Many pond owners are familiar with the problems caused by filamentous algae. The stringy, fast-growing algae can cover a pond with slimy, lime-green clumps or mats in a short period of time. Fortunately, products are available to assist the owner in controlling filamentous Planktonic algae are another group of algae common to ponds. These algae are critical to a ponds food chain as they provide food for the microscopic animals that in turn are eaten by freshly-hatched fish fry. Occasionally, planktonic algae can bloom to nuisance levels which may necessitate using control methods.

These algae are microscopic free-floating plants. They are suspended in the top few feet of water of a pond or lake where light is bright enough for them to produce food by photosynthesis. The planktonic algae community in ponds is typically composed of green algae, blue-green algae, diatoms, and euglenas. Some species of planktonic algae, primarily blue-greens, can be toxic to animals and impart an odor or taste to the water.

Planktonic algae are least abundant in winter when cold water temperatures inhibit their reproduction and growth. This is why most ponds are their clearest in winter. As ponds warm in April, reproduction by algae increases greatly and the spring algae bloom occurs. The ponds water becomes distinctly less clear, with water color becoming greenish or brownish depending on the algae species present. In late April and May, reproduction by various microscopic animals (rotifers and daphnia, for example) creates large populations of these animals which begin to crop the algae bloom. The water becomes clearer. Once water temperatures reach about 72 degrees F, the microscopic animal population declines rapidly through decreased reproduction and predation by small fish fry. This allows the planktonic algae to rebound, but usually not to levels of April. In most ponds, planktonic algae levels remain stable throughout the summer unless there is a sudden, unexpected source of new nutrients to cause a summer bloom. As ponds cool in fall, abundance slowly declines to winter levels. Many ponds become noticeably clearer during fall as algae abundance declines.

Planktonic algae blooms are rarely a problem for Ohio pond owners, but when they do occur the blooms cause considerable concern. Algae blooms cause the water to appear pea soup green or brown in color. Severe blooms often create the appearance of green paint being spilled on the ponds surface. For the pond owner who desires a clear pond for swimming or for aesthetic reasons, this situation is unacceptable.
Severe algae blooms can cause problems for fish. If the bloom dies-off suddenly, a fish summer kill can result due to oxygen depletion. Bloom die-off can be caused by weather changes, a sudden decrease in nutrient levels, or treatment of the pond with an herbicide.

Like all plants, algae require nutrients to grow and reproduce. Because algae are free-floating, they must get those nutrients from the water. They have no ability to obtain them from the pond bottom. Thus, the higher the nutrient level in the pond, the more algae you will have. At slightly higher nutrient levels, the algae community is often dominated by filamentous algae. This is particularly true during summer. At very high nutrient levels, the algae blooms are typically composed of planktonic algae rather than filamentous algae. Fortunately, few Ohio ponds have very high nutrient levels unless they are receiving unwanted nutrients from the ponds watershed.

There are many possible sources of excess nutrients that might enter a pond and cause a nuisance bloom. Some of the more common sources are fertilizing grass around ponds, too many geese, run-off from livestock operations, and leaking septic systems. Another common cause of summer planktonic algae blooms is the sudden release of nutrients from dying aquatic vegetation or filamentous algae that has been treated with an herbicide. Again, the more excess nutrients added, the more likely the resulting bloom will reach nuisance levels.

Prevention—Preventing an algae bloom is always preferable to the expense of treating with an algaecide to eliminate it. If a pond owner wishes to avoid a nuisance bloom, he or she should investigate the ponds watershed and determine potential sources of excess nutrients. If sources are found, then a plan to eliminate the source should be developed and implemented. This may require working closely with neighbors.

If a pond is experiencing a severe nuisance algae bloom, the owner should perform the above investigation but may want to postpone implementation until winter if fish are important. As mentioned previously, the sudden depletion of nutrients to a pond with a severe ongoing algae bloom could cause a summer fish kill. In this case, it makes sense to allow cooler fall weather to decrease the algae bloom and then implement a plan to prevent new blooms the next year.

