Flowers and Garden Home The owner's goal for a pond is an important factor in determining when to control aquatic plants.  
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Browse Articles: Ponds
When to Apply Aquatic Herbicides
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Reference: Ohio State University Extension

Aquatic vegetation often reaches nuisance levels in many 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.

Emergent Vegetation
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 a commonly controlled emergent plant. Best control is achieved when the seed head is just beginning to turn brown and for several weeks thereafter. 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.

Submerged Vegetation
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 plant's 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.

Floating Vegetation¨Water Lilies, Water Lotus, Pennywort, and Watershield
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.

Floating Vegetation¨Watermeal and Duckweeds
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 pond's 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 pond's 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.

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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.