness. Other herbicides must be absorbed by the plant to
be effective and are called "translocated" herbicides. The following can
be used as general guidelines for the application of herbicides to the
four aquatic weed groups:
Since algae is usually dispersed throughout the body of water, control
usually requires treatment of the total volume of water. Exceptions to
this are treatments of floating (filamentous) algae mats which may be spot
treated, or treatment of small portions of the pond at two week intervals.
The two-week interval is necessary to reduce the possibility of a fish
kill which may be caused by oxygen depletion resulting from the decomposition
of dead plants.
Control of floating weeds usually requires a surface spray application,
as the least expensive method, as compared with treatment of the total
volume of water.
Emersed (above water) Weeds
Control of emersed weeds may permit a choice of methods, depending upon
the specific weed. If a large portion of the leaf area is above water,
surface spray applications may be the most effective and least expensive
method. Application of aquatic granules or pellets may also be substituted,
but this is usually more expensive. Treatment of other weeds may permit
gravity flow application, where the undiluted herbicide is poured slowly
into the pond from a boat propelled across the surface in a regular pattern.
Wind and water currents disperse and dilute the herbicide to the desired
Submersed (underwater) Weeds
Control of submersed weeds is not usually practical with surface spray
applications. Gravity flow application is sometimes utilized but may be
wasteful of herbicide. Herbicides are more effective in an aquatic situation
if placed close to the root system or leaf area of the plant. Granule or
pellet formulations are more frequently used to accomplish this. Another
variation is the use of specialized equipment, used by commercial applicators,
which places liquid herbicides in the "bottom acre-foot" of water near
the site of uptake by the plant. The greater specific gravity of the herbicide
holds the treatment in higher concentration near the plant for improved
Rates and Time of Application
If it is necessary to use aquatic herbicides in the management of a pond,
it is best to apply them in the spring or early summer. Most aquatic herbicides
can be effectively applied when water temperatures are above 55°F or
when aquatic weeds show signs of new growth. Advantages of spring or early
summer treatment are: 1. There is less of a chance of oxygen depletion
and fish kills. 2. For some herbicides, a smaller amount of chemical is
needed for control. 3. Many herbicides are more effective on plants in
the early growth stages. Also, total plant mass is less, so less herbicide
is required. 4. Early application of herbicides often facilitates later
non-herbicide control measures such as fertilization.
Application rates should always be based upon label recommendations.
However, if a pond has a heavy infestation of weeds it is wise to treat
a fraction of the weeds in the pond at a time, waiting two or three weeks
between applications. During the warm summer months, never treat the entire
pond with a herbicide at one time, no matter how minor the weed infestation.
Dead and decaying plants consume oxygen from the water. Treating a fraction
(Figure 49) of the pond will allow the fish to escape to oxygenated water
during the decay process and reduce the chances of a fish kill. When using
herbicides, never wait until a pond has become choked by aquatic weeds.
Instead, spot treat areas of weed growth early, before they become a problem.
Figure 49. Partial herbicide treatment.
Precautions and Warnings
Specific information concerning precautions and warnings is printed on
the herbicide label. These precautions and warnings are listed to help
insure that no health hazard occurs because of their use. The following
are applicable to any aquatic herbicide:
ATTENTION: Herbicide Precautions
Observe all directions, restrictions and precautions on herbicide labels.
It is dangerous, wasteful and illegal to do otherwise.
Store all herbicides in original containers with labels intact and behind
Use herbicides at correct dosage and intervals to avoid illegal residues
or injury to animals.
Apply herbicides carefully to avoid drift.
Bury surplus herbicides and destroy used containers so that contamination
of water and other hazards will not result.
We are grateful to the following people and organizations for their assistance
and suggestions with this publication: Dr. David R. Bayne, Auburn University;
Mr. Gladney Davidson, U.S. Fish and Wildlife Service; Dr. Thomas K. Hill,
The University of Tennessee; Dr. John W. Jensen, Auburn University; Mr.
Donald Lee, Louisiana Department of Wildlife and Fisheries; Mr. Wayne Thomaston,
Georgia Department of Natural Resources, and Dr. Thomas L. Wellborn. Jr.,
Mississippi State University who also supplied photographs 5, 6, 21, 22,
43. Funds for this publication were provided, in part, by the U. S. Department
of the Interior, Fish and Wildlife Service.
Burkhalter, A.P. et. al. 1978. Aquatic Weed Identification Control Manual.
Bureau of Aquatic Plant Research and Control, Florida Department of Natural
Resources, 100 pages.
Godfrey, R.K. and J.W. Wooten, 1979. Aquatic and Wetland Plants of Southeastern
United States. The University of Georgia Press, 712 pages.
Robinette, D.L. Aquatic Pest Control Training Manual, Pesticide Training
Series 5. Cooperative Extension Service, Clemson University, 105 pages.
Wellborn, T.L. 1979. Aquatic Weed Identification and Control. Cooperative
Extension Service, Mississippi State University, Information Sheets 10251037.
Winterringer, G.S. and A.C. Lopinot, 1966. Aquatic Plants of Illinois,
Illinois State Museum of Science Series Vol. Vl, 142 pages.