In some situations, it may be impossible to eliminate or substantially reduce unwanted nutrients from entering a pond. Uncooperative neighbors is a common reason for this. A preventative control method is the use of an aquatic dye, such as Aquashade, to prevent initial growth early in spring. The dye must be added by April 1 to insure prevention of that years algae blooms. A drawback to the use of dyes is that reducing the production of planktonic algae impacts the ponds food chain. Less production at the bottom of the food chain can mean fewer pounds of fish in the pond. Many owners of very nutrient-rich ponds consider this an acceptable outcome when using the dyes.

Algaecide—A nuisance planktonic algae bloom can be quickly reduced with the use of liquid chelated copper compounds, such as Cutrine-Plus or Algae-Pro. Again, it is important to note that a sudden summer die-off of algae can cause a fish kill. Planktonic algae have a very high reproductive rate, so re-bloom may occur in just a few weeks following treatment. Several treatments may be necessary for seasonal control.

Aquatic vegetation often reaches nuisance levels in many Ohio ponds, prompting owners to initiate control activities. Frequently they turn to aquatic herbicides and/or algaecides to achieve their desired pond conditions. Often overlooked in the use of these products is the timing of application. Timing is important for two reasons. First, application at the correct time can greatly minimize the possibility of a fish kill and second, applying at the wrong time can result in less than satisfactory results in the control of nuisance vegetation. Poor timing can result in additional, expensive applications later on. Knowing the correct time periods to apply the various control products can save the pond owner money and frustration.
The owner’s goal for a pond is an important factor in determining when to control aquatic plants. This goal and the owner’s personal tolerance for various aquatic plants determine whether or not to treat, as well as when to treat. In ponds where fish are a high priority or the owner simply does not want to potentially see dead fish, timing is critical to avoid a fish kill. Ponds used primarily for swimming, where plants typically are not desired, need to be treated as early as possible in order that quality swimming can occur by early June. The same scenario applies to ponds used for irrigation to avoid clogging pumps by early June. Ponds that do not contain fish or where the owner is willing to accept an increased risk of a fish kill can be treated throughout the summer. While the following are general guidelines for determining when to treat various types of aquatic plants, the final decision ultimately rests with the pond owner.

Examples of commonly controlled emergent plants include cattails, bulrushes, spike-rushes, smartweeds, and arrowhead.

A general rule of thumb for eradicating emergent plants is to apply a herbicide when the plant is finishing its flowering stage and beginning to set its seed head. This is particularly true for cattails, bulrushes, and arrowhead. During and just after seed head formation, these plants begin to send food to their root systems for root growth. Infusion of a systemic herbicide into the root system at this time is very damaging to the plants. Cattails are the most commonly controlled emergent plant in Ohio. Best control is achieved when the seed head is just beginning to turn brown and for several weeks thereafter. In Ohio, this typically occurs in June and July, depending on seasonal temperatures. Contact herbicides, such as diquat products, can be used at any time when plants are actively growing; however contact herbicides do not kill the root system. These products provide temporary control, but re-growth from the roots will occur.
Applications on emergent plants are not recommended prior to seed head formation unless a contact herbicide is being used. Contact herbicides can also be used on emergent plants throughout the summer. September and October applications are not recommended, as colder fall weather will cause a natural dieback.

Examples of commonly controlled submerged aquatic plants include pondweeds, naiads, Eurasian watermilfoil, elodea, and coontail.

Most submerged plant species begin active growth in May, reaching maximum biomass in July and August. Whole pond treatments, with contact or systemic herbicides, to control submerged plants should occur in May or June before maximum biomass is attained. Generally, treatments should not occur until after water temperatures reach 60 degrees F and will remain there for the remainder of the spring. The exceptions to this rule are if the problem plants are Eurasian watermilfoil or Curlyleaf pondweed. These invasive European species should be treated in April during their fast growth period.

A common lament by pond owners is that aquatic submerged plants aren’t a problem until July and August. Their follow-up question is, How do I know in May or June if I’m going to have a problem later in the summer? If the pond has had a problem with submerged plants in the past, it is likely they will pose a problem in the current year. The owner may want to treat preemptively and avoid later problems. Another strategy is to check submerged plant growth during late May, even though submerged plants may not be easily visible. A pair of polarized sunglasses may allow the owner, on a calm day, to see deeper into the water and determine if plants are visible. The presence of young, submerged plants throughout the shallow areas of a pond indicates a potential problem later in summer. Another alternative is to use a rake and assess submerged plant growth in all depths that the owner can reach. The presence of plants on the rake each time it is pulled along the bottom indicates a potential problem. Whether or not to treat is up to the owner and his/her goals, but it is better to treat in May and June than later in the summer.

So why not treat later? Successful treatment of submerged plants should strive to minimize the amount of decaying vegetation in the pond during the summer. For ponds containing fish, it is generally not recommended that liquid herbicides be used for the first time after late June if the abundance of submerged plant material is high. The chances of a summer fish kill due to oxygen depletion are greatly increased after whole pond herbicide treatments in July, August, and September. Even June can be risky if it is abnormally warm. Warmer water contains less oxygen and the decomposition of dead plant material requires large amounts of oxygen to complete. The more plant material killed, the more oxygen required for decomposition, which results in less oxygen available to fish and other aquatic animals. Additionally, eradication of submerged green plants eliminates a major source of pond oxygen from photosynthesis.
It is possible to do summer and late summer herbicide applications for submerged vegetation with granular formulations if done carefully. To be successful, treat about one-fourth of the vegetation with a granular contact herbicide every two to three weeks until the entire pond has been treated. This allows the treated plants to decay while the remainder of the vegetation continues to produce oxygen for fish and the decomposition process. Also, consider treating only the more important areas (i.e., swimming area) and leaving the remaining areas untreated.

One herbicide is currently available that allows for whole pond treatment of submerged plants during the summer. The active ingredient is fluridone, a systemic herbicide that slowly eliminates a plants ability to photosynthesize food. Thus, the plant slowly starves to death. For most submerged plants, it takes 45 to 90 days for a complete kill. This lengthens the decomposition period as all plants do not die at one time. While some plants are decomposing, surviving plants are still producing oxygen needed to aid the decomposition process and prevent a fish kill. Using fluridone requires the pond owner’s willingness to tolerate a slow decrease in submerged plant abundance. If a quick eradication is desired, May and June treatments with other products should be considered.

The same recommendations provided for emergent plants can be applied to plants whose leaves have reached the surface. Systemic glyphosate herbicides should be applied immediately after the peak flowering period has passed. Prior to their leaves reaching the surface, 2,4-D granular herbicides can be used to kill these floating plants. Apply the recommended rate just as the first leaves are below the surface. Once most leaves are floating, 2,4-D is not a recommended control product for floating plants.

Watermeal and duckweeds have an explosive reproductive potential, and can easily cover a pond in a few weeks. This generally occurs in mid- to late summer but occasionally occurs earlier.

These small floating plants should be treated as soon as they become noticeable in the shallow areas of the pond. At this time, any product labeled for controlling these plants can be used. Once watermeal and/or duckweeds have covered most of the pond, a quick, total kill should not be done because a fish kill can occur. A scenario similar to that described for submerged plants should be followed. Either use a fluridone product to slowly eliminate the floating watermeal/duckweed problem over a 45 to 90 day period or treat small areas (one-fourth of the pond at a time) every two weeks with other recommended products.

There are many types of algae. Planktonic algae are microscopic in size, and are the foundation of a ponds food chain for fish and other animals. They are generally considered desirable, and only occasionally attain nuisance abundance. Filamentous algae forms dense mats of hair-like strands that can rise to and subsequently cover the ponds surface during late spring and summer. Bottom, mat-forming algae grows initially on the bottom, but often breaks away to create numerous small floating pads on the surface. A pond can have thousands of these pads, making for an unsightly mess. Chara, or muskgrass, resembles a submerged plant but is actually an alga. Unfortunately, many algae species are what are called blue-green algae, which are more resistant to algaecides than green algae species. Nuisance levels of any algae species are an indicator of excessive nutrients in the pond. A pond owner would be well advised to look for and eliminate unwanted nutrient sources prior to applying algaecides. Common sources of unwanted nutrients are too many geese and ducks, lawn fertilizer, agricultural runoff, animal manure, and septic systems. Lowering nutrient levels may eliminate the need for algaecides.

Most alga species begin active growth in April, reaching maximum biomass in June, July, and August depending on pond nutrient levels. As a rule of thumb, it is recommended that treatments with algaecides begin in May or early June before maximum biomass is attained. Generally, treatments should not occur until after water temperatures reach 60 degrees F and will remain there for a week. However, good control of filamentous algae in very shallow areas has been attained in early spring with granular copper compounds. Shallow areas can get very warm on quiet, sunny spring days while the main area of the pond is still too cold for a general copper application. Simply sprinkle the granules on the early season growth of filamentous algae in the shallow areas. This early season control may preclude a whole pond treatment later once water temperatures exceed 60 degrees F.

Chara, the plant-like alga, should be treated in May if at all possible. Chara has a high affinity for calcium, which calcifies this alga species by mid-June. Control of Chara becomes less successful as summer progresses. Copper compounds are best applied to Chara infestations when the alga is young and supple.
As for submerged plants, treating a heavy growth of algae after late June increases the risk of a fish kill. Contrary to submerged plants, however, algae often returns four to eight weeks after an algaecide application has been applied. Initial applications should be made in spring to control algal growth, followed by additional small-scale treatments to keep the algae at low levels of abundance. Otherwise, algae could again reach nuisance levels even though a spring application was made. Weekly inspections of the pond are a good strategy to assess algal abundance.

When confronted with an overabundance of aquatic vegetation, many pond owners know the importance of choosing the correct herbicide or algaecide to use and calculating the correct application rate. Product labels provide the needed information to accomplish this. However, these owners rarely give serious consideration to when to apply the chosen product so as to maximize effectiveness and decrease problems associated with its use.

Effectiveness is greatly enhanced by treating a plant problem early, before it reaches serious nuisance levels. Emergent (cattails) and large-leaved floating vegetation (water lilies) are best controlled just after flowering when roots are weakest and the plant begins to send nutrients down to the root system. Submerged plants, duckweeds, watermeal, and algae should be initially treated in late spring, before high biomass levels are attained. In the case of algae, additional treatments may be necessary throughout the summer as re-blooms are quite common.

The main problem associated with mid and late season treatment of dense areas of submerged plants, duckweeds, watermeal, and algae is the increased risk of causing a fish kill. Decomposition of dying plants requires large amounts of oxygen at a time when oxygen production in the pond has been reduced due to the elimination of living green plants. This risk is greatest during hot weather because warmer water naturally contains less oxygen. After mid-June, the pond owner should consider spot-treating portions of the pond rather than performing a whole pond treatment. Control of emergent vegetation throughout the summer rarely causes a fish kill because it only grows in the shallowest water, decreasing the amount of dying vegetation in the pond.

When applied at the correct time, using aquatic herbicides and algaecides can be done in a cost-effective manner with little risk of a fish kill. Many product labels provide important information related to when to apply herbicides and algaecides. These labels should be read carefully.

Like all plants, submerged aquatic plants and algae require nutrients and sunlight to flourish. These plants can grow only where sufficient light reaches the bottom of the pond or lake. This zone is called the photic zone. Beyond this zone, usually in deeper waters, plants cannot grow. Many factors affect how deep the photic zone extends, including plankton density, water color, and even wind.
In general, the more stained or colored the water, the shallower the photic zone. In some lakes and ponds, the water can be stained so dark by tannic acid from leaves or colored by a dense phytoplankton bloom that no submerged plants can grow except in the shallowest water.

