Flowers and Garden Home The most common aquatic weed problem in many ponds is filamentous algae.  
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Browse Articles: Ponds
Controlling Filamentous Algae in Ponds
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Reference: Ohio State University Extension

The most common aquatic weed problem in many ponds is filamentous algae. Its presence can degrade water quality and recreational enjoyment. Excessive algae can cause an oxygen depletion leading to a fish kill when it decomposes as a result of natural die-off or herbicide application. Early and regular control measures will help reduce the problems associated with filamentous algae.

The Plant
Filamentous algae, also called "moss" or "pond scum," forms dense mats of hairlike strands. Its growth begins on submerged objects on the pond bottom. As it grows, the algae gives off oxygen that becomes entrapped in the mat of strands. This gives it buoyancy and causes it to rise to the surface where it frequently covers large areas of the pond. Filamentous algae is often a persistent problem because it reproduces by plant fragments, spores and cell division.

There are many species of filamentous algae and microscopic examination is usually required to make an exact identification. However, some of the more common forms can be distinguished by their texture. Spirogyra is bright green and slimy to the touch; Cladophora has a cottony feel; and Pithophora is often referred to as "horse hair" algae because its coarse texture resembles that of horse hair and it may feel like steel wool.

Mechanical Control
Filamentous algae can be controlled by physically removing large floating clumps with a rake. This will prevent the algae from decomposing in the pond and consuming dissolved oxygen. Algae that has been removed can be piled for composting or used in a garden as mulch.

Steepening the sides of the pond to achieve a 3:1 slope will eliminate shallow water areas so that sunlight cannot reach bottom-growing algae. However, if the entire pond has filled in as a result of sedimentation or decaying vegetation, a dragline or dredge may be needed to deepen the pond.

Biological Control
Biological control involves disrupting plant growth by modifying the aquatic environment through natural manipulation, or it can mean introducing a living organism that is capable of controlling aquatic vegetation.

One method of biological control is maintaining a fertility level that fosters the development of a microscopic plant and animal population, which prevents sunlight penetration. This requires intense management and more time than the average pond owner may wish to devote to the pond. Sunlight penetration to the pond bottom where the algae begins to grow can also be reduced by introducing an inert dye (usually blue).

The addition of triploid white amur (a vegetation-eating fish) as a biological control measure may have mixed results. Filamentous algae is not a preferred food, but will be eaten if no other vegetation is present. If other aquatic plants such as water milfoil or coontail are readily available, the filamentous algae may be ignored and continue to flourish.

Chemical Control

Copper Sulfate
Most species of algae can be controlled with very low concentrations of copper sulfate. It is available in crystalline nuggets the size of rock salt or as a finely ground "snow" grade (Figure 1). The recommended treatment rate is 2.7 pounds per acre-foot of water. [Acre-feet is a volume measurement of the pond. It is determined by multiplying average depth (feet) X surface area (acres). When uniformly applied, this will result in a 1 part per million (ppm) concentration throughout the volume of the pond. For very hard water (more than 12 grains or 200 parts per million of hardness), this rate should be doubled.

Figure 1. Copper sulfate is available as nuggets
(left) or as finely ground crystals.

The method of application will determine what size of copper sulfate crystals to purchase. The important principle to keep in mind is that actual contact of the copper sulfate with the algae is necessary in order to achieve satisfactory control. For best results, dissolve copper sulfate in water and spray it directly on floating algal mats or on the water surface above submerged algae. Finely ground, "snow grade" copper sulfate is best for1 this method as it dissolves easier. Mix the desired amount of copper sulfate with enough water to cover the area to be treated, and apply with a sprayer or bucket and dipper. Because copper is corrosive to galvanized metal, application equipment and mixing containers should be made of plastic or stainless steel.

In large ponds and when spray equipment is not available, it may be easier to treat with copper sulfate by placing the larger crystals of this chemical in a burlap bag and towing the bag through the water until all the crystals have been dissolved in the area to be treated.

One application of copper sulfate is unlikely to provide season-long control. Re-treatment may be necessary at 3-4 week intervals.

There are no water-use restrictions associated with the use of copper sulfate. When applied at the proper rate, the water may be used immediately for swimming, drinking, fishing, irrigation and livestock. However, since copper sulfate has a metallic odor, pond owners may want to suspend drinking, swimming and livestock watering uses for 12 hours.

Copper Chelate
Copper is also available in a chelated, or buffered, formulation, which is manufactured as a liquid or granule. This provides some advantages during application. The liquid form needs only to be mixed with water and sprayed out over the pond surface; there are no crystals to dissolve. The granular formulation consists of a clay granule impregnated with copper chelate. As the granule breaks down, the copper is released into the water. This formulation is especially useful when spot treatment is desirable. Granules are best suited for application early in the growing season because of the time required (2-3 weeks) for them to dissolve and release the chemical. There are no water-use restrictions associated with either formulation of copper chelate.

Diquat dibromide
This is a contact herbicide that will control some, but not all, species of filamentous algae. It is applied by pouring directly from the container or by diluting with water and injecting below the water surface. For best results, it should be applied before algae growth reaches the surface. Diquat dibromide should not be used in muddy water. There are water-use restrictions associated with this material. Read the label.

The amine salt formulation of endothall (sold as Hydrothol 191) is labeled for algae control. It is available as a liquid or granular material. Endothall is a contact herbicide and is most effective in waters 65 degrees F and above. Fish are extremely sensitive to this material. Read the label for water-use restrictions.