Aquatic dyes work similarly by coloring the water a dark blue. The absorption and scattering of sunlight in the blue water significantly reduces the depth of the photic zone. This limits submerged plant and algae growth to only the shallowest areas of the pond or lake. For some pond owners, this reduction in plant biomass may be sufficient to meet their desired pond condition. Other pond owners may need to spot treat with herbicides and algaecides or introduce grass carp (white amur) to eliminate remaining plants.
Applying Aquatic Dyes
Aquatic dyes must be applied in March or early April to be effective for plant and algae control. The key to controlling aquatic plants is to prevent germination and limit early season growth. Because this occurs in April for most submerged plants and algae, the dye must be present to prevent germination. Late applications that occur after plants have grown several feet off of the pond bottom yield poor results. Sufficient sunlight will reach the top of the plants, allowing them to continue growing.

Application is as easy as pouring the dye into the water. It will quickly spread throughout the pond, usually within a few hours. Because the dye will naturally diffuse throughout the water, there is no need to use a sprayer. The applicator should take precautions to avoid getting the undiluted dye on skin or clothes, as it will stain.

The initial application in March will begin to fade in May or June, sometimes even earlier. Color is gradually lost due to rainwater dilution, photodegradation, and biodegradation. As the dye fades, sunlight reaches deeper water and can trigger plant germination there. This leads to a late summer aquatic plant or algae problem. Maintaining the correct level of blue color throughout the growing season can eliminate a late-season plant problem.

How does a pond owner know when more dye is required? The pond owner should regularly assess how much blue color is present in the water. This can be accomplished by measuring how far down a weighted white object on a string can be seen as it is lowered into the water. These readings should always be taken at the same location and at the same time of day. The first reading should be taken 48 hours after the initial application in March. This will serve as the baseline color level for the recommended application rate.

Every two weeks or so, the pond owner can re-measure the depth at which the white object disappears. Once the measured depth increases by roughly 25%, more dye should be added to bring the measured depth back to the baseline measurement. For example, if the baseline measurement in March was 24 inches, an increase to 30 inches in May should prompt additional dye being added to the pond.
Pond owners should try to maintain the desired color through August to prevent late summer plant or algae problems. As water cools in September, submerged plant and algae growth will slow considerably, and the residual color will continue to inhibit their growth during fall.

A pond with an existing abundant submerged plant or algae community should not be treated with dyes for the first time in late spring or summer. This will cause a substantial die-off of plants in deeper water where sunlight no longer reaches. These dying plants begin to decay and can cause a fish kill due to oxygen depletion.

Ponds that have been dyed should not be used as a drinking water source for humans; however, consumption by livestock is permitted. Water can also be used for irrigation of all crops. Fish from dyed ponds are safe to eat, and recreational swimming is permitted. Swimming in dyed ponds once the dye has diluted to the proper level will not result in stained clothes, skin, or hair.
Dyes do not inhibit growth of all aquatic plants found in Ohio ponds. Dyes are recommended for use on submerged plants, such as pondweeds, watermilfoil, naiads, coontail, and elodea. Dyes also inhibit growth of filamentous algae, mat-forming algae, and single-cell planktonic algae. Dyes do not inhibit the growth of emergent plants, such as cattails, and are not effective in controlling floating-leaved plants (duckweeds and water lilies) once their leaves are floating on the surface.

A major limitation in the use of aquatic dyes is the water exchange rate associated with the pond. Ponds with a substantial watershed receive considerable flow during rain events, and it becomes difficult to maintain the required blue color to inhibit plant growth. The owner would incur considerable expense in the continual addition of dye to compensate for dye lost through the overflow pipe.
Aquatic dyes are most effective in ponds receiving very little inflow water, even during heavy rain. For these reasons, dyes are very effective in excavated ponds receiving little runoff water. Dyes are not recommended for embankment ponds (dam) that retain water from a large watershed.

Aquatic dyes are a viable tool in managing many submerged plant and algae species. Dyes work best in excavated ponds receiving very little runoff water. They inhibit plant growth by limiting how deep sunlight can penetrate into the water, which allows plant growth only in areas of very shallow water. The keys to effectively using pond dyes are to make the initial application in early spring prior to germination and then add supplemental applications as needed to maintain the desired color. Ponds already having an overabundance of submerged plants or algae that contain fish should not be treated with a dye as the dark blue water will begin to kill plants in deeper water and can trigger a fish kill.