Inert dyes
Inert dyes can be used to control algae. The color they turn the water, usually blue, reduces sunlight penetration, which in turn reduces growth of algae and submerged weeds. These dyes are not effective in water less than 2 feet deep or if the algae is floating on the water surface. Most inert dyes are labeled for all water uses except domestic drinking water supplies. Check the label.

Table 1. Aquatic Herbicides for Filamentous Algae Control (except Pithophora)
Labeled Herbicides Waiting Period Before Water Used For:
Chemical NameHumanAnimalIrrigation
DrinkingSwimmingFishingDrinking Turf Food Crops
0 days0 days0 days0 days0 days0 days
(Inert dye)Not permitted24 hours
(note 1)
0 days0 days0 days0 days
CoppersulfateCopper sulfate0 days (note 2)0 days
(note 2)
0 days0 days (note 2)0 days0 days
(liquid and
Copper chelate0 days0 days0 days0 days0 days0 days
(note 3)
Diquat dibromide14 days24 hours0 days14 days14 days14 days
Endothall7-14 days (note 4)24 hours3 days7-14 days (note 4)Not permitted7-14 days
(note 4)
Notes to Table 1: 1 - Wait for complete dispersal before swimming.
2 - No required waiting period. 24-hour waiting period recommended to allow for dissipation of metallic odor.
3 - Controls some species of algae: Spirogyra and Pithophora.
4 - Varies by application rate used.
5 - Copper is toxic to fish eggs and fish fry.
6 - Production of this material has been terminated, but it may be purchased and used until supplies are exhausted.

Table 1. (Continued) Aquatic Herbicides for Filamentous Algae Control (except Pithophora)
Labeled HerbicidesCharacteristics
Trade NameChemical NameMin/Max
Water Temp.
BiodegradableFish Toxicity at
60 F/NonePartialnote 57-10 days
(Inert dye)NoneYesNoVaries
CoppersulfateCopper sulfate60 F/NoneNonote 53-5 days
(liquid and
Copper chelate60 F/NonePartialnote 57-10 days
(note 3)
Diquat dibromide60 F/NoneAdheres to soilNo7 days
Endothall65 F/NoneYesYes3-14 days
Notes to Table 1: 1 - Wait for complete dispersal before swimming.
2 - No required waiting period. 24-hour waiting period recommended to allow for dissipation of metallic odor.
3 - Controls some species of algae: Spirogyra and Pithophora.
4 - Varies by application rate used.
5 - Copper is toxic to fish eggs and fish fry.
6 - Production of this material has been terminated, but it may be purchased and used until supplies are exhausted.

Copper-Resistant Algae
One form of filamentous algae, Pithophora, can be especially troublesome because it is resistant to normal applications of copper compounds. Although it is not widespread, scattered reports of Pithophora in ponds are received every year. If, after a normal treatment with copper sulfate, there is algae remaining that does not appear to be affected, it may be Pithophora.

Pithophora is extremely difficult to control. Its unique cell wall structure and the tight clumping of filaments inhibit the penetration by copper. Additionally, large numbers of resilient spore-like bodies, called akinetes, germinate and provide a continuous source of new plants. Partial, short term control can usually be achieved with either of the following herbicide mixtures:

Rate of Mixture
Cutrine Plus Liquid
and Diquat/Reward
1:12 gallons per acre-foot
Cutrine Plus Liquid
and Hydrothol 191 Liquid
2:11 gallon per acre-foot

Additionally, Cide-Kick, a nonionic spray adjuvant, should be added to the mixture at the rate of 1-2 gallons per surface-acre. This material acts as a cell wall penetrant to increase the effectiveness of the herbicides.

Special Precautions
Fish are extremely sensitive to Hydrothol 191. To reduce the hazard of a fish kill, start application at the shoreline and move outward so that fish can escape from treated areas. Select another product if fish toxicity is a concern.

Copper sulfate is corrosive to galvanized containers. Therefore, the solution should be mixed in wooden, earthenware, plastic, stainless steel or copper-lined containers. If a sprayer is not available, you may broadcast the solution with a plastic watering can or bucket and dipper.

If the algae is so abundant that it covers more than half of the total pond surface, a complete treatment may result in an oxygen depletion and fish kill. This hazard is greatest during very hot, overcast weather. When these conditions exist, treat only half the pond and wait 10-14 days before treating the other half.

Copper compounds applied at the recommended rates are lethal to fish eggs and some species of newly hatched fish. These materials should not be applied during spawning periods, unless it is desirable to destroy the eggs and the new hatch. Bass will begin to construct shallow depressions in the pond bottom when the water reaches 60 degrees F. Eggs are deposited by the female and guarded by the male for 3-14 days. Within a couple of weeks after the bass have spawned and when the water temperature reaches 70 degrees F, bluegill and redear sunfish will be seen building nests in the shallow areas. As with the bass, the male guards the nest after the eggs have been deposited. These eggs will hatch in a few days. Bass will only spawn once in the spring, but forage fish (bluegill, redear sunfish and minnows) will spawn throughout much of the summer and some individuals may spawn several times in a single season. To avoid the application of copper compounds during the spawning season, monitor the water temperature and look for active nests in the shallow areas of the pond.

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  Data Source: Ohio State University Extension. Articles and resource may contain pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide and it is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